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Mission Statement 

The purpose of FLAPS-2-APPROACH is two-fold:  To document the construction of a Boeing 737 flight simulator, and to act as a platform to share aviation-related articles pertaining to the Boeing 737; thereby, providing a source of inspiration and reference to like-minded individuals.

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Journal Archive (Newest First)

Entries in Flight Training (10)

Monday
Aug042014

Boeing 737-800 Takeoff Procedure (simplified)

One aspect novice ‘virtual pilots’ find difficult to grasp is the correct method of flying the aircraft, especially the takeoff and transition to climb and cruise. 

LEFT:  Captain-side B737 trim tab lightplate with backlighting turned on (OEM throttle quadrant).  Setting the correct trim prior to takeoff is important; an incorrect trim may cause pitch issues during rotation.

The sheer volume of information available on the Internet often results in ‘information overload’ and it is understandable that many become bewildered to the correct way of completing a task.  The boundaries between fact and fiction quickly become blurred.  Add to this that many articles on the Internet have not been peer reviewed and you have a recipe set for disaster!

In this article,  I will instruct on the basic procedures used to takeoff, climb and transition to cruise.

I will not discuss before and after takeoff checklists, the overhead, how to determine aircraft weights, or how to use of the Control Display Unit (CDU).   I will assume that this has been done.

I have attempted to try and simplify the procedure as much as possible.   However, the automated systems that can be used on the Boeing aircraft are complicated, can be used fully or in part, and can easily generate confusion. 

Add to this that procedures can be different between an automated and manual takeoff, and that some procedures are dependent upon what software is used by the Flight Management System (1), and the display of specific items, such as the speed references on the Primary Flight Display (PFD) will only be propagated if the CDU is correctly set-up prior to takeoff - there is a challenge to write something that is complicated in a simple way.

To minimise wordiness in this article I have used acronyms and footnotes.  Refer to the end of the article for a list of acronyms and their meaning.

Variability Allowed

The first point to take on board is that there is no absolute correct method for takeoff and climb.  Certainly, there are specific tasks that need to be completed; however, there is an envelope of variability allowed.  This variability may relate to how a particular flight crew flies the aircraft, environmental considerations (ice, rain, wind, noise abatement, obstacles, etc.), flight training, or a specific airline policy.

Whenever variability is injected into a subject you will find those who work in absolutes having difficulty.  If you’re the kind of person who likes to know exactly what to do at a particular time, then I’d suggest you find a technique that fits with your liking and personality. 

Table 1 is a ‘ready reckoner’ that explains much of what occurs during the takeoff roll, climb out and transition to altitude.  Like anything there are some specific terms that you need to remember and more importantly understand.

Peer Review

The information in the below chart has been peer reviewed by B737 Captain.  He agreed with the content; however, reiterated the variability allowed by flight crews when flying the B737. 

TABLE 1: Condensed points that need to be addressed during a takeoff and climb.  The table content is generic and does not reflect any particular airline operation.  It is for reference only. 

Procedures (generic)

The following procedures assume other essential elements of pre-flight set-up have been completed (for example, correct set-up of CDU).

1.    Using the Mode Control Panel (MCP), dial into the altitude window an appropriate target altitude, for example 13,000 feet.

2.    Command speed is set in the MCP speed window.  The speed is set to V2.  The V2 speed is determined by the CDU and is based on aircraft weight and several other parameters.

Important Points:

  • V2 is the minimum takeoff safety speed and provides at least 30° bank capability with takeoff flaps set.
  • On the speed tape of the PFD, a white-coloured airspeed bug is propagated at V2 +15/20. V2+15 knots provides 40° bank capability with takeoff flaps set.   The bug is a visual aid to indicate the correct climb-out speed (this bug is discussed later on).
  • A flight crew may fly either +15/20 knots (maximum +25 knots) above V2 command speed to lower/increase pitch during takeoff depending on the weight of the aircraft and other environmental variables.  Company policy frequently dictates whether +15/20 knots is added to V2. (this assumes both engines operational).

3.    Turn on (engage) the Flight Director (FD) switches (pilot flying side first).

4.    Set flaps 5 and trim the aircraft using the electric trim on the yoke to the correct trim figure for takeoff. The trim figure is shown on the CDU (for example, 5.5 degrees) and is calculated dependent upon aircraft weight with passengers and fuel.  Normally the trim figure will place the trim tabs somewhere within the green band on the throttle quadrant.

5.   Arm the A/T toggle (the airline Flight Crew Operations Manual (FCOM) may indicate different timings for this procedure).

6.    Release the parking brake and advance the throttle levers manually to around 40%N1 (some FCOM’s differ to the %N1 recommended – for example, 60% N1).  

Important Points:

  • You do not have to stop the aircraft on the runway prior to initiating 40%N1.  A rolling takeoff procedure is often recommended for setting takeoff thrust.  This is because it expedites the takeoff (uses less runway length), and reduces the risk of foreign object damage or engine surge/stall due to a tailwind or crosswind.
  • After thrust has reached 40% N1, wait for it to stabilise (roughly 2-3 seconds).  Look at the N1 thrust arcs and the EGT gauge (on the EICAS display).  Both N1 arcs must be stable and the EGT values decreasing slightly.  In the real aircraft, the EGT should reduce between 10C-20C after N1 has stabilised at 40%.  If the engines are NOT allowed to stabilise, before advancing the thrust levers, the takeoff distance can be adversely affected.

