What determines the "acceleration" and "thrust reduction" heights?

Question

Thrust reduction height is the height at which automatic thrust reduction from takeoff thrust to climb thrust happens. And acceleration height is the height at which you start accelerating the plane from takeoff config to clean config.

But is the acceleration height always the same as the thrust reduction height, just sometimes, or will it depend on the company's SOP (standard operating procedure)?

Answer 1

(Highlight mine.)

Short answer: airport briefing.

If no restrictions, then company SOP.

Thrust reduction is usually set to activate once the plane clears the noise abatement and/or MSA (Minimum Sector Altitude).

VNAV takes care of acceleration when activated as long as the SID is programmed. It also warns the crew if constraints will not be met.

Boeing 737NG VNAV. (Highlight mine.)

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From Skybrary:

Thrust Reduction and Acceleration Procedures

On take-off, in order to position the aircraft to a safe height away from terrain and obstacles (i.e. a flight path of maximum height and minimum ground distance desired), the engine thrust is set to a high ‘take-off power’ setting (although this is not necessarily full power) and the aircraft attitude is pitched up to maintain a specific speed (usually in the region of V2 + 15kts). Once the ‘safe height’ is reached the engine thrust can therefore be reduced to a more appropriate (i.e. efficient) setting and the aircraft flight path can be changed to a more appropriate (i.e. efficient) flight path.

In order to achieve that described above and also to provide minimum noise disturbance to the area surrounding the airport, regulatory procedures require aircraft to fly one of the two profiles given below during every departure. The aerodrome briefing pages will state which one of the two profiles is to be flown at each particular airport:

• At 1,500ft aal reduce thrust to climb power and accelerate the aircraft towards 250 knots, retracting flaps and slats.
• At 1,500ft aal reduce thrust to climb power maintaining V2 + 15kts. At 3,000ft accelerate the aircraft.

Operator’s SOPs must therefore enable the above procedures to be flown.

Typical Boeing 737 climb profile:

Acceleration here would happen at (2) when VNAV is engaged. Default acceleration height depends on the airline, manual entry is possible from 400 to 9999 feet.

The default thrust reduction height is 1500 feet (lowest manual entry is 800).

Answer 2

The Airbus reference Getting to Grips with Aircraft Performance provides a description in section 4.1 Takeoff Flight Path.

They describe the takeoff flight path as four segments as shown in the following figure;

They provide the following discussion:

So, below 400 feet, the speed must be maintained constant to a minimum of V₂. Above 400 feet, the aircraft must fulfill a minimum climb gradient, which can be transformed into an acceleration capability in level flight. Therefore, the regulatory minimum acceleration height is fixed to 400 feet above the takeoff surface.
Nevertheless, during the acceleration segment, obstacle clearance must be ensured at any moment. Therefore, the operational minimum acceleration height is equal to or greater than 400 feet (Figure C16).

The Maximum Acceleration Height is described as follows:

JAR/FAR 25.111 (c)(3) At each point along the takeoff flight path, starting at the point at which the aeroplane reaches 400 ft above the takeoff surface, the available gradient of climb may not be less than:
• 1.2% for a two-engined airplane
• 1.7% for a four-engined airplane

The Maximum Takeoff Thrust (TOGA) is certified for use for a maximum of 10 minutes, in case of an engine failure at takeoff, and for a maximum of 5 minutes with all engines operating. The Maximum Continuous Thrust (MCT), which is not time-limited, can only be selected once the enroute configuration is achieved (i.e. when the aircraft is in clean configuration at green dot speed). As a result, the enroute configuration (end of the third segment) must be achieved within a maximum of 10 minutes after takeoff, thus enabling the determination of a maximum acceleration height (Figure C16).

The Acceleration phase ends when the aircraft reaches green dot speed in a clean configuration. Based on the requirement to maintain a minimum climb gradient during this phase, the Maximum Acceleration Height will be above the Minimum Acceleration Height.

The biggest caveat in the definition is that heights and climb gradients must comply with adequate obstacle clearances.