777 APU & flight control questions

[Check Airman]

Hi all,

A couple of 777 questions for those with the knowledge.

1. When selecting an APU to Pack takeoff in the fms, why is only one pack powered? If it can supply both at the gate, what’s the logic for only one during takeoff, and would a crew manually configure the system for the APU to supply both packs on a hot day?

2. When doing the flight control check, it why do you not select the flight control synoptic page? How do you know the surfaces are moving as required?

[FullWings]

1. Don’t have that option in any of our aircraft, so can’t comment on the FMS. We can do packs on or packs off, APU not involved. In the FCOM it says to use both packs on the ground if available, as the fuel burn is the same as using one. “AUTO” is a good setting for many of the controls.

2. The answer to this one that I got from a Boeing engineer was that the system automatically checks that the control surfaces are moving correctly and relative to inputs on the controls. If there is a problem, you will get a Caution/Status. Much in the same way that if any other monitored aircraft parameter goes out of normal operating range, you will get some kind of notification.

[8che]

Hi all,

A couple of 777 questions for those with the knowledge.

1. When selecting an APU to Pack takeoff in the fms, why is only one pack powered? If it can supply both at the gate, what’s the logic for only one during takeoff, and would a crew manually configure the system for the APU to supply both packs on a hot day?

2. When doing the flight control check, it why do you not select the flight control synoptic page? How do you know the surfaces are moving as required?

1. Because the hydraulic C1 and C2 air driven demand pumps need air to work. The APU cannot supply 2 packs and hydraulics for take off. If 2 packs are on with APU that would conflict with the right
bleed air. The left and right air systems have to be isolated for the APU to give the bleed air relief and hence the take off performance improvement.

2. the DSP plays no part in any non normal procedures for B777 and is stated as such in the FCTM. Only an EICAS message indicates a problem. For the flight control check you are looking for the EICAS FLIGHT CONTROL message to generate with the full and free movement. Missing indications on the DSP are no indication of a failure and as such should not be used for diagnosis. Again it’s all in the FCTM.

[Check Airman]

1. Don’t have that option in any of our aircraft, so can’t comment on the FMS. We can do packs on or packs off, APU not involved. In the FCOM it says to use both packs on the ground if available, as the fuel burn is the same as using one. “AUTO” is a good setting for many of the controls.

2. The answer to this one that I got from a Boeing engineer was that the system automatically checks that the control surfaces are moving correctly and relative to inputs on the controls. If there is a problem, you will get a Caution/Status. Much in the same way that if any other monitored aircraft parameter goes out of normal operating range, you will get some kind of notification.

Thanks. I find that a little strange, particularly in light of the Airbus incident a few years ago where the sidestick wires were crossed, and so the roll control was reversed.

I imagine Boeing has found a way around that, as it’s worked for the last almost 30 years though. Appreciate it.

[Check Airman]

1. Because the hydraulic C1 and C2 air driven demand pumps need air to work. The APU cannot supply 2 packs and hydraulics for take off. If 2 packs are on with APU that would conflict with the right
bleed air. The left and right air systems have to be isolated for the APU to give the bleed air relief and hence the take off performance improvement.

2. the DSP plays no part in any non normal procedures for B777 and is stated as such in the FCTM. Only an EICAS message indicates a problem. For the flight control check you are looking for the EICAS FLIGHT CONTROL message to generate with the full and free movement. Missing indications on the DSP are no indication of a failure and as such should not be used for diagnosis. Again it’s all in the FCTM.

Many thanks. That makes sense now. My systems knowledge isn’t quite there yet. Appreciate you sharing yours.

[FullWings]

Thanks. I find that a little strange, particularly in light of the Airbus incident a few years ago where the sidestick wires were crossed, and so the roll control was reversed.

I imagine Boeing has found a way around that, as it’s worked for the last almost 30 years though. Appreciate it.
I think the only reasonably foolproof way to check if the surfaces are working would be have somebody look at them while you were exercising the controls. You are relying on sensors otherwise. The “carry away” I got from the discussion I had was that it was important to exercise the controls through their full range of motion (to the stops) to pick up anything untoward; the comparison logic is active as soon as things are powered up, so “control checks” are ongoing throughout the whole flight.

[Check Airman]

I think the only reasonably foolproof way to check if the surfaces are working would be have somebody look at them while you were exercising the controls. You are relying on sensors otherwise. The “carry away” I got from the discussion I had was that it was important to exercise the controls through their full range of motion (to the stops) to pick up anything untoward; the comparison logic is active as soon as things are powered up, so “control checks” are ongoing throughout the whole flight.

(putting on my un-earned engineering hat)

I imagine the systems display would get information from the surface position sensor.

I’m imagining a failure mode where you turn the wheel left, but computers get a right turn signal, and move the surface for a right turn. I imagine it would pass the internal check in that case.

I suppose Boeing has controlled for that eventuality.

(pilot hat back on)

Thanks for the info. Good to know why we’re doing what we’re doing.

[Uplinker]

It does seem risky getting electronics to monitor electronics - either of which could be cross-wired by a tired or overworked engineer. I would prefer to see the control surface movement and make my own decision as to whether it is what I or PF was demanding.

But I take the point that the flight control display does rely on sensors too, and obviously the '777 system works.