Interesting Point:

  • After configuration is completed, and with the parking brake in the off position, some crews move the thrust levers quickly forward and then aft.  They do this to see if the configuration horn will sound.  

7.   Once the throttles are stabilized, advance the thrust levers to takeoff thrust (if flying manually) or depress one or both TOGA buttons.  If TOGA is used, the thrust levers will  automatically begin to advance to the correct %N1 output calculated by the Flight Management System. 

Important Points:

  • Do not push the thrust levers forward of the target %N1 - let the A/T do it (otherwise you will not know if there is a problem with the A/T).  See point 10 concerning hand placement.
  • Ensure target %N1 is initiated by 60 knots ground speed.

8.   Maintain slight forward pressure on the control column to aid in tyre adhesion. Focus on the runway approximately three-quarters in front of the aircraft.  This will assist you to maintain visual awareness and keep the aircraft on the centreline.  Use rudder and aileron input to control crosswind.

9.    During initial takeoff roll, the pilot flying should place their hand on the throttle levers in readiness for a rejected takeoff (RTO). The pilot not flying should place his hand behind the throttle levers.  Hand placement facilitates the least physical movement should an RTO be required. 

10.    The pilot not flying will call out ‘80 Knots’.  Pilot flying should slowly release the pressure on the control column so that it is in the neutral position.  Soon after the aircraft will pass through the V1 speed (this speed is displayed on the speed tape of the PFD.  Takeoff is mandatory at V1 and Rejected Takeoff (RTO) is now not possible.  The pilot flying, to reaffirm this decision, should remove their hands from the throttles; thereby, reinforcing the ‘must fly’.

11.    At Vr (rotation), pilot not flying calls ‘Rotate’.  Pilot flying slowly and purposely initiates a smooth continuous rotation at a rate of no more than 2 to 3 degrees per second to an initial target pitch attitude of 8-10 degrees (15 degrees maximum).

Important Points:

  • Normal lift-off attitude for the B737-800 is between 8 and 10 degrees.  This provides 20 inches of tail clearance at flaps 1 and 5.  Tail contact will occur at 11 degrees of pitch if still on or near the ground.
  • Takeoffs at low thrust setting (low excess energy) will result in a lower initial pitch attitude target to achieve the desired climb speed.
  • The correct liftoff attitude is achieved in approximately 3 to 4 seconds (depending on airplane weight and thrust setting).

12.    After lift-off, continue to raise the nose smoothly at a rate of no more than 2 to 3 degrees per second toward 15 degrees of pitch attitude.  The Flight Director (FD) cues will probably indicate approximately15 degrees. 

Important Points:

  • Be aware that the cues provided by the Flight Director may on occasion be spurious; therefore, learn to see through the cues to the actual aircraft horizon line.
  • The Flight Director pitch command is not used for rotation.

13.    You will also need to trim the aircraft to maintain minimum back pressure (neutral stick) on the control column.  The B737 is usually trimmed to enable flight with no pressure on the control column. 

  • It is normal following rotation to trim down a tad to achieve neutral loading on the control column.  Do not trim during rotation.     

14.    When positive rate has been achieved, and double checked against the speed and vertical speed tape in the PFD, the pilot flying will call ‘Gear Up’ and pilot not flying will raise the gear to minimize drag and allow air speed to increase.  The pilot not flying will also announce when the gear has been retracted successfully.

15.    The Flight Director will command a pitch to maintain an airspeed of V2 +15/20.  Follow the Flight Director (FD) cues (pitch bar), or target a specific vertical speed.  The vertical speed will differ widely when following the FD cues as it depends on weight, fuel, derates, etc. If not using the FD cues, try to maintain a target vertical speed (V/S) of ~2500 feet per minute. 

Important Points:

  • V2+15/20 is the optimum climb speed with takeoff flaps.  It results in maximum altitude gain in the shortest distance from takeoff.
  • If the FD cues appear to be incorrect, or the pitch appears to be too great, ignore the FD and follow vertical speed guidance. 
  • Bear in mind that vertical speed has a direct relationship to aircraft weight - if aircraft weight is moderate to low, use reduced takeoff thrust (derates) or Assumed Temperature Method to achieve recommended vertical speed.
  • If LNAV and VNAV were selected prior to takeoff, LNAV will provide FD inputs at 50 feet and VNAV will engage at 400 feet.
  • It’s common practice for a flight crew to manually engage (push VNAV button on MCP) at 400 feet if VNAV has not been preset on the MCP prior to takeoff.

16.    Follow and fly the cues indicated by the Flight Director.   Maintain a command speed at V2 +15/20 until you reach a predefined altitude called the Acceleration Height (AH).  AH is often stipulated by company policy and is usually between 1000-1500 feet ASL.

17.    At Acceleration Height, the nose of the aircraft is lowered (pitch decreased).  This will increase airspeed and lower vertical speed.  A rough estimate to target is half the vertical speed used at takeoff.  Press N1 on the MCP (if you have been manually flying and this is desired).  Follow FD cues to flaps UP speed. 

Important Points:

  • N1 is automatically selected at thrust reduction altitude (if autothrottle system has been used during takeoff – this is usually at 1500 feet RA).
  • When N1 is selected, the autothrottle will control the speed of the aircraft to the N1 limit set by the Flight Management System (FMS).  Selecting N1 ensures the aircraft has maximum power (climb thrust) in case of a single engine failure. 
  • N1 mode does not control aircraft speed. The autothrottle will set maximum N1 power.   Speed is controlled by aircraft pitch attitude.
  • Selecting N1 on the MCP does not provide any form of speed protection. 

