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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.

I am not a professional journalist.  Writing for a cross section of readers from differing cultures and languages with varying degrees of technical ability, can at times be challenging. I hope there are not too many spelling and grammatical mistakes.


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I use the words 'modules & panels' and 'CDU & FMC' interchangeably.  The definition of the acronym 'OEM' is Original Equipment Manufacturer (aka real aicraft part).


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Journal Archive (Newest First)
« Take Off / Go Around (TOGA) - Explained | Main | Simulator Construction Update - June 2013 »

Avoiding Confusion: Acceleration Height, Thrust Reduction Height, Derates, Noise Abatement and the Boeing Quiet Climb System

During preparation for takeoff, three similar functions that deal with how the autothrottle calculates N1 thrust can be altered in the CDU: Acceleration Height (AH), Thrust Reduction Height (TRH) and the Quiet Climb System (QCS).  Although there are similarities, each function is used independently of each other. 

Confusion can also occur deciphering the different methods used to alter N1 thrust, such as: Derated Thrust, Assumed Temperature and Derated Thrust Climb.

Acceleration Height (AH) 

Acceleration Height is the altitude above ground level (AGL) that a pilot accelerates the aircraft by reducing the aircraft’s pitch, to allow acceleration to a speed safe enough to raise flaps and slats, and then reach the desired climb speed.

LEFT: Thompson B738NG transitioning to Acceleration Height, Manchester, UK.  Click to see full size.

Part 23 of Federal Aviation Regulations (USA) dictates that the airplane is able to climb at a certain rate in this configuration up to a safe altitude.

The acceleration height is the altitude that the aircraft transitions from takeoff speed (V2+15/20) to climb out speed.  This altitude is usually between 1000 and 1500 feet, but may be as low as 800 feet; however, can differ due to noise abatement, airline policy, or airport specifics such as obstacles, etc.

The reason for acceleration height is to allow a safety envelope below this altitude should an engine problem develop after rotation; engines are set to maximum thrust, and the plane is pitched for V2 safety speed (V2+15/20).

Acceleration Height is altered in the CDU 'Init/Ref Index/Takeoff Ref Page (lsk4R) Accel HT ---- AGL'

Practical Application

Once the Acceleration Height has been reached, the pilot flying will reduce attitude pitch by pushing the yoke forward to increase speed.  As the speed increases Flaps 5 is retracted.  At this time the speed will need to be increased in the MCP speed window from V2 to climb speed, followed by further flap retraction on schedule. 

Although crews use slightly varying techniques; I find the following holds true for a non-automation climb to 10,000 feet AGL.

  • Set the MCP to V2
  • Fly the Flight Director cues to Acceleration Height (which will be at V2+15/+20).
  • At Acceleration Height, push yoke forward reducing pitch.
  • As forward speed increases you will quickly pass through the schedule for initial flap retraction – retract flaps 5.
  • Dial into the MCP speed window the appropriate 'clean up' speed (reference the top bug on the speed tape of the PFD, usually 210-220 kias).
  • Continue to retract flaps as per schedule.
  • After flaps are retracted, engage automation (if wanted) and increase speed to 250 kias or as indicated by Air Traffic Control.

NOTE:  If the acceleration height has been entered into the CDU, then the Flight Director bars will command the decrease in pitch when the inputted altitude (RA) has been reached - all you do is follow the FD bars.

Thrust Reduction Height (TRH)

The main wear on engines, especially turbine engines, is heat. If you reduce heat, the engine will have greater longevity. This is why takeoff power is often time limited and a height established that thrust is reduced. The difference between takeoff thrust and climb thrust may only be a few percent, but the lowering of EGT reduces heat and extends engine life significantly. 

Thrust Reduction Height (TRH):  The thrust reduction height is where the transition from takeoff to climb thrust takes place.  TRH can be altered in the CDU 'Init/Ref Index/Takeoff Ref Page (lsk1R) Reduction AGL-- AGL'

The height usually used for thrust reduction, not taking into account noise abatement, can vary; but, 400 feet AGL is the minimum allowed. 

LEFT:  Figure showing Thrust Mode Display (TMD).  In this example it is displaying CRZ (cruise). Figure copyright FCOM.

Once takeoff has occurred, examination of the Thrust Mode Display (TMD) will alert the flight crew to the type of climb that has been choosen.  The TMD will display the acronym TO (takeoff) or R-TO (reduced takeoff thrust) and will alter to CLB (climb) once the Thrust Reduction Height has been reached.

Confusion between Acceleration Height and Thrust Reduction Height

Newcomers are often confused between the two similarly-sounding terms, possibly because they both occur at the interface between takeoff and climb-out.  Simply written:

Acceleration Height is when the nose is to be lowered to allow the aircraft to accelerate. When the aircraft starts accelerating is when the flight crew will retract flaps as per the schedule.  Thrust Reduction Height is when the autothrottle will decrease the engine power to the preselected climb thrust; thereby reducing engine wear and tear.  Both may occur simultaneously or at differing heights above ground level.  Both can be configured in the CDU.

Differing Methods to Alter Thrust:  Derated Thrust (CLB-1, CLB-2), Assumed Temperature & Derated Thrust

There are several methods available to flight crews to alter N1 thrust controlled by the autothrottle system, and with the exception of the N1 speed reference knobs on the Main Instrument Panel (MIP), all are accessed via the CDU interface.

Derated Thrust (Derates):  Derate is a term used for derated thrust (or reduced thrust). 

