I was asked today by a very good first officer for a “5 star” airline the difference between Minimum Acceleration Height and Maximum Acceleration Height as seen on RTOW charts. For example, the RTOW chart for an A319 with the IAE V2522-A5 engines from Bangalore’s old HAL Airport (VOBG) shows a min and max acceleration height of 436ft and 1333ft, for a departure from runway 09. What does this mean?
Opening Airbus A319 FCOM 2.02.10 describes the acceleration height as that which “ensures that the net flight path clears the highest obstacle by at least 35ft when accelerating in level flight to Green DOT (Maximum lift to drag ratio speed in clean configuration) speed after engine failure, in the most adverse of conditions.” That can only be the minimum acceleration altitude. The acceleration altitude never goes below 400ft. Anything above 400ft is to ensure operational minimum acceleration height.
So what is the maximum acceleration height in that case?
A take off has four segments: First, Second, Third, and Final.
First Segment: from 35ft AGL to Gear Retraction, with takeoff thrust at V2 minimum.
Second Segment: from gear retraction to acceleration height, at V2 minimum, with take off thrust.
Third Segment: from acceleration height to reaching green dot speed. This segment must end at a maximum of 10 minutes from the start of the take off roll.
Final segment: A climb at green dot speed till 1500ft AGL with MCT (Maximum Continuous Thrust).
What determines the maximum acceleration height are the following:
a) A minimum of 1.2% climb gradient when above the acceleration height.
b) Green Dot speed must be reached 10 minutes after starting take off roll.
For this, you must construct the path backward.
1. Determine the time taken for the aircraft to accelerate from V2 to GDOT with a) Single Engine TOGA b) Worst case RTOW c) 1.2% climb gradient. Call this “t1“.
2. Determine the time taken for initiation of take off roll to attaining V2 with one engine failing at V1. (35ft above the ground). Call this “t2“.
3. Determine the time taken for aircraft to completely retract gear, from 35ft above the ground (First Segment). Call this “t3“. The height attained in this first segment may be considered height “h1“. All this is with one engine inoperative, with the other generating TOGA thrust.
4. The time remaining for the second segment is “t4” = (10 – (t1+t2+t3)). 10 minutes is considered as that is the maximum duration for which a single engine may be operated at TOGA. Note that the worst case assumption is for a take off at TOGA, with worst case RTOW (Heaviest), and engine failure at V1. With these parameters of physics, the climb gradient is calculated for the second segment at V2. With the climb gradient, the height attained in time t4 is “h2“.
5. Thus, with the height the aircraft attains, above the ground, at the end of the second segment, with TOGA thrust on one engine, other engine inoperative at V1, and the worst and heaviest weight restricted by the runway, is the maximum acceleration height.
If a pilot attempts to climbout beyond the maximum acceleration height maintaining V2, the live engine may exceed the 10 minute TOGA limit, risking engine failure and consequently total loss of thrust, before the Green Dot Speed is reached.