18.    The following should also be done at, or passing through Acceleration Height. 

  • Flaps should be retracted on schedule.  If noise abatement is necessary, flaps retraction may occur at Thrust Reduction Height.  Retract flaps as per the flaps schedule (retract each degree of flaps as the aircraft's speed passes through the next flap increment setting as observed on the PFD speed tape).
  • When flaps retraction commences, the airspeed bug will disappear from the speed tape on the PFD.
  • Do not retract flaps unless the aircraft is accelerating, and the airspeed is at, or greater than V2+15/20 - this ensures the speed is within the manoeuvre margin to allow for over-bank protection.  Do not retract flaps below 1000 feet RA.
  • If flying manually without VNAV selected, set the speed in the speed window of the MCP to a speed that corresponds to the flaps UP speed.  The flaps UP speed can be found displayed on the speed tape on the PFD.  This is often referred to as ‘Bugging Up’.
  • If VNAV has been selected at takeoff, the flaps UP speed will automatically be displayed on the speed tape on the PFD. 
  • It’s important to note that if VNAV has been selected, the flaps UP speed is displayed on the PFD; the speed is NOT displayed in the MCP speed window.

19.    When the aircraft flies through the flaps UP speed, and after the flaps have been fully retracted, the desired climb speed is dialled into the speed window of the MCP (assuming manual flight).  If VNAV has already been selected, the climb speed will be displayed in the PFD.  The speed is NOT displayed in the MCP speed window. 

At this stage, you can fly manually to altitude, or select full or part-automation (pitch and roll mode) controlled by either CWS or the autopilot (CMD A/B).

Important Points:

  • Unless you select a different mode, the TOGA command mode that was engaged at takeoff (assuming you used the autothrottle system), will remain engaged until you each the assigned altitude on the MCP.      
  • Selecting N1 on the MCP does not disengage TOGA mode.  If you want to disengage TOGA mode prior to reaching 400 feet ASL, then both Flight Director switches need to be turned off.
  • Some flight crews when reaching Acceleration Height call 'Level Change, Set Top Bug'.  This ensures that TOGA speed is disengaged and causes the Flight Director (FD) cues to lower on the PFD; thereby, increasing speed as Level Change increases thrust.
  • Other flight crews may engage Control Wheel Steering (CWS) following flaps UP and fly in this mode to 10,000 feet before engaging the autopilot (CMD A/B).  Whatever the method, it is at the discretion of the pilot in command and the method is often stipulated by company policy.

20.    The aircraft is usually flown at a speed no faster than 250 KIAS to 10,000 feet.  At 10,000 feet increase your speed to 275 KIAS (or whatever is desired based on environmental factors).  If using VNAV, the climb speed is automatically populated.  The same will occur for cruise speed. 

If the aircraft is being flown by hand (manually), then the appropriate climb and cruise speeds will need to be dialed into the MCP.  At 10,000 feet, dial 270 KIAS into the MCP speed window and then at 12,000 feet dial in 290 KIAS.  Follow the Flight Director cues or maintain roughly 2000-2500 fpm vertical speed.  At cruise altitude, transition to level flight and select on the MCP speed window 290-310 KIAS or whatever the optimum speed is (see CDU).

Guidelines (FCOM Do Differ)

The above guidelines are general.  Specific airline policy for a particular airline may indicate otherwise.  Likewise, there is considerable latitude to how the aircraft is flown, whether it be manually or with part of full automation selected.

LEFT:  Qantas Airways departs Queenstown, New Zealand.

Confusion

It is very easy to become confused during the takeoff phase - especially in relation to automation, V speeds and how and when to change from TOGA to MCP speed.  The takeoff phase occurs quickly, and there is a lot to do and quite a bit to remember - there is little time to consult a manual or cheat sheet. 

One way to gain a little extra time during the takeoff transition, is to select an appropriate derate.  Apart from being standard practice in the real-world for many takeoffs, a derate will also help control over-pitching and high vertical speeds which are common when the aircraft is light due to minimal fuel load and cargo.

Understanding % N1

N1 is a measurement in percent of the maximum rpm, where maximum rpm is certified at the rated power output for the engine (most simple explanation).  Therefore, 100%N1 is maximum thrust while 0%N1 is no thrust.  During the initial takeoff, if the autothrottle is used, thrust (N1) is automatically selected when you engage the TOGA buttons. 

At 80 knots the automated system will engage thrust to N1.  This will be at a percentage commensurate with the settings that have been set in the CDU (aircraft weight, climb, derate etc.). 

Important Points:

  • When the autothrottle is used, and TOGA selected as the command mode, the automated system will control the speed of the aircraft with pitch. 
  • To determine what is controlling the thrust of the aircraft, always refer to the Flight Mode Annunciations (FMA) in the PFD (Refer to Table 2 for a quick overview of annunciations during the takeoff).       

Common Practice (what to select)

It’s not the purpose of this article to rewrite the FCOM or FCTM.   Needless to say, there are several combinations that can be selected at varying stages of flight.  All are at the discretion of the pilot flying, or are stipulated as part of airline policy.

After acceleration height has been reached, the nose lowered to increase speed, and flaps retracted, it’s common practice to use LVL CHG, V/S or LNAV and VNAV. 

LNAV can be selected at, or after 50 feet.   VNAV can be selected at, or after 400 feet.

Theoretically, a crew can fly the F/D cues at V2+15/20 to the altitude set in the MCP; however, there will be no speed protection available.  If the pitch recommendation (Flight Director cues) are not followed, then airspeed may be either above or below the optimal setting.

The aircraft will remain in TOGA command mode (thrust controlled by N1)  until the altitude set in the MCP is reached, unless another mode is selected.  To deselect TOGA as the command mode, another mode such as LVL CHG, VNAV or V/S will need to be selected.  Altitude Hold (ALT HOLD) also deselects TOGA as does engaging the autopilot.

Flight crews typically fly the aircraft manually until the flaps are retracted and the aircraft is in clean configuration.  A command mode is then selected to continue the climb to altitude.

Important Point:

  • It’s important to understand what controls the various command modes.  For example, LVL CHG is controlled by N1 and pitch.  In this mode the autothrottle will use full thrust, and the speed will be controlled by pitch.   

TABLE 2:  PFD and FMA annunciations observed during takeoff and climb.

Speed Protection

One of the advantages in using the automated systems in the Boeing aircraft is the level of speed protection the systems can provide.  Speed protection means that the aircraft will not have its speed degraded to a value below what has been set.  This ensures that the aircraft ‘should not’ be placed in a situation in which it can stall or be below manoeuvring speed.

Speed protection is relevant only when automation is selected.  Protection, depends upon the FMS software in use, the automation mode selected, and whether the flaps are extended or fully retracted.

For example, when you select LVL CHG, the speed window will open allowing you enter a desired speed.  LVL CHG is speed protected, meaning that the aircraft's speed will not precede past the speed set in the speed window of the MCP.  LVL CHG is controlled by N1, and will adjust the pitch of the aircraft to match the speed set in the MCP.  

VNAV also has speed protection, but not with flaps extended.  The speed in VNAV is defined by the value (speed) set in the CDU (this differs depending upon which FMS software is in use). 

Vertical Speed, in contrast, provides no speed protection.  This is because V/S holds a set vertical speed.  In V/S, if you are not vigilant, you can easily encounter an overspeed or under speed situation. 

Selecting N1 by depressing the N1 button on the MCP (without any other mode selected) does not provide speed protection.  Using the N1 mode only ensures maximum thrust (as set in the FMS) is generated. 

Important Point:

  • It’s imperative that you carefully observe (scrutinise) the Flight Mode Annunciator (FMA) to ensure the aircraft is flying the correct mode.     

Always Think Ahead

The takeoff can be very fast, especially if you have an aircraft which is light in weight (cargo, passengers and fuel).

LEFT: B737 CDU showing Takeoff page.  A takeoff can occur without the completion of data; however, some automation features such as VNAV and LNAV will not be available.  Additionally, the V1, V2 and Vr speed bugs will not be propagated on the speed tape of the PFD (click to enlarge).

Soon after rotation (Vr), the aircraft will be at acceleration height and beyond…  It’s important to remain on top of what’s happening, and try to think one step ahead of the automated system that is flying the aircraft.

If the aircraft is light, flight crews often limit the takeoff thrust by using one of several means.  Typically, it is by using a thrust derate and selecting either CLB 1 or CLB 2, or entering an assumed temperature thrust reduction - both done in the CDU. 

Selecting either option will cause a longer takeoff roll, delay the rotation point (Vr) and cause a less aggressive high pitch climb-out, than observed if these variables were not altered.

Final Call

Reiterating, the above guidelines are generalist only.  Flight crews use varying methods to fly the aircraft and often the method used, will be chosen based on company policy, crew experience, aircraft weight and other environmental factors, such as runway length, weather and winds.

Additional takeoff information - mainly in relation to acceleration height, thrust reduction height, and derated thrust can be read on these pages.

Disclaimer

The content in this post has been proof read for accuracy; however, explaining procedures that are  convolved and subjective can be challenging.  Errors on occasion present themselves.  if you observe an error (not a particular airline policy), please contact me so it can rectified.

Footnotes

(1): For example, there are different protocols between FMC U10.6 and FMC U10.8 (especially when engaging VNAV and LNAV prior to takeoff).  I have purposely not addressed all these differences because they can be confusing (another article will do this).  As at writing, ProSim-AR uses U10.8A.

Acronyms and Glossary

AFDS – Autopilot Flight Director System
AH - Acceleration Height.  The altitude above sea level that aircraft’s nose is lowered to gain speed for flap retraction.  AH is usually 1000 or 1500 feet and is defined by company policy.  In the US acceleration height is usually 800 feet RA.
CDU / FMC – Control Display Unit / Flight Management Computer (term used interchangeably on this website).  The visual part of the Flight Management System (FMS)
CLB 1/2 – Climb power
Command Mode – The mode of automation that controls thrust
EICAS – Engine Indicating and Crew  Alerting System
F/D – Flight Director (Flight Director cues/crosshairs)
FMA – Flight Mode Annunciation located upper portion of Primary Flight Display (PFD)
KIAS – Knots Indicated Air Speed
LNAV – Lateral Navigation
LVL CHG – Level Change Command Mode
MCP – Mode Control Panel
N1 &N2 - N1 and N2 are the rotational speeds of the engine sections expressed as a percentage of a nominal value. ... The first spool is the low pressure compressor (LP), that is N1 and the second spool is the high pressure compressor (HP), that is N2. The shafts of the engine are not connected and they operate separately. Often written N1 or %N1.
RTO – Rejected Take Off
T/O Power – Takeoff power
Throttle On & Off-Line – Indicates whether the throttle is being controlled by the A/T system.
TOGA – To Go Around Command Mode
TRA - Thrust Reduction Altitude.  The altitude that the engines reduce in power to increase engine longevity.  The height is usually 1500 feet; however, the altitude can be altered in CDU
V/S – Vertical Speed Command Mode
V1 – is the Go/No go speed.  You must fly after reaching V1 as a rejected take off (RTO) will not stop the aircraft before the runway ends
V2 – Takeoff safety speed.  The speed at which the aircraft can safely takeoff with one engine inoperative (Engine Out safe climb speed)
VNAV – Vertical Navigation
Vr – Rotation Speed.  This is the speed at which the pilot should begin pulling back on the control column to achieve a nose up pitch rate