The CDU displays a list of fixed-rate derates which may differ between aircraft, the reason being that each airframe may have a different powered engine.

Derates can be accessed from the N1 Limit Page.

Assumed Temperature:  This method calculates thrust based on a higher than actual air temperature and requires the crew to input into the CDU a higher than normal outside temperature.  This will cause the on-board computer to believe that the temperature is warmer than what it actually is; thereby, reducing N1 thrust.

The outside air temperature can be altered in the N1 Limit Page (lsk1L) or from the Takeoff Ref Page 2/2 (lsk4L).

Derated Thrust Climb (CLB-1 & CLB-2):  Selecting CLB-1 or CLB-2 commands the autothrottle to reduce N1 thrust during any climb phase to a higher altitude.  

Rather than use maximum climb or rate, crews often select CLB-1 which is approximately a 10% derate of climb thrust (climb limit reduced by 3% N1), while  CLB-2 is approximately a 20% derate of climb thrust (climb limit reduced by 6% N1).   Flight crews routinely preselect a lower than maximum climb thrust before departure.

CLB-1 and CLB-2 can be accessed from the N1 Limit Page. 

The reduced climb thrust setting, no matter which method used, gradually increases to full rated climb thrust by 15,000 feet.

Quiet Climb System (QCS) - Abiding with Noise Abatement Protocols 

Boeing has developed the Quiet Climb System, an automated avionics feature for quiet procedures that causes thrust cutback after takeoff.  By reducing and restoring thrust automatically, the system lessens crew workload and results in a consistently less noisy engine footprint, which helps airlines comply with noise abatement restrictions. There are two variables to enter: Altitude reduction and altitude restoration.

During the take-off checklist procedure, the pilot selects the QCS and enters the altitude at which thrust should be reduced.  The thrust reduction altitude is greater or equal to 800 ft AGL and the thrust restored altitude is typically 3000 feet AGL, however the altitudes may alter depending on obstacle clearance and the noise abatement required. 

With the autothrottle system engaged, the QCS reduces engine thrust when the cutback altitude is reached to maintain the optimal climb angle and airspeed. When the airplane reaches the chosen thrust restoration altitude (typically 3,000 ft AGL or as indicated by noise abatement procedures), the QCS restores full climb thrust automatically.  Note that the minimum altitude that the QCS can be set is 800 feet AGL.  This allows the safety envelope dictated by Acceleration Height to remain active.

Multiple Safety Features for Disconnect 

The Quiet Climb System incorporates multiple safety features and will continue to operate even with system failures. If a system failure does occur, there are several methods for exiting QCS.  In the most common method, the pilot selects the takeoff/go-around (TOGA) switches on the throttle control levers. The pilot can also take control of the throttles easily by disconnecting the auto throttle and controlling the thrust manually.

The Quiet Climb System, also known as 'cutback' can be accessed from the Takeoff Ref Page (lsk6R).  You will observe the name cutback with on/off.  You can also enter an altitude that you wish the system to restore full thrust.

For completeness, below is a copy of the current Noise Abatement Departure Procedures (NADP).  A copy of these procedures can be downloaded from the Training and Documents section on this website.  Click image for larger view.

Similarity of Terms

When you look at each of the above-mentioned three functions they appear similar in many respects. 

The way I remember them is as follows:

Acceleration Height (AH) is the altitude above ground level (AGL) that is set to ensure take-off speed (V2+15/20) is maintained for safety reasons. 

Thrust Reduction Height (TRH) is the altitude above ground level (AGL) that is set to reduce take-off thrust a few percent to maintain and increase engine life.

The Quiet Climb System (QCS) allows a minimum and maximum altitude to be set in the FMC; thereby, reducing engine power and engine noise.  The restoration altitude is the altitude that full climb power is restored.  The QCS is used only for noise abatement. 

Thrust Reduction Caveat

It must be remembered that any thrust reduction made within the CDU is accumulative.  For example, if you select a lower fixed-rate derate and then select a reduced N1 by the assumed temperature method, the thrust reductions will be added.  It is imperative that the crew actually look at the N1 power settings to ensure they are suitable for the weight of the aircraft, environmental conditions, and length of the runway.  To check and confirm the N1 settings, look at the Thrust Mode Display or the appropiate page in the CDU.

I urge you to read further by downloading the following documents located in the Training and Documents section on this site.

ProSim 737

As of June 2013, the ProSim737 avionics suite incorporates the Boeing Quiet Climb System and Thrust Reduction Height.  Acceleration Height is yet to be modelled.

Quality Assurance (QA)

This has been a long post dealing with items that are often confusing in their own right.   Rather than separate the similar topics into individual posts, I thought it easier to deal with them together.

When explaining procedures, I  attempt to keep the writing style simple and easy to understand for a wide range of audiences.  If I have failed, or you discover a mistake, please contact me so this can be rectified.

Acronyms Used

AH – Acceleration Height
AGL – Above Ground Level
CDU – Control Display Unit
CLB-1 & CLB-2 – Climb 1/2
DERATE – De-rated Thrust
FMC – Flight Management Computer
LSK1R – Line Select 1 Right (CDU)
PFD - Primary Flight Display
QCS – Quiet Climb System
R-TO – Reduced Takeoff (thrust)
RTC – Reduced Takeoff Climb
TRH – Thrust Reduction Height
TO – Takeoff (thrust)
TMD – Thrust Mode Display

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Reader Comments (1)

Many thanks for this very complete article !! It helps me a lot !

August 10, 2013 | Unregistered CommenterAir59

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