What does 'in-trim speed' mean with respect to Boeing 737 mis-trimming correction?

Question

This question was prompted by discussions of the recent Boeing 737 MAX crashes, but it is not specifically about that variant.

In Boeing's 737 NG Flight Crew Training Manual, in Chapter 8 (Non-Normal Operations), under the heading 'Manual Stabilizer Trim' on page 8.16, it states:

Excessive airloads on the stabilizer may require effort by both pilots to correct the mis-trim. In extreme cases it may be necessary to aerodynamically relieve the airloads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually.

In this context, does 'in-trim speed' mean the speed it is mis-trimmed to, the speed it would be trimmed to when put back in trim, or something else? For example, if the airplane is mis-trimmed nose down, and therefore, presumably, for a higher speed than intended, what action would relieve the airloads on the stabilizer, and why?

Additional information about what 'extreme cases' might require the airloads to be relieved in this manner would also be appreciated.

Answer 1

"In-trim" speed means whatever speed the airplane will naturally seek if you let go of the controls. If you set the stab to position X, the airplane will naturally seek speed Y, for a given weight, thrust, flaps, and C of G condition.

So say the stab is at position X and this makes the airplane want to pitch to 240kt. It will seek and maintain that speed if you don't touch the controls. If you want to fly slower, you can add up elevator by pulling the column back, increasing the tail's downforce and pitching the nose up, until it slows to a speed where the forces are back in balance with the elevator held fixed at that position. Say it's 200kt.

You are now 40 kt below trim speed, and having to hold the elevator displaced to keep it there. If you want to be able to let it fly hands off at 200kt, you have to move the stab itself so that the tail's downforce with the elevator at neutral is the same as when the elevator was being displaced. So you trim out the elevator displacement (by applying trim and easing the column back to neutral in little trial and error inputs) and now you're in trim at 200kt, hands off.

The 737 has a stab screw jack with a manual operation feature with a cable circuit between the screw jack and the trim wheels in the cockpit that let the crew turn the jack directly with the trim wheel, with no electric input, when in manual trim mode.

If you are way out of trim, that is, holding a lot of elevator to make the airplane fly slower or faster than its trim speed, it puts a large rotational moment within the horizontal tail which increases the up or down load on the screw jack itself. These jacks are usually acme screws with dual load path threads that have a fair bit of internal friction under load, and the effort to drive the screw when it's highly loaded can be more than you can apply with your hand on the little wheel on the sides of the center console.

So say you're in manual trim and the airplane is trimmed to 280 kt and you want to slow to 200kt, and pull the nose up with elevator to slow down and you don't retrim as you're slowing down. At some point, if the out of trim condition and resulting the load on the screwjack is high enough, you can't move the trim wheel because you can't apply enough force to turn the jackscrew. To avoid this, you should be retrimming the stab as you slow down.

Their procedure is to try to offload some of the out of trim load on the stab jack by moving the elevators back toward the original trim speed. If you were holding the column way back to slow to 200kt while the airplane was still trimmed for 280kt, you'd release some of the pull, letting the airplane pitch over a bit and accelerate, while trying to move the trim wheel. At some point you'll have unloaded the stab screw jack enough to make the wheel move. You don't want to pitch over any more than you have to so you just relieve the elevator enough to get the wheel to move, and keep working it along until it works normally.

In this context, "extreme cases" refers to the case of a large difference between existing speed created by elevator input and existing trim speed.

Answer 2

In layman's terms it's saying the control forces might be heavier than normal when using only manual stabilizer trim.

The manual stab trim section of the Boeing book is explaining that if manual stabilizer trim is necessary (likely due to the electric stab trim having a failure), and the air loads are great, instead of ham-fisting the aircraft controls, it might be better to accelerate or decelerate to the trimmed speed, and then move the manual trim wheels.

Here's a hypothetical: If a 737 for whatever reason is trimmed for 250 knots, but for whatever reason is flying at 200 knots, the aircraft will try to regain its trimmed speed. This is absent any automation/autopilot. This is basic aerodynamics inherent to any stable design from a Cessna 172 to a Boeing 777. It is all due to positive static and positive dynamic stability. If the thrust does not increase, the only way to increase speed is to begin a dive and the aircraft will nose down & accelerate toward 250. (If allowed to accelerate, the aircraft would get real close to 250 and hold that speed closely, perhaps after a few oscillations).

If the pilots do not desire a descent, they might first react by pulling back to raise the nose and thus the speed will be lower than the in trim speed (which is also what the Boeing book is calling "mis-trim" speed). Due to the aerodynamic loads on the stab, it may be very difficult to get the stab to move toward nose-up with the manual trim wheel.

Avoiding this difficulty is why Boeing's guidance is to get closer to the in-trim speed (even though it is NOT the desired speed and may even result in a unwanted climb or descent) before attempting to trim manually.

The final paragraph on that page:

"Anticipate the trim changes required for the approach. Configure the airplane early in the approach. When reaching the landing configuration, maintain as constant a trim setting as possible. If a go-around is required, anticipate the trim changes as airspeed increases."

Boeing is saying in layman's terms, stay ahead of the airplane with speed/configuration changes, don't waterski 2 miles behind the airplane (or bad things might happen).