747 hydraulic failure

[tcasblue]

According to the QRH, if both Systems 1 and 4 are Inoperative, the speedbrake lever does not extend past the flight detent until the nose gear is on the runway and thrust reversers are only available after the nose gear is on the runway. But there is nothing in the systems description that I saw that explains why.

The Air/Ground Sensing System is based on main gear tilt while the nosegