Crosswind Landing Techniques Part Two - Calculations

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

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.

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.

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 complied with:

  1. Glide slope angle tolerance - maximum 3.25 degrees / minimum 2.5 degrees;

  2. Engines 1/2 operational;

  3. Maximum​ tailwind - 15 kts​;

  4. Maximum headwind - 25 kts​;

  5. Maximum crosswind - 20​ kts ;

  6. Maximum tailwind at flaps 30 - 12 knots (winglets); and,

  7. 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 glide slope, 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.

 

table 1: wind limits for 737-800. the table provides a good summary of what has been written in the article

 

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.