In this article I would like to explain to you the often confusing entries for Thrust Reduction, Thrust Acceleration and Engien Out Acceleration in the MCDU. These often lead to quite a bit of confusion, but are explained quite quickly and easily. In this article I will explain the following terms and topics to you:
- Thrust Reduction Height (TRH) – Thrust reduction height
- Thrust Acceleration or Acceleration Height (AH) – acceleration height
- Noise Abatement – noise reduction
- Engine Out Acceleration
Thrust Reduction Height (TRH)
Thrust Reduction Height (TRH) is the height above the ground (AGL – Above Ground Level) at which the take-off thrust (TO/GA or MAN FLEX) is switched to the CLIMB (CLB). This reduces thrust by a few percent to maintain and extend engine life. As soon as the aircraft has reached the altitude, the display shows LVR CLB – the request to move the throttle to the CLB position.
The absolute minimum for TRH is 400ft, although TRH is typically 1000-1500ft. In most cases, TRH and AH correspond to the same height.
Example: Frankfurt/Main EDDF
- Airport Elevation: 364ft MSL
- AH: 1500ft AGL
- At 1870ft to 1900ft (MSL) the throttle should be moved from TO/GA or FLX to CLB
Thrust Acceleration or Acceleration Height (AH)
Acceleration altitude is the height above ground level (AGL) at which a pilot accelerates the aircraft by reducing the aircraft’s pitch to allow acceleration to a speed safe enough to deploy flaps and slats ( flaps & slats) and then achieve the desired rate of climb.
The takeoff is carried out with the so-called takeoff speed (V2 + 10/20kt). From the acceleration level, the speed is increased to Climb Speed. This is usually the “Green Dot Speed” unless there is a constraint (speed limit) in the way. The AH is usually 1000 to 1500ft but can also be 800ft. In most cases, TRH and AH correspond to the same height.
The acceleration altitude is necessary because it is safer to first let the aircraft climb strongly (at a lower speed) in order to have reached a certain altitude in the event of an engine failure. The aircraft should then be accelerated.
Example: Frankfurt/Main EDDF
- Airport Elevation: 364ft MSL
- AH: 1500ft AGL
- At 1870ft to 1900ft (MSL) the nose should be lowered and the aircraft accelerated
Noise abatement
There are often so-called noise abatement (noise reduction) instructions for the climb phase at night or for certain departure directions. Up to about 1500ft AGL there is a normal climb phase with the speed V2 + 10/20kt. This 1500ft AGL is considered the THR (Thrust Reduction Altitude). This means that the aircraft’s thrust lever is now switched from TO/GA or FLX to CLB. The thrust is now a little less, which means that the pitch (rate of climb) has to be lowered a little. However, it will continue to rise with V2 + 10/20kt up to mostly 3000ft AGL. The AH (acceleration altitude) only occurs at an altitude of 3000ft AGL.
Example: Frankfurt/Main EDDF
- Airport Elevation: 364ft MSL
- TRH: 1500ft AGL
- AH: 3000ft AGL
- At 1870ft to 1900ft (MSL) the throttle should be set from TO/GA or FLX to CLB and the rate of climb should be reduced slightly so that V2 + 10/20kt are maintained. At 3370ft to 3400ft (MSL) the rate of climb is further reduced allowing the aircraft to accelerate and flaps to retract.
Engine Out Acceleration
The Engine Out Acceleration is the altitude at which acceleration is carried out with only one engine. As a rule, it is a fixed value for airlines at around 1000ft AGL. However, nearby obstacles still have to be considered, so the height definitely has to be adjusted.
The aircraft should be leveled (hold altitude) at Engine Out Acceleration and thereby increase speed quickly. The flaps should then be retracted according to plan.
FAQ
The most important questions and terms are summarized again in this section. You can find more topics on the overview page for advisors.
What is Thrust Reduction Height?
The thrust reduction level is when the TO/GA or FLX thrust should be reduced to the CLB thrust. In addition, wear and tear on the engines is significantly reduced. To do this, the aircraft’s throttle is switched back to CLB mode.
What is Engine Out Acceleration?
Engine Out Acceleration holds the aircraft at the selected altitude and then accelerates.
What is Thrust Acceleration?
The acceleration altitude (AH) is when the nose of the aircraft must be lowered in order for the aircraft to accelerate. During acceleration, the aircraft’s flaps are retracted.
What is Acceleration Height?
The acceleration altitude (AH) is when the nose of the aircraft must be lowered in order for the aircraft to accelerate. During acceleration, the aircraft’s flaps are retracted.
What does AH mean?
AH is the Acceleration Height or Thrust Acceleration. Acceleration altitude is when the nose of the aircraft must be lowered in order for the aircraft to accelerate. During acceleration, the aircraft’s flaps are retracted.
What does AGL mean?
AGL is an indication of altitude and means Above Ground Level or height above the ground and is measured with a radar altimeter. As opposed to MSL or AMSL (altitude above sea level). AGL is usually referred to as a height (HT), AGL as an altitude.
What does TRH mean?
Thrust Reduction Height is the height at which an airplane’s throttle is shifted from TO/GA or FLX to CLB mode. This significantly increases the service life of the engine.