[Boeing 777] Trim reference for speed

[Frusciante]

Good evening everyone!


I have flown the 777 (PMDG) for couple months. As I come from the 737, I still a bit skeptical about how the trimming should be performed. So far, I have read FCOM's, I have checked on YouTube videos about it, also I have checked across PPRuNe, but I have not been able to really understand the logic which is behind.





Statement: I understood that on 777, pushing the trim switch located on the yoke does not make the stab moving directly -- as it does on 737 --. Instead, it changes the trim reference speed, the speed which is targeted by the current configuration by the trim. For instance : I am at FL080, and I want to descent and keeping the speed, so I just reduce the throttle and push forward. Now I want to make the same but I want to reduce the speed then I push forward and trim downwards -- because I want the reference speed to be lower!-- at the same time (seems contradictory, isn't it ? On a C172 I would trim upwards to get the nose down...), so in this situation I don't know what to do...



If someone who knows the aircraft could explain me the correct way of using the trim reference speed, I would be very happy.



Thank you very much for your help
Have great flights ! :)

[highflyer40]

Checked you tube and pprune... Well there you have it.

[B-HKD]

Checked you tube and PPRuNe... Well there you have it.

Why don't you give him the answer then you muppet!

Unbelievable, why even bother to reply with such a useless answer to a set of perfectly valid questions?

Statement: I understood that on 777, pushing the trim switch located on the yoke does not make the stab moving directly -- as it does on 737 --. Instead, it changes the trim reference speed, the speed which is targeted by the current configuration by the trim. For instance : I am at FL080, and I want to descent and keeping the speed, so I just reduce the throttle and push forward. Now I want to make the same but I want to reduce the speed then I push forward and trim downwards -- because I want the reference speed to be lower!-- at the same time (seems contradictory, isn't it ? On a C172 I would trim upwards to get the nose down...), so in this situation I don't know what to do...

From the B777 FCOM

In the normal mode, primary pitch trim operates differently on the ground than it does in flight. On the ground, the stabilizer is directly positioned when the pilot uses the pitch trim switches. In flight, the pitch trim switches do not position the stabilizer directly, but make inputs to the PFCs to change the trim reference speed.

The trim reference speed is the speed at which the airplane would eventually stabilize if there were no control column inputs. Once the control column forces are trimmed to zero, the airplane maintains a constant speed with no column inputs. Thrust changes result in a relatively constant indicated airspeed climb or descent, with no trim inputs needed unless airspeed changes.

When pilot trim inputs are made, the PFCs automatically move the elevators to achieve the trim change, then move the stabilizer to streamline the elevator. Stabilizer motion may also automatically occur to streamline the stabilizer and elevator for thrust and configuration changes.

By pressing the trim switches you have the ability to increase/decrease the reference speed at which, "the airplane would eventually stabilize if there were no control column inputs". You are properly trimmed on the B777 when the aircraft is maintaining a constant speed without any control column inputs.

Example, you are flying at 250kts and you are having to push the control column slightly forward to fly level. In this case your trim reference speed is "too slow" hence a few clicks of down trim, will increase the trim reference speed until no further downward control column input is needed to maintain level flight at 250kts.

Set the desired pitch, then increase or decrease trim reference speed to the point where it matches the speed you are trying to fly at with a neutral column input.

[c100driver]

B-HKD

You are over thinking this.

Fly-by-wire aircraft are not B737's with a computer moving the control surfaces directly. Unfortunately with the Boeing control wheel it gives this impression.

The control wheel or the side stick in the airbus could be seen as "aircraft desired state requests" i.e. tell the aircraft what you want and the FBW will manipulate the surfaces iaw the designers algorithm to achieve this state.

The FCTM.The primary difference from a conventional airplane is in the pitch axis. A control column input commands a pitch maneuver instead of commanding a surface to move, so anything else that tries to change the pitch attitude or flight path will be countered by the flight control system.

The FCOM. When pilot trim inputs are made, the PFCs automatically move the elevators to achieve the trim change, then move the stabilizer to streamline the elevator.

In reality just fly it like an aeroplane trim out the forces as required.
You are not trimming the stabiliser but the the elevator and the stabiliser then moves to streamline the system.

[JammedStab]

For the 777, can you confirm what will happens during manual level flight at say 250 knots with hands off the control column(I assume as the FBW is maintaining altitude) when one activates forward trim on the control column while carefully ensuring that the control column position was not moved.

Do the engines spool up and the aircraft increases speed while maintaining level altitude.

[c100driver]

The unique 777 implementation of maneuver demand and speed stability in the pitch control laws means that: An established flight path remains unchanged unless the pilot changes it through a control column input, or if the airspeed changes and the speed stability function take effect.
Trimming is required only for airspeed changes and not for airplane configuration changes.

Your particular situation is not covered in the available literature and I have not tried it in the simulator or the aircraft. Just a guess but would surmise that you have changed the reference speed so the auto thrust will increase to achieve the new ref speed.

[singleseater]

You are indeed overthinking this. It flys like a normal aircraft, and the trim effectively works the same.
The auto throttle has nothing to do with this function.
If you are flying level, the flight control system is maintaining an attitude to maintain a speed, the trim ref. speed.
If you trim nose down, the trim ref sped is increased, the attitude is adjusted to maintain the new ref. speed and the aircraft will descend, maintaining an attitude to maintain the new trim ref speed.
If you change the attitude but do not trim, the ref speed is unchanged, so eventually, the flight control system will find and maintain an attitude to maintain the trim speed. In this case it will be the same attitude and speed as before the disturbance.
All this assumes constant pwr/ config.
If you just increase the pwr. The ref speed is unchanged and the aircraft will fly an attitude to maintain it, ie climb.

[Rick777]

Another nice feature is that you don't have to get it exactly in trim like other planes. Just get it close and hold it for a few seconds and it does the fine tuning. If you keep going back and forth a click OT two at a time it doesn't do any good.