Vr+15/20 – Rotation speed plus additional knots (defined by company policy)

Tuesday
Jul012014

Crosswind Landing Techniques Part Two - Calculations 

Determining Correct Landing Speed (Vref)

Vref is defined as landing speed or the threshold crossing speed, while Vapp is defined as the approach speed with wind/gust additives.

LEFT:  Final Approach:  Finals using 'crab approach' (Airbus A320)

When landing with a headwind, crosswind, or tailwind the Vref and Vapp must be adjusted accordingly to obtain the optimal speed at the time of touchdown.  Failure to do this may result in the aircraft landing at an non-optimal speed causing runway overshoot, stall, or floating (ground affect).

Mathematical calculations can be used to determine Vref and Vapp based on wind speed, direction, and gusts.

This is the second segment of a two-segment post.  The first segment dealt with methods used to land in cross wind conditions  - Crosswind Landing Techniques Part One Crab and Sideslip.

Normal Conditions

When using the autothrottle, position command speed to Vref+5 knots.

If the autothrottle is disengaged or is planned to be disengaged prior to landing, the recommended method of approach speed correction to obtain Vapp (approach speed), is to add one half of the reported steady headwind component plus the full gust increment above the steady wind to the reference speed.

One half of the reported steady headwind component can be estimated by using 50% for a direct headwind, 35% for a 45-degree crosswind, zero for a direct crosswind, or interpolation between.

When making adjustments for wind additives, the maximum command speed should not exceed Vref+20 knots or landing flap placard speed minus 5 knots, whichever is lower.

The minimum command speed setting with autothrottle disconnected is Vref+5 knots.  The gust correction should be maintained to touchdown while the steady headwind correction should be bled off as the airplane approaches touchdown. 

It is important to note that Vref+5 knots is the speed that is desired when crossing the threshold of the runway - it is NOT the approach speed.  The approach speed (Vapp) is determined by headwind with/without gusts.  If the wind is calm, Vref+5 knots will equal Vapp.

When landing in a tailwind, do not apply wind corrections. Set command speed at Vref+5 knots (autothrottle engaged or not engaged).

Non-Normal Conditions

When Vref has been adjusted by the non-normal procedure, the new Vref is called the adjusted Vref and is used for the landing.  To this speed is added the wind component (if necessary).

For example, if a non-normal checklist specifies 'Use flaps 15 and Vref 15+10 for landing', the flight crew would select flaps 15 and look up the Vref 15 speed (in FCTM or QRH) and add 10 knots to that speed.  The adjusted Vref does not include wind corrections and these will need to be added.

If the autothrottle is disengaged, or is planned to be disengaged prior to landing, appropriate wind corrections must be added to the adjusted Vref to arrive at command speed.  Command speed is the safest speed used to fly the approach (Vapp).  If the speed is above command speed then it will need to be bleed off prior to touchdown.

An interesting publication (powerpoint presentation) concerning the use of the autothrottle can be read here autothrottle usage - training alert.  Search for Autothrottle Usage - Training Alert.

Autoland Limitations

 If using autoland (CAT II & CAT IIIA) the autothrottle remains engaged and the command speed is set to Vref+5.

The following autoland limitations must be abided by:

(i)     Glideslope angle tolerance - maximum 3.25 degrees / minimum 2.5 degrees;
(ii)    Engines 1/2 operational;
(iii)   Maximum​ tailwind - 15 kts​;
(iv)   Maximum headwind - 25 kts​;
(v)    Maximum crosswind - 20​ kts ;
(vi)   Maximum taliwnd at flaps 30 - 12 knots (winglets); and,
(vii)  Landing in gusty​ wind​ or windshear​ conditions is not approved during CAT II and CAT IIIA operations.

Guideline (an easy way to remember the above - cheat sheet)

This information assumes the autothrottle will be disengaged prior to landing.

  • Headwind less than 10 knots:  Vref+5
  • Headwind greater than 10 knots:  Vref + headwind / 2 (half your headwind) - This is your Vref
  • If Vref is > 20 knots, then:  Vref+20 (as per placard guide)

With Gusts

  • Formula (Wind < 10 knots):  Vref+5 + gust – headwind
  • Formula (Wind > 10 knots):  Vref + headwind/2 (half your headwind) + gust – headwind

Calculating Directional Wind

A wind component will not always be at 90 Degrees or straight on to your landing direction.  The following calculation is often used to determine the directional component.  One half of the reported steady headwind component can be estimated by using 50% for a direct headwind, 35% for 45 degree crosswind, zero for a direct crosswind and interpolation in between.

Tail Winds 

Tail winds are very challenging for conducting a stabilized approach.  Because of the increased ground speed caused by a tail wind, Boeing does not publish Vref correction factors for tail winds. 

