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

Thompson B738NG transitioning to Thrust Reduction Height, Immediately following this will be acceleration height when the aircraft’s nose is lowered, flaps are retracted and climb thrust commences, acceleration will be reached, Manchester, UK (Craig Sunter from Manchester, UK, Boeing 737-800 (Thomson Airways) (5895152176), CC BY 2.0)

The takeoff phase of a flight is one of the busiest and most critical periods. During this time, several distinct functions occur in rapid succession. While each function serves a unique purpose, they are intricately linked by the changing altitude of the aircraft. Because they unfold so quickly, these functions often cause confusion for those unfamiliar with the process.

In this article, we will explore the following:

  • Acceleration Height;

  • Thrust Reduction Height;

  • Derated Takeoff Thrust;

  • Assumed Temperature Method (Thrust Reduction);

  • Derated Climb Thrust; and,

  • The Quiet Climb System (often called cutback).

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.

The acceleration height is the altitude that the aircraft transitions from takeoff speed (V2+15/20) to climb speed.  This altitude is typically 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 three main reasons for acceleration height are:

  1. It enables a safety envelope below this altitude should there be an engine failure; engines are set to maximum thrust, and the aircraft’s attitude is set to maintain V2 safety speed (V2+15/20);

  2. It provides a safe height (AGL) at which the aircraft’s airspeed can be increased (transition to climb speed) and the flaps retracted; and,

  3. It provides a noise buffer concerning noise abatement. Below acceleration height the engines will be targeting V2 safety speed (V2+15/20) and will be generating less engine noise.

The Acceleration Height can be changed in the CDU (Init/Ref Index/Takeoff Ref Page (lsk4-L) ACCEL HT ---- AGL).

Practical Application

Once acceleration height has been reached, the pilot flying will reduce the aircraft’s attitude by pushing the yoke forward; thereby, increasing the aircraft’s speed.  As the speed increases to climb speed, the flaps can be retracted as per the flaps retraction schedule. It is important not to retract the flaps until the aircraft is accelerating at the speed indicated by the flaps retraction schedule (green-coloured flaps manoeuvring speed indicator ) on speed tape in the Primary Flight Display).

At this time, the autothrottle will be commanded by the autoflight system to increase the aircraft’s speed to climb speed. if manually flying the aircraft, the flight crew will need to increase the speed from V2 to climb speed (by dialing a new speed into the MCP speed window).

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

  1. Set the MCP to V2;

  2. Fly the Flight Director cues to Acceleration Height (which will be at V2+15/+20);

  3. At Acceleration Height, push yoke forward reducing the aircraft’s attitude (pitch);

  4. As forward speed increases you will quickly pass through the schedule for initial flap retraction (as indicated by the green-coloured flaps manoeuvring speed indicator – retract flaps 5;

  5. Dial into the MCP speed window the appropriate 'clean up' speed (reference the top white-coloured carrot on the speed tape of the PFD, typically 210-220 kias);

  6. Continue to retract flaps as per schedule; and,

  7. 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, the flight director bars will command the decrease in pitch when the selected altitude has been reached - all you do is follow the flight director 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 vary only be a few percent, but the lowering of EGT reduces heat and extends engine life significantly. 

The thrust reduction height is where the transition from takeoff to climb thrust takes place.  Acceleration height comes soon after.

Figure 1: upper display unit.. The green-coloured N1 reference bug reads 87..7. 1. indicates the thrust mode display (TMD). (image copyright FCOM)

The height used for thrust reduction, not taking into account noise abatement, can vary and be dependent on airline policy. Typically it falls between 800-1500 feet AGL. 

Possible reasons for selecting a higher thrust reduction altitude may be obstacle clearance (such as buildings, towers, etc) or environmental factors

When the aircraft reaches the thrust reduction height, the resultant loss of N1 from the engines will be noticeable. This decrease in N1 is displayed on the N1 RPM indicationd in the upper display unit of the EICAS. The N1 is displayed in large white numerals (87.7) and is also indicated by the green-coloured reference N1 bug.

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:

  • 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; and,

  • Acceleration Height is when the nose of the aircraft is lowered to increase airspeed. The flaps are then retracted as per the flaps retraction schedule.

The thrust reduction height can be altered in the CDU (Init/Ref Index/Takeoff Ref Page 2/2 (LSL-5L) REDUCTION THR -- AGL).


