A question on 737 simulator stab trim fidelity re engine failure V1/Go on runway
In the back of my mind was the understanding that the correct setting of the CG/Stab Trim from the load-sheet meant the aircraft was designed to be in-trim at V2+15 with both engines operating. A former inspector qualified to conduct simulator fidelity checks on the Boeing 767 told me that the stab trim setting from the load sheet on the 767 is based upon an engine failure at V1 - accelerate to VR on one engine - and be in trim at V2 +15.
The discussion arose over stick forces in one particular 737 simulator following an engine failure at V1 - accelerating to VR - and rotating. Experience with this simulator is that with the V1 and Go situation allowing say a 10 knot spread between V1 and VR, the back stick force to achieve rotation to around 12 degrees initially on one engine, is very heavy requiring a determined pull and by the time the aircraft has reached gear selected up height of about 100 fet agl, it is necessary to use considerable back stab trim to the order of at least seven units indicated. The stab trim was initially set to 5.2 Units for take off. The take off weight was 50 tonnes with flap 5 sea level standard temp.
On a normal two-engine take off the stab trim of 5.2 Units poses no problems as far as I recall. If the airworthiness certification of the simulator stab trim forces means in-trim at V2+15 with two engines then I can understand that following a engine cut at V1 and the decision is made to continue, the absence of the failed engine to provide a vertical component of thrust as the nose rises, will give the effect of a nose heavy aircraft that requires significant stab trim aft to get back into trim. This is what we experience on the engine failure on the runway case (V1) with continued acceleration on the live engine. It is not uncommon for pilots in the simulator to sink back onto the runway as they are caught by surprise by the significant pull needed to rotate on one engine and get the climb going from VR.
The question therefore relates to the simulator airworthiness meaning of the normal take off stab trim setting. Is it designed to be in trim for V2+15 on two engines? Or does the stab trim setting from the load sheet assume engine failure at V1 and continue? Once that fact is known then it becomes a simulator fidelity discussion re stick forces on VR.
Our load sheet gives a stab trim (in accordance with Boeing performance methods) that should give reasonable control forces and an approximate in trim condition for V2+20 all engines and V2 one engine out. I am told and can understand that a substantial pull is required for the EF case as you stated, the nose up ten dance is less with one engine.
I can't speak for your 737 sim, but when we did simulated EFTOs (idle thrust on OB engine selected) on B707 during base training, then it needed more NU trim than a normal all engine climb out.
Your logic with "If the airworthiness certification of the simulator stab trim forces means in-trim at V2+15 with two engines then I can understand that following a engine cut at V1 and the decision is made to continue, the absence of the failed engine to provide a vertical component of thrust as the nose rises, will give the effect of a nose heavy aircraft that requires significant stab trim aft to get back into trim." is correct.
How much extra nose up trim does it require to fly 15 kts slower? I suggest if you doubled that (for the loss of the nose up couple from the failed engine) - then you'd be about right.
The mach trim is supposed to indirectly help to increase nose up during take off in case an engine fails...to avoid such issue as a nose heavy feeling as mentionned above. It is called the FCC Controlled Neutral Shift Enable region.(FCNSE).
I've a feeling that the stab trim is set (on 738) for 2 eng, gear up & V2+20. We're encouraged by our senior trainers to teach the guys to avoid trimming until gear is up for that reason, and for the engine failure case to expect to be out of trim for the reasons outlined by rrrat. As far as giving the CC a big heave, am not sure that's the answer (of course you do what you have to), you're trying to achieve a slightly slower rotation rate eng out, so the control force should be similar. It is interesting however the two sims that we use require entirely different techniques to get airborne on one engine! So perhaps I better refer to the FCC Controlled Neutral Shift Enable region!
Last edited by Fredairstair; 31st Jan 2013 at 14:11.
Centaurus: I would suggest that you ask the people who maintain and certify the simulator you train on if you can review IQTG test 1B5 (critical engine failure at takeoff): this particular objective test covers precisely this situation and offers time histories of all the relevant flight paramenters that have to match with Boeing flight test data recorded from the real aircraft.
Maybe use correct speeds from an approved perf table when using flex take offs,ensure CG in FMC takeoff page is correct. Also the rotation rate should be the same as for two engines,if the pilot either delays rotation(surprise and or analysing factor) and delays rotation,speed will build up and higher pulling force may be necessary.