View Full Version : Performance Question

29th Jul 2002, 01:23
Given a company acceleration altitude of 1500 AGL and TOGA power approval of 10 minutes for a B-737 NG, could you apply wing anti-ice after an engine failure at 400 feet AGL and still meet the second segment minimum 2.4% climb gradient?:confused:

29th Jul 2002, 05:14
Considering that you have ALL ENGINE performance from V1 - 400 feet, if the engine failed at that point, your climb gradient would decrease towards the 2.4% minimum gradient and slightly less if you then selected Anti-Ice.

The average gradient by the time that you reached 1500 feet MIGHT be 2.4%. But there are no guarantees!

If you need Anti Ice that badly, you should insist that your takeoff weights are calculated with ANTI-ICE ON.


29th Jul 2002, 08:21
As Mutt says, if you THINK you will need WING OR ENGINE anti-ice before you are clean, the performance calcs should reflect this.

29th Jul 2002, 08:51
Thanx for that, but it is still not very clear. According to the AFM, the WTAI is supposed to come on when you have MCT on the good engine and you are into your third climb segment so to speak. Also, suppose you have already taken into account the weight penalty of the engine and wing anti-ice on the performance calculations and the ATOW is at or below that. Accoding to the FAR part 25.111 (d)(2) ... the W1 and power/thrust must be constant throughout each segment; by putting the WTAI you are effectively reducing your available thrust by approximately 3% N1, give or take some.
One opinion that was given to me, was that since the FAR legal second segment had been complied with, you could put it on as only a 1.2% gradient was needed after 400 feet AGL (FAR Part 25.111 (c)(3)).
Now, I still think that since the Operator's acceleration altitude is set at 1500 feet AGL, you cannot afect the thrust till reaching that that altitude regardless of the FAR minimum legal 400 feet altitude. Is that correct or not?

29th Jul 2002, 21:53
Is the 1500ft acceleration height there for obstacle clearance or is it part of the company/ICAO standard noise abaitment procedure.

For performance requirements;

400ft is the lowest that the acceleration height can be.
The acceleration height may be higher in order to clear a relatively close in obstacle. The maximum acceleration height (maximum level off height) is the highest you can climb and accelerate to final segment climb speed within the take-off power time limit.

If the 1500ft is for obstacle clearance then I would agree with those who say that anti-ice should be allowed for in advance if required.....after all, you can not plan that your engine will fail at 400ft for performance A requirements.

If on the other hand the 1500ft is a noise limit then after checking the performance data, you may find that the minimum acceleration height for that airfield's obstacle environment is lower.

Just a guess.


31st Jul 2002, 17:10

I am not specifically familiar with the NG systems so I will presume that Finalveridict is correct in saying that "According to the AFM, the WTAI is supposed to come on when you have MCT on the good engine".

The selection of MCT normally occurs at the start of the 4th segment and is an acceptable power reduction in accordance with FAR 25.121 (c)(1). Provided that there are no 4th segment obstacles to consider and the minimum certification climb performance requirement is met, there is no practical or legal problem and the references to the TOGA thrust limit and acceleration altitude are quite irrelevant.

The AFM data will allow you to calculate a TOW such that the first three segments are completed within the TOGA thrust limit using whatever acceleration altitude the operator nominates. As that acceleration altitude gets further away from 400 ft, TOW often must be reduced to allow the extended 2nd segment and 3rd segment acceleration to be completed. With high thrust engines in icing conditions, an acceleration altitude of 1500 ft may not require any TOW reduction at all - the answer will be in front of you in the TOW data.

I suggest that you read all of the relevant provisions, including subpart G and FAR 25.1533 to appreciate that 400 ft is the minimum, but not the only, height to demonstrate compliance with the take-off flight path provisions.

4th Aug 2002, 08:34
The segment in question is the second climb only, neither noise abatement nor obstacle is an issue here. The 1500 foot altitude was set to standarize operations across the airline's fleets.
According to a performance engineer of the company, the second segment here, ends at 1500 feet and the application of WTAI after 400 ft would definitely affect the 2.4% minimum gradient in case the TOW would be at the maximum permitted under the conditions.
Furthermore, given that the sytems design's objective is to deice rather than anti-ice, I would assume that it would be safe to continue the V2 climb to the acceleration altitude of 1500 feet without the WTAI on and then aplly it after flap retraction is completed and MCT set, as it was supposed to be used and the procedure calls for. The ice accretion would be dealt with at the fourth segment. Is this a sound asumption or not?

11th Aug 2002, 20:59
From the standpoint of performance requirements, it is pretty clear that the minimum climb gradients have to be complied with. If the second segmnent is, for whatever reason, extended above 400 ft GND, than the requirements for second segment still apply - until acceleration altitude, when third segment starts and other rules apply. So if anti-ice is needed during that period, this will affect the take-off performance calculation. Otherwise, if an engine would fail at, say, 600 ft which anti-ice now ON, the climb gradient could in theory be less than 2,4% gross and therefore less than 1.6% net, which is not a good idea (and of course that would be illegal for any FAR/JAR 25 aircraft).
On the other hand I assume that the performance of the 737 NG is so good that in most cases the aircraft will not be at the minimum 2.4% gross climb limit with temperatures in a such a low range that anti-ice is required. But I could be wrong on this one.

12th Aug 2002, 11:58
I must have missed a salient point here, perhaps.

Why would we be wanting to turn on the anti-ice half way through the takeoff sequence ? regardless of OEI/AEO considerations ?

... instead of having it selected on (regardless of where the system might permit the selection to take effect) throughout ?

.. .and if one expected to need it for some reason half way through, then surely the appropriate RTOW penalty would have been taken for the takeoff ?

in which case the original question is addressed in the ops eng sums for the degraded performance case ?

Please do ease my confusion, someone ....