View Full Version : B737 stab trim setting v V2

21st May 2015, 13:52
In another forum there has been discussion on rotation technique and in particular why is it in the 737 at least, a normal rotation rate results in the aircraft "pausing" as it reaches 10 degrees and the pilot has to pull a bit harder to get the rotation going again toward the initial 15 degree nose up mark.

Opinions vary and the facts are hard to pin down but I always thought the unintentional pause at 10 degrees was a stick force change as the tail went into ground effect. Someone else said it was a combination of centre of gravity through the wheels still on the ground and the centre of pressure moving forward as angle of attack (?) was increased at VR.

Disregarding the above my question is on certification take off tests on the 737 and the applicable stab trim setting. I understood (probably incorrectly) that provided the stab trim setting was correct on load sheet figures, the stab trim calculation is based upon the 737 reaching 35 feet at V2 speed all engines and in trim.

On the other hand, another view was the certification stab trim calculation was based upon the aircraft losing an engine at V1 and continuing the take off roll to VR and being in trim at V2 at 35 feet with one engine inoperative.

Having thankfully never experienced a real engine failure at V1 and thus only simulator experience to fall back on, I can say with some authority that in the 737 various simulators I have "flown" the stab trim setting certainly was way out at V2 one engine inoperative. In fact one needed a prolonged and heavy pull force at VR when an engine was failed at V1 and the take off run continued. The stab trim setting for the all engines case meant rotation forces at VR were normal passing V2 but in my experience were out of whack by at least two units on the engine failure and go case at V1. This was common to all the 737 simulators I have operated.

Judging from the 737 Classic simulators I have operated in several countries, the same comments I made earlier held true. Does this mean that it is correct or incorrect to say that the setting of the stab trim (certification flight tests) from the load sheet ensures the trim is perfect passing V2 at 35 feet all engines? Or are all the simulators lacking fidelity in that area of operations?

21st May 2015, 20:02
I don't know about the certification side of things, but if you lose an engine before rotation then I would expect a greater pull being required as you have lost half of the pitch up moment due to engine thrust with under slung engines.

Capt Chambo
21st May 2015, 23:35
As ever a thought provoking post from you Centaurus.

A couple of random thoughts..

I seem to recall way back that the stab trim setting was only valid for a full thrust take off. Not sure if that was in the FCOM, FCTM, or a Boeing performance or loading manual. If true then any de-rate, either fixed de-rate or ATM de-rate means the stab trim setting is not optimal.

I also seem to recall a Bulletin advising that the installation of winglets also rendered the stab trim setting inaccurate, though I believe that may have been resolved.

In any instance as you point out there always seems to be a "null" point during rotation and one nearly always ends up "blipping" some aft trim after T/O!

22nd May 2015, 07:57
In any instance as you point out there always seems to be a "null" point during rotation and one nearly always ends up "blipping" some aft trim after T/O!

The term "Null" really does describe the sensation of the pause succinctly. Thanks for that and I will remember to use it during simulator briefings on engine failures during the take off phase.

During type rating and recurrent training (simulator of course), it is quite common to observe the pilot caught by surprise (despite being briefed earlier) at the heavy stick force needed to initially rotate on one engine and while dragging the control column back through to 10 degrees.

Often the aircraft will gently sink back on to the runway as they inadvertently relaxed the heavy back pressure during the one engine rotation instead of maintaining the back pressure through the "pause" period of a couple of seconds. Sometimes pilots will then use the stab trim to help with rotation forces.

That brings with it its own problems as we see pilots relying on the stab trim to "fly" the aircraft instead of using elevators to set the required attitude followed by stab trim to fine tune stick forces. It takes several of these manoeuvres (repositioning each time) until pilots are confident they can handle engine failure at V1 and continue to VR on one engine and climb away without momentarily allowing the wheels to hit the runway mid rotation.

Lord Spandex Masher
22nd May 2015, 16:29
The trim setting will change in the FMS if you change either the derate or the TAss.

I've had the benefit of performing upto 4 take offs on the same aircraft, on the same day, at the same weight and the trim setting has been pretty spot on for each thrust setting (from full chat to max derate/assumed), with maybe a small blip or two either way - probably due to that inaccurate pointer or being too impatient for the gear to get up!

Not an engine failure but we do airborne engine slam acceleration checks one engine at a time and the stick force change between TOGA and idle is about 15 odd pounds (subjectively, I haven't used a force meter) which is not insignificant.

I've no idea about certification but I'd suggest that from my experience that the trim is valid only for the AEO case.

I've heard several explanations for the rotation pause - I'd go with ground effect personally but I also quite like elevator blanking by the wing. It's probably a combination of both and more.

Capt Chambo
23rd May 2015, 00:44
Another couple of random thoughts that have occurred to me. The stab. trim setting must be a compromise setting particularly on the -NG. The "normal" flap setting for T/O I would suggest is Flap 5, however I have over the years frequently used Flap 15, and less frequently Flap 1 or Flap 25, when the performance has required it.

Our T/O MAC trim comes from a DCS load sheet which doesn't know what flap setting we are going to use, so we use the same setting irrespective of flap setting. IIRC the "classics" had a correction on the back of the QRH to allow for different flap settings. In practice I have only ever used Flap 5 or 15 on the classic.

There are of course other load and trim sheet providers, from a manual load sheet where pax and cargo are in zones, right through to Boeing propriety tools like the OPT, some of which will know flap settings and most will be able to calculate the pax loads based on which row of seats they are in. These should be able to provide a more accurate trim sheet.

Which brings me on to a final point on how the stab. trim setting must be a compromise. Various airlines use different weight assumptions for pax and bags. Most operators I have worked for use a standard pax weight and a standard bag weight. My present mob use a standard pax weight but the actual baggage weight. How many of us, or our pax are a standard weight though?

All this leaves me to believe that the stab trim is usually "about right" for an all engines take off, but is woefully inadequate when practising an EFATO, where a big heave is required!

23rd May 2015, 11:27

FMC & OPT will give most accurate stab trim when CG is entered in TO page, having selected Flaps, Thrust and Derate.
AFM & QRH & FPPM have reference tables which adjust "uncorrected" trim to stab trim based on flap and thrust setting only. In these tables there is no disparity between e.g. Flap 1 & 5, which if selected on FMC does generate a change. Thus these tables are simplified.

Best way to fly the 737 in trim: use OPT or FMC in combination with actual CG for takeoff as per loadsheet.

Always a little out of trim when using simplified referenced figures.

23rd May 2015, 12:35
My present mob use a standard pax weight but the actual baggage weight. How many of us, or our pax are a standard weight though?

In another era, Air Nauru (Central Pacific airline in the 1970-80's using the 737-200) operated a weekly charter from Noumea, New Caledonia (French), to Wallis island (a French protectorate). The Wallis islanders are seriously big Pacific islanders like the Samoans and Tongans. Morbidly obese is a good description.

Departing Noumea one day at max structural of 53,000 kgs with P&W JD8-17 engines, the 737 laboured to reach 31,000. Unusually long take off run too. In cruise using load sheet weights to set cruise thrust, the airspeed would continually lose ten knots. We would increase to climb thrust to get back to 0.74 and slowly the speed would fall back 10 knots IAS.

On arrival Wallis, I had each of the approximately 80 passengers weighed as they stepped off the aircraft. First female was 135 kgs. Second 110 kgs. Repeat for most of the others. And that didn't count hand baggage which averaged 15 kgs per huge gaily coloured bag.

The French agent at Noumea used standard weights of 75 kgs for male and 65 kgs female and hand baggage not weighed. Turned out we were roughly 1500 kgs over max structural BRW for the 737-200. After that, we convinced the French agents at Noumea and Wallis to weigh each passenger and also weigh the hand luggage.

At Samoa in the South Pacific region, it was common for Samoans to have "cousins" everywhere; including the staff at the check-in counter where luggage was weighed. Frequently one would observe the man weighing the luggage slip his big toe under the scales and lift a bag or suitcase with the usual result the bags weighed less on the scales depending the strength of the big toe. V1 became useless..

24th May 2015, 05:08
Centaurus: During training, this was a question of mine as well. I was told the trim was set with V2 initial climb (all engines) being the certification (and subsequent SOP) target. I was also told that optimum stab trim settings will always require nose up input.

As to the secondary discussion here, it is interesting to hear theories on the need for additional back stick at around 9-10 degrees pitch. I believe the phenomenon occurs on most if not all transport category aircraft although the DC-9 does not require nearly as much additional back pressure as those aircraft I've flown with conventional tailplanes. Ground effect and wing downwash are the two theories I have heard over and again, but never has there been a definitive answer, although the aerodynamicists I've talked to favored GE as the cause for the 'pause.'

24th May 2015, 06:53
Stab trim is set for a TO with all engines operating. If there is a discrepancy in the stick pressure, you trim, unless you rely on speed trim. That accounts also for the unlikely event if you loose an engine.
Why would anybody in the world plan for a 2 engine take off, and set the trim for an engine failure case?
The different seat distribution, and actual versus calculated weights on a given station will most likely be the reason for the slight out-of-trim condition.

27th May 2015, 14:55
The trim setting will change in the FMS if you change either the derate or the TAss.

Correct for the Derate change requiring a stab trim change. One of the things to check when a new runway is given if the performance ends up changing the thrust rating. Easily missed.