Reduced Thrust Derates

Engine derates on a Boeing 737 refer to the intentional reduction in engine thrust during certain flight conditions to optimise engine performance, increase safety, and preserve the longevity of the engines. A derate involves limiting the maximum available thrust that an engine can produce under specific conditions.

Purpose of Engine Derates:

  1. Safety and Engine Longevity: Derating can help prevent engine overstress and prolong the life of the engine, especially during takeoff and climb phases.

  2. Performance Optimisation: It can help maintain more efficient fuel burn, reduce engine wear, and manage high temperatures.

  3. Environmental Conditions: In cases of high ambient temperature or high-altitude airports, derating helps reduce the engine's demand on performance.

The takeoff performance available from an aircraft is typically in excess of that required, even when accounting for an engine failure. As a result, airline management encourage flight crews to use a derate, when possible. A derate enables the engines to operate at less than maximum thrust while still maintaining normal safety margins. In aviation terms, reducing takeoff or climb thrust is referred to as a derate.

It is routine for a flight crew to select a lower than maximum climb thrust (derate) before departure. This is done to minimise engine wear and tear, increase engine longevity, and to lower overall maintenance costs.

Derates to change the %N1 thrust commanded by the autothrottle system for takeoff and climb are found on the N1 Limit Page in the CDU. The following derates, applied singly or in combination, are possible:

  • Derated Takeoff Thrust (fixed derate).

  • Assumed Temperature Method (ATM) ; and,

  • Derated Climb Thrust (CLB-1 & CLB-2).

When To Use a Derate

Possible reasons for using or not using a derate are:

  • Environmental considerations (runway condition, weather, wind, etc);

  • Ambient temperature;

  • Airport’s height above sea level;

  • The weight of the aircraft’s load including fuel;

  • Consideration to airline management;

  • The length of the runway; and,

  • Noise abatement.

A derate is not idly selected by the flight crew, although if the aircraft is light in weight, the runway long, and the environmental conditions are favorable, a fixed derate may be selected ‘on the fly’. It is standard practice to use the Electronic Flight Bag (EFB) or another approved source to input various parameters (such as aircraft weight, wind direction and speed, runway length and direction, etc.) into the software. The software then calculates the necessary computations and provides a suitable derate. If an EFB is unavailable, the aircraft performance tables in the Flight Crew Operating Manual (FCOM) must be consulted, and the calculations done manually.

Using a derate is not always an option. For example, in high-performance scenarios, such as heavy takeoffs, high-density altitudes, or congested airspace, full thrust may be required. Similarly, a derate may not be suitable if the weather is extremely hot, or if the aircraft is heavy and the runway is short. The final decision on whether to use a derate rests with the Captain of the aircraft.

Thrust Mode Annunciations

When a derate is used, the thrust mode annunciation (coloured green) will be displayed (coloured green) on the upper display unit on the EICAS.

Possible displays are as follows:

  • TO – takeoff (displayed if no derate is used)

  • TO 1 – derated takeoff one.

  • TO 2 – derated takeoff two.

  • D-TO – assumed temperature reduced thrust takeoff (ATM).

  • D-TO 1 – derate one and assumed temperature reduced thrust takeoff (ATM).

  • D-TO 2 – derate two and assumed temperature reduced thrust takeoff (ATM).

1 - Derated Takeoff Thrust (Fixed Derate)

A fixed derate is a certified takeoff rating lower than a full rated takeoff thrust. In order to use a fixed derate, takeoff performance data for a specified fixed derate is required (Boeing FCTM 2023).

The N1 Limit page in the CDU displays three fixed-rate engine derates: 22000, 24000 and 26000 (22K, 24K and 26K). Selection of a derate will command the software to limit the maximum thrust of that engine to whatever has been selected; nothing is altered on the actual engine. Selecting a derated engine thrust can only occur when the aircraft is on the ground.

The %N1 for the selected derate is displayed on the NI Limit page, the TAKEOFF REF page (LSK-2L) and in the N1 RPM indications in the upper display unit (%N1 RPM readout and %N1 reference bug) on the EICAS.

Thrust Limitation (Fixed Derate)

When using a fixed derate, the takeoff thrust setting is considered a takeoff operating limit since minimum control speeds (VMCG and VMCA) and stabilizer trim setting are based on the derated takeoff thrust (Boeing, 2023 FCTM; 3.17).

The thrust levers should not be advanced beyond the N1 RPM indications unless conditions during takeoff require additional thrust on both engines (such as in the case of windshear). If the thrust levers are advanced beyond the N1 RPM indications (for example, in the event of an engine failure during takeoff), any increase in thrust could result in a loss of directional control.

