Elevator force when mis-trimmed?

[Will]

I was playing around at the extremes ends of the PSX envelope, and was intrigued to see that with max-aft center of gravity, and maximally mis-trimmed aircraft (nose up), a takeoff was noisy but still easily flyable.

But of course, I wasn't having to push as hard on the joystick as a real life crew would have to push on the elevator.

Lion Air got me thinking. Any idea how much yoke force is needed to control a 747 when maximally mistrimmed?

[John H Watson]

The maintenance manual offers no clues. It's a complicated mix of airspeed and trim setting.

As the stabilizer goes more to A/C nose up trim the elevator feel pressure is reduced. The faster the aircraft, the more the elevator feel pressure is increased.

The Maintenance Manual only has random values. e.g.

At 237kts, the force required to pull the column 5/6ths back (approx. 10 degrees) with the stab trim at around 5 units is 53~73lbs.  (The stabilizer is at neutral at 3 units).

At 204 kts, with 5 units of trim, with the column fully aft, the pressure is 72~106lbs. With the stab trim at full nose up (with the alternate switches), i.e. 14.75 units, the force is 40~55lbs.

[Jeroen Hoppenbrouwers]

Any values for column forward?

[skelsey]

I can't find any numbers anywhere, but anecdotally I have seen 737 pilots following Lion Air describe their experiences of runaway stab trim in the sim as involving both pilots pulling with both hands and often feet up on the MIP for additional leverage... so I would interpret that as implying that the forces involved are potentially very substantial.

CS25 contains the absolute limits for control force application in any aircraft, and there are various limits for different phases of flight.

CS25.255 deals with out of trim characteristics:

(a) From an initial condition with the aeroplane trimmed at cruise speeds up to VMO/MMO, the aeroplane must have satisfactory manoeuvring stability and controllability with the degree of out-of-trim in both the aeroplane nose-up and nose-down directions, which results from the greater of –

(1) A three-second movement of the longitudinal trim system at its normal rate for the particular flight condition with no aerodynamic load (or an equivalent degree of trim for aeroplanes that do not have a power-operated trim system), except as limited by stops in the trim system, including those required by CS25.655 (b) for
adjustable stabilisers; or

(2) The maximum mistrim that can be sustained by the autopilot while maintaining level flight in the high speed cruising condition.

<yada yada yada>

(f) In the out-of-trim condition specified in sub-paragraph (a) of this paragraph, it must be possible from an overspeed condition at VDF/MDF, to produce at least 1·5 g for recovery by applying not more than 556 N (125 lbf) of longitudinal control force using either the primary longitudinal control alone or the primary longitudinal control and the longitudinal trim system. If the longitudinal trim is used to assist in producing the required load factor, it must be shown at VDF/MDF that the longitudinal trim can be actuated in the aeroplane nose-up direction with the primary surface loaded to correspond to the least of the following aeroplane nose-up
control forces:

(1) The maximum control forces expected in service as specified in CS 25.301 and 25.397.
(2) The control force required to produce 1·5 g.
(3) The control force corresponding to buffeting or other phenomena of such intensity that it is a strong deterrent to further application of primary longitudinal control force.

In general terms the maximum allowable control forces are up to 75lb in pitch, 50lb in roll and 150lb (!) in yaw (these are short-term limits -- there are much lower limits for sustained application).

It's worth thinking about: 150lb is not vastly less than I weigh! Clearly it's not possible to put one's full weight on a rudder pedal or the control wheel itself; the best one can do is brace oneself against the seat back and push hard, or in the case of pulling, brace oneself against the MIP and pull hard.

Obviously the actual forces in any particular aircraft could be significantly less than these absolute limits, but still...

[Britjet]

I've done this in the sim to demo the effect of trim on beginner pilots.
With full nose-down trim applied it's difficult to hold with one hand.
A jammed stab approach usually benefits from the NHP holding some of the backforce by means of clasping hands behind the vertical pole of the column, while still allowing the HP some movement.
It's quite a force.

Peter.

[Jeroen Hoppenbrouwers]

The horror. Imagine the choice you make: pull as hard as you can with two, or one lets loose to try to wind the trim wheels back up, fighting the motors. With three seconds to go.

Brrrrrrrr.

[cagarini]

The horror. Imagine the choice you make: pull as hard as you can with two, or one lets loose to try to wind the trim wheels back up, fighting the motors. With three seconds to go.

Brrrrrrrr.

Piece of cake for you ;-)  I know !

[ahaka]

Maybe this is slightly off-topic, but:

Aerowinx PSX is the pinnacle of desktop/home built flight simulation. Basically Level-D software, the only one that is accessible not just by professionals, but enthusiasts as well.

But when are we going to see a USB column yoke that simulates the forces and feel of the real yoke on the 747? For years I've been waiting for something like this to come out, but we are still stuck with the spring loaded/gas damper ones.

Although perhaps the forces would be so strong that mounting this kind of yoke would be problematic unless you can bolt it to some concrete floor...

[Jeroen Hoppenbrouwers]

Yoke thread:
http://aerowinx.com/board/index.php?topic=5195

[Toga]

Maybe this is slightly off-topic, but:

Aerowinx PSX is the pinnacle of desktop/home built flight simulation. Basically Level-D software, the only one that is accessible not just by professionals, but enthusiasts as well.

But when are we going to see a USB column yoke that simulates the forces and feel of the real yoke on the 747? For years I've been waiting for something like this to come out, but we are still stuck with the spring loaded/gas damper ones.

Although perhaps the forces would be so strong that mounting this kind of yoke would be problematic unless you can bolt it to some concrete floor...

For the 3 minutes or less worth of flying on a typical 12 hour flight, you've got to ask, is it worth it?

[Hardy Heinlin]

For the many hours of manual flight training it's worth it.

Who flies 12 hours in normal automatic cruise mode in a training sim?

[Toga]

It's a good point and a credit to PSX in itself. I have no idea how you've managed to create such voodoo magic, but it's most pleasing.