Typically, to maintain the proper approach speed and rate of descent while maintaining glideslope, thrust must be decreased which minimizes the available safety envelope should a go-around be required.  If a go-around is required, precious seconds might be lost as the engines accelerate; the aircraft would continue to descend and might touch down on the runway before the engines produce enough thrust to enable a climb.

The International Civil Aviation Organization (ICAO) recommends that the tail wind component must not exceed 5 knots plus gusts on a designated runway; however, adherence to this recommendation varies among members.  Several airlines have been certified for operation with a 15 knot tailwind. 

In the United States, Federal Aviation Administration (FAA) sets the tail wind component limit for runways that are clear and dry at 5 knots, and in some circumstances 7 knots, however FAA allows no tail wind component when runways are not clear and dry.  Note, that many manuals subscribe to the 10 knots no tailwind rule (see table below).

Crosswind components can be variable dependent upon flight crew discretion and airline policy; therefore, the above is to be used as a 'guide' only.

The below table (limitations) summarizes much of what has been written above.

The CDU if configured correctly can provide information concerning wind components.  Press the key on the CDU named 'PROG' followed by 'PREV PAGE'.  This page provides an overview of the wind component including head, tail and crosswind.

Wind Correction Field (WIN CORR)

The approach page in the CDU has a field named WIN CORR (Wind Correction Field or WCF).  Using this field, a flight crew can alter the Vref+ speed (additive) that is used by the autothrottle.  The default reading is +5.   Any change will alter how the FMC calculates the command speed that the autothrottle uses; changes are reflected in the LEGS page.  It's important to update the WIND CORR field if VNAV is used for the approach, as VNAV uses data from the FMC to fly the approach.  If hand flying the aircraft, it's often easier to to add the Vref additive to the speed window in the MCP.

WIND CORR Explained

The WCF is a handy feature if a flight crew wishes to increase the safety margin the autothrottle algorithm operates.

Boeing when they designed the autothrottle algorithm programmed a speed additive that the A/T automatically adds to Vref when the A/T is engaged.  The reason for adding this speed is to provide a safety buffer to ensure that the A/T does not command a speed equal to or lower than Vref.   (recall that wind gusts can cause the autothrottle to spool up or down depending upon the gust strength).  

A Vref+ speed higher than +5 can be inputted when gusty or headwind conditions are above what are considered normal.  By increasing the +speed, the  speed commanded by the autothrottle will not degrade to a speed lower than that inputted.

Important Points

  • During the approach, V speeds are important to maintain.  A commanded speed that is below optimal can be dangerous, especially if the crew needs to conduct a go-around, or if winds suddenly increase or decrease.  An increase or decrease in wind can cause pitch coupling.
  • If using VNAV, it's important to update the WIND CORR field to the correct headwind speed based on conditions.  This is because VNAV uses the data from the FMC.

If executing an autoland (rarely done in the B737), the WIND CORR field is left as +5 knots (default).  The autoland and autothrottle will command the correct approach and landing speed.

Tuesday
Jun172014

Crosswind Landing Techniques Part One - Crab and Sideslip

This video very clearly illustrates my point that landing in a strong crosswind can be a challenging and in some cases downright dangerous (Video © CargoSpotter (with thanks); courtesy U-Tube).

Generally, flight crews use one of two techniques or a combination thereof to approach and land in crosswind conditions.  If winds exceed aircraft tolerances, which in the 737-800 is 33 knots (winglets) and 36 knots (no winglets), the flight crew will divert to their alternate airport (Brady, Chris - The Boeing 737 technical Guide).

Maximum crosswind figures can differ between airlines and often it's left to the pilot's discretion and experience.  Below is an excerpt from the Landing Crosswind Guidelines from the Flight Crew Training Manual (FCTM).  Note that FCTMs can differ depending on date of publication.

LEFT:  Although not as dramatic as the video clip, the screen shot illustrates the ‘crab approach’.  Wind is right to left at 16 knots with aircraft crabbing into the wind to maintain centerline approach course.  Just before flare, left rudder will be applied to correct for drift to bring aircraft into line with centerline of runway.  This technique is called 'de-crabbing’. During such an approach, the right wing may also be lowered 'a tad' (cross-control) to ensure that the aircraft maintains the correct alignment and is not blown of course by a 'too-early de-crab'.  Right wing down also ensures the main gear adheres to the runway during the roll out.
 
There are several factors that require careful consideration before selecting an appropriate crosswind technique: the geometry of the aircraft (engine and wing-tip contact and tail-strike contact), the roll and yaw authority of the aircraft, and the magnitude of the crosswind component.  Consideration also needs to be made concerning the effect of the selected technique when the aircraft is flared to land.

Crosswind Approach and Landing Techniques

There are four techniques used during the approach and landing phase which center around the crab and sideslip approach.  The crab and sideslip are the primary methods and most commonly used while the de-crab and combination crab-sideslip are subsets that can be used when crosswinds are stronger than usual.

It must be remembered that whatever method is used it is at the discretion of the pilot in command.

1.    Crab (to touchdown).
2.    Sideslip (wing low).
3.    De-crab during flare.
4.    Combination crab and sideslip.

1:  Crab (to touchdown)

  • Airplane maintains a crab during the final approach phase.
  • Airplane touches down in crab.
  • Flight deck is over upwind side of runway (Main gear is on runway center).
  • Airplane will de-crab at touchdown.
  • Flight crew must maintain directional control during roll out with rudder and aileron.