Important Point:

  • A fixed derate can be used on a runway that is either wet, has standing water, or has slush, ice or snow ( provided the performance data supports use of such a derate).

2 - Assumed Temperature Method (ATM)

This method calculates thrust based on a assumed higher than actual air temperature and requires the crew to input into the CDU a higher than actual 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. This reduces the need for full thrust, achieving a quieter and more fuel-efficient takeoff.

Using ATM, the desired thrust can be be adjusted by incrementally changing the temperature to a higher or lower value. Often this is an advantage as a flight crew can fine tune the thrust setting to exactly what is required, rather than using a fixed derate.

The desired thrust level is obtained through entry of a SEL TEMP value on the N1 Limit Page (LSK-1L) or from the Takeoff Ref Page 2/2 (LSK-4L).

To delete the assumed temperate use the CDU’s delete key.

Thrust Limitation (ATM)

When using ATM, the takeoff thrust setting is not considered a takeoff operating limit since minimum control speeds (VMCG and VMCA) are based on the full rated takeoff thrust. At any time during takeoff, thrust levers may be advanced to the full rated takeoff thrust (Boeing, 2023 FCTM; 3.17).

Furthermore, a reduced takeoff thrust (ATM) may be used for takeoff on a wet runway provided the takeoff performance data (for a wet runway) is used. However, reduced takeoff thrust (ATM) is not permitted for takeoff on a runway contaminated with standing water, slush, snow, or ice.

Important Point:

  • During an ATM takeoff, the yoke may require additional back pressure during rotation and climb.

Combined Derate (ATM and Fixed Derate)

A fixed derate can be further reduced by using assumed temperature (ATM). However, the combined derate can not exceed a reduction of 25%.

Thrust Limitation (ATM & Fixed Derate Combined)

When conducting a combined ATM and fixed derate takeoff, takeoff speeds consider VMCG and VMCA only at the fixed derate thrust level. The thrust levers should not be advanced beyond the fixed derate limit unless conditions during takeoff require additional thrust on both engines, such as in the case of windshear(Boeing, 2023 FCTM; 3.18).

If the assumed temperature method is applied to a fixed derate, additional thrust should not exceed the fixed derate N1 limit. Otherwise, there may be a loss of directional control while on the ground.

3 - Derated Climb Thrust (CLB-1 & CLB-2) 

This method commands the autothrottle to reduce N1 thrust during any climb phase to a higher altitude.  There are three settings: CLB, CLB-1 and CLB-2. The settings are accessed from the N1 Limit page in the CDU. The derates are as follows:

  • CLB: Normal climb thrust (no derate);

  • CLB-1: Approximately a 10% derate of climb thrust (climb limit reduced by 3% N1; and,

  • CLB-2: Approximately a 20% derate of climb thrust (climb limit reduced by 6% N1).

Selecting CLB on the N1 Limit page after selecting either CLB-1 or CLB-2 will override the derate. The preferred method when deselection a climb derate is to delete the derate using the CDU delete key.

Figure 2: upper display unit.  1. points to the thrust mode display (TMD). In this example, it is displaying crz (cruise). If a climb derate is set it will display the derate (clb-1 or clb-2). The green-coloured arrow is the %N1 reference bug (image copyright FCOM)

The selected climb derate (after selection) and the %N1 for the derate is displayed on the NI Limit page and on the TAKEOFF REF page (LSK-2L). The specific N1 setting is displayed on the N1 RPM indicator and by the N1 reference bug). After takeoff, the climb derate is also displayed on the Climb page in the CDU.

Examination of the Thrust Mode Display (TMD) in the upper display unit of the EICAS will display the selected climb type.  The TMD will display the acronym TO (takeoff without a derate) or R-TO (reduced takeoff thrust) and when the thrust reduction height has been reached the display will change to whatever climb derate was selected (CLB, CLB-1 or CLB-2).

No matter whether an assumed temperature thrust reduction or a reduced climb setting is used, the FMC will automatically calculate a corresponding climb speed that will be less or equal to the takeoff thrust. Therefore, it is imperative that the flight crew actually look at the N1 power settings to ensure that they are suitable for the weight of the aircraft, environmental conditions, and length of the runway.  The reduced climb thrust setting, no matter which method used, gradually increases to full rated climb thrust by 15,000 feet. To deselect either CLB-1 or CLB-2, the delete key is used in the CDU.

Important Caveat:

It is important to be watchful that the climb thrust does not become greater than the takeoff thrust. This will occur, if a derated climb has been selected and the flight crew select CLB after takeoff; selecting CLB will apply climb thrust and does not take into account any speed compilation made by the FMC when the derated climb was selected. if this was to occur, the aircraft may not be flying at a speed commensurate with V2+15/20.

Important Points:

  • If the Assumed Temperature Method is selected in addition to another thrust derate setting, the calculation for takeoff thrust is accumulative. Selecting more than one derate can affect the power that is available for takeoff and significantly increase roll out distance for takeoff.

  • Prior to takeoff, a flight crew must check and confirm the N1 settings for the takeoff thrust and climb. These are displayed on the TAKEOFF REF and NI LIMIT pages in the CDU and on the upper display unit.

  • if strong winds or windshear is expected during the takeoff, a full rated takeoff thrust is recommended. Furthermore, if the runway is contaminated (standing water, slush, snow or ice) a derate (not to be confused with a fixed derate discussed above) should not be used.

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

The Boeing Quiet Climb System (often called cutback and referred to by line pilots as ‘hush mode’), is 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 takeoff 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.

The heights referenced by the Quiet Climb System can be changed in the CDU in the TAKEOFF REF 2/2 page (LSK-5R). The system can be selected or unselected at LSK-6L (on/off).

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, two ways of exiting the QCS are to:

  1. Select the takeoff/go-around (TOGA) switches on the throttle control levers; or,

  2. Disconnect the autothrottle and control thrust manually.

ProSim737

The quiet climb system was a component of the ProSim737 avionics suite, however, from release 3.33 has been removed. It is now only available in the professional version of ProSim737 - not in the domestic version.

Figure 2: For completeness, and to provide an example of the altitude above ground level (AGL) that a noise abatement proceedure uses.

Figure 2: Noise Abatement Departure Procedures (NADP). (click image for larger view).

Similarity of Terms

When you look at the intricacies of the above mentioned functions there is a degree of similarity. This is because all the functions center around altitude above ground level, in what is a time critical phase of flight (the takeoff and initial climb)

The way I remember them is as follows:

Thrust Reduction Height is the altitude above ground level (AGL) that is set to reduce takeoff thrust a few percent to maintain and increase engine life. it is when the autothrottle will decrease the engine power to the preselected climb thrust;

Acceleration Height is the altitude above ground level (AGL) at which the aircraft’s nose is lowered to increase airspeed. Flap retraction typically begins at acceleration height;

Assumed Temperature Method (ATM) is when the N1 is lowered by changing the ambient temperature to a higher value in the CDU. This is done prior to takeoff;

Derated Takeoff Thrust is when the N1 of the engines is reduced (22K, 24K or 26K). This is done prior to takeoff;

Derated Climb Thrust (CLB-1 & CLB-2) is when the N1 is changed to a lower power setting during climb below FL150. It can be done prior to takeoff or when the aircraft is airborne; and,

The Quiet Climb System enables 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.

Final Call

Acceleration height, thrust reduction height, and derates are critical elements in optimising the takeoff performance of the Boeing 737.

Acceleration height is the altitude at which the aircraft’s nose is lowered to gain speed and the flaps are retracted, while the thrust reduction height determines at what height above ground level (AGL) to reduce engine power, from takeoff thrust to a lower setting. By adjusting the engine thrust settings and applying derates, operators can enhance engine longevity, improve fuel efficiency, and reduce noise during takeoff.

Understanding and properly applying these settings not only ensures compliance with performance regulations, but also contributes to operational efficiency.. Ultimately, these parameters enable operators to maximize safety, minimise fuel consumption, and optimise aircraft performance during takeoff.

Acronyms Used

AH – Acceleration Height

AGL – Above Ground Level (height)

CDU – Control Display Unit

CLB-1 & CLB-2 – Climb 1 and Climb 2

DERATE – De-rated Thrust

FMC – Flight Management Computer

LSK-1R – Line Select 1 Right (CDU)

‘On The Fly’ – ‘On the fly’ is an idiomatic expression often used in casual or conversational contexts to mean something done quickly, without preparation, or while in motion.

PFD - Primary Flight Display

QCS – Quiet Climb System

TMD – Thrust Mode Display

Updates

19 February 2025

Gallery: Various screen grabs from the CDU showing the effect on %N1 for various fixed derates and Assumed temperate (ATM)