With wings level, the crew will use drift correction to counter the effect of the crosswind during approach.  Drift correction will cause the aircraft to be pointing in a direction either left or right of the runway heading, however, the forward energy of the aircraft will be towards the centerline.  This is called the crab because the aircraft is crabbing at an angle left or right of the aircraft's primary heading.

Most jetliners have the ability to land in a crab, however, it must be remembered that landing in a crab places considerable stress on the main landing gear and tyre side-walls, which in turn can cause issues with tyre and wheel damage, not too mention directional control.

The later is caused by the tandem arrangement of the main landing gear that has a strong tendency to travel in the direction that the nose of the aircraft is pointing at the moment of touchdown.  This can result in the aircraft travelling toward the edge of the runway during the roll out.  To counter this, and align the nose of the aircraft with the centreline of the runway, the pilot flying must apply rudder input when lowering the nose wheel to the runway surface.

A reference to the maximum amount of crab that can be safely applied in the B737 was not found, other than maximum crosswind guidelines must not be exceeded.  The crab touchdown technique is the preferred choice if the runway is wet.

2:  Sideslip (wing low)

  • Upwind wing lowered into wind.
  • Opposite rudder (downwind direction) maintains runway alignment.
  • In a sideslip the aircraft will be directly aligned with the runway centerline using a combination of into-wind aileron and opposite rudder control (called cross-controls) to correct the crosswind drift.

The pilot flying establishes a steady sideslip (on final approach by applying downwind rudder to align the airplane with the runway centerline and upwind aileron to lower the wing into the wind to prevent drift.  The upwind wheels should touch down before the downwind wheels touch down.

The sideslip technique reduces the maximum crosswind capability based on a 2/3 ratio leaving the last third for gusts.  However, a possible problem associated with this approach technique is that gusty conditions during the final phase of the landing may preempt a nacelle or wing strike on the runway.

Therefore a sideslip landing is not recommended when the crosswind component is in excess of 17 knots at flaps 15, 20 and 30, or 23 knots at flaps 40.

The sideslip approach and landing can be challenging both mentally and physically on the pilot flying and it  is often difficult to maintain the cross control coordination through the final phase of the approach to touchdown.  If the flight crew elects to fly the sideslip to touchdown, it may also be necessary to add a crab during strong crosswinds.

3:  De-crab During Flare (with removal of crab during flare)

  • Maintain crab on the approach.
  • At ~100 foot AGL the flight crew will de-crab the aircraft; and,
  • During the flare, apply rudder to align airplane with runway centreline and, if required slight opposite aileron to keep the wings level and stop roll.

This technique is probably the most common technique used and is often referred to as the 'crab-de-crab'.

The crab technique involves establishing a wings level crab angle on final approach that is sufficient to track the extended runway centerline.  At approximately 100 foot AGL and during the flare the throttles are reduced to idle and downwind rudder is applied to align the aircraft with the centerline (de-crab). 

Depending upon the strength of the crosswind, the aircraft may yaw when the rudder is applied causing the aircraft to roll.  if this occurs, the upwind aileron must be placed into the wind and the touchdown maintained with crossed controls to maintain wings level (this then becomes a combination crab/sideslip - point 4).

Upwind aileron control is important, as a moderate crosswind may generate lift by targeting the underside of the wing. Upwind aileron control assists in ensuring positive adhesion of the landing gear to the runway on the upwind side of the aircraft as the wind causes the wing to be pushed downwards toward the ground.

Applied correctly, this technique results in the airplane touching down simultaneously on both main landing gear with the airplane aligned with the runway centerline.

4:  Combination Crab and Sideslip

  • De-crab using rudder to align aircraft with runway (same as point 3 de crab during flare).
  • Application of opposite aileron to keep the wings level and stop roll (sideslip). 

The technique is to maintain the approach in a crab, then during the final stages of the approach and flare increase the into-wind aileron and land on the upwind tyre with the upwind wing slightly low.  The combination of into-wind aileron and opposite rudder control means that the flight crew will be landing with cross-controls.

The combination of crab and sideslip is used to counter against the turbulence often associated with stronger than normal crosswinds.

As with the sideslip method, there is the possibility of a nacelle or wing strike should a strong gust occur during the final landing phase, especially with aircraft in which the engines are mounted beneath the wings.

FIGURE 1:  Diagram showing most commonly used approach techniques (copyright Boeing).

Operational Requirements and Handling Techniques

With a relatively light crosswind (15-20 knot crosswind component), a safe crosswind landing can be conducted with either; a steady sideslip (no crab) or a wings level, with no de-crab prior to touchdown.
With a strong crosswind (above a 15 to 20 knot crosswind component), a safe crosswind landing requires a crabbed approach and a partial de-crab prior to touchdown.

For most transport category aircraft, touching down with a five-degree crab angle with an associated five-degree wing bank angle is a typical technique in strong crosswinds.

The choice of handling technique is subjective and is based on the prevailing crosswind component and on factors such as; wind gusts, runway length and surface condition, aircraft type and weight, and crew experience.

Touchdown Recommendations

No matter which technique used for landing in a crosswind, after the main landing gear touches down and the wheels begin to rotate, the aircraft is influenced by the laws of ground dynamics.

Effect of Wind Striking the Fuselage, Use of Reverse Thrust and Effect of Braking 

The side force created by a crosswind striking the fuselage and control surfaces tends to cause the aircraft to skid sideways off the centerline.  This can make directional control challenging.

Reverse Thrust

The effects of applying the reverse thrust, especially during a crab ‘only’ landing can cause additional direction forces.  Reverse thrust will apply a stopping force aligned with the aircraft’s direction of travel (the engines point in the same direction as the nose of the aircraft).  This force increases the aircraft’s tendency to skid sideways.

Effects of Braking

Autobrakes operate by the amount of direct pressure applied to the wheels.  In a strong crosswind landing, it is common practice to use a combination of crab and sideslip to land the aircraft on the centerline.  Sideslip and cross-control causes the upwind wing to be slightly down upon landing and this procedure is carried through the landing roll to control directional movement of the aircraft. 

The extra pressure applied to the ‘wing-down’ landing gear causes increased auto-braking force to be applied which creates the tendency of the aircraft to turn into the wind during the landing roll.  Therefore, a flight crew must be vigilant and be prepared to counter this unwanted directional force.

If the runway is contaminated and contamination is not evenly distributed, the anti-skid system may prematurely release the brakes on one side causing further directional movement.

FIGURE 2:  Diagram showing recovery of a skid caused by crosswind and reverse thrust side forces (source: Flight Safety Foundation ALAR Task Force).

Maintaining Control - braking and reverse thrust

If the aircraft tends to skew sideward from higher than normal wheel-braking force, the flight crew should release the brakes (disengage autobrake) which will minimize directional movement.  

To counter against the directional movement caused by application of reverse thrust, a crew can select reverse idle thrust which effectively cancels the side-force component.  When the centerline has been recaptured, toe brakes can be applied and reverse thrust reactivated.

Runway Selection and Environmental Conditions

If the airport has more than one runway, the flight crew should land the airplane on the runway that has the most favourable wind conditions.  Nevertheless, factors such as airport maintenance or noise abatement procedures sometime preclude this.

I have not discussed environmental considerations which come into play if the runway is wet, slippery or covered in light snow (contaminated).  Contaminated conditions further reduce (usually by 5 knots) the crosswind component that an aircraft can land.

Determining Correct Landing Speed (Vref)

Vref is defined as landing speed or threshold crossing speed.

When landing with a headwind, crosswind, or tailwind the Vref must be adjusted accordingly to obtain the optimal speed at the time of touchdown.  Additionally the choice to use or not use autothrottle must be considered. Failure to do this may result in the aircraft landing at a non-optimal speed causing runway overshoot, stall, or floating (ground affect).

This article is part one of two posts.  The second post addresses the calculations required to safety land in crosswind conditions - Crosswind Landing Techniques Part Two Calculations.

Friday
Jun212013

Searching for Definitive Answers - Flight Training

Learning to operate the B737 is not a matter of 1, 2, 3 and away you fly; there’s a lot of technical information that requires mastering for successful and correct flight technique.  Searching for a definitive answer to a flight-related question can become frustrating.  Whilst respondents are helpful and want to impart their knowledge, I’ve learnt through experience that often there isn’t a definitive answer to how or why something is done a certain way.  

Typical Pilot-type Personalities

Typical pilot personalities nearly always gravitate towards one answer and one correct method; black or white, right or wrong – virtual pilots or “simmers” behave in a similar fashion.  They want to know with certainty that what they are doing replicates the correct method used in the "real-world". 

In reality, the Boeing 737NG is flown by different crews in different ways all over the globe every minute of the day.   Often the methods used are not at the discretion of the crew flying but are decided by airline company policy and procedures, although the ultimate decision rests with the Captain of the aircraft.   Just ask the __________ (you fill in the nationality or airline) and they will tell you that they are the best and fly the correct way.

For example, climb out procedures vary between different airlines and flight crews.  Some crews verify a valid roll mode at 500’ (LNAV, HDG SEL, etc) then at 1000’ AGL lower pitch attitude to begin accelerating and flap retraction followed by automation.  Others fly to 1500' or 3000’ AGL, then lower pitch and begin to "clean up" the aircraft; others fly manually to 10,000’ AGL before engaging CMD A. 

LEFT:  First Officer conducts pre-flight check list & compares notes.  Whilst check lists are essential in ensuring that all crews operate similarly, there is considerable variance in how flight crews actually fly the 737 (click for larger view)

Another example is flying an approach.  Qantas request crews to disengage automation at 2500’ AGL and many Qantas crews fly the approach without automation from transition altitude (10,000’ AGL).  This is in contrast to European counterparts in Ryanair which request crews use full automation whenever possible.  A further example is the use of Vertical Navigation, Level Change and Vertical Speed; there are several possibilities.

Considerable Variance Allowed

I have been told by a Qantas pilot, that there is "a huge amount of technique allowed when flying the B737".  "There are certainly wrong ways to do things; but, there is often no single right way to do something".

Therefore; when your hunting for a definite answer to a question, remember there are often several ways to do the same thing, and often the method chosen is not at the crew’s discretion but that of the airline.

Wednesday
Apr242013

Video Lecture on Automation Dependency, Loosing The Ability To Fly

This is an excellent lecture video that discusses the reasons for and the solution to "Automation Dependency".  As aircraft become increasingly complex, higher levels of automation are made available to allow pilots to minimise task loading.  In the late 1990's pilots were often referred to as  flight managers, meaning they supervised how and when various automated systems controlled the aircraft.  Several air crashes during the last decade has revealed that pilots are loosing the raw ability to actual fly an aircraft and are relying increasingly on automation to solve issues during time of critical flight stress.  This lecture video discusses the implications arising from automation dependency and how airlines are attempting to solve the issue.  I found the presentation to be very informative, amusing in parts and helps to explain "What's it doing now"......   The video has been embedded directly from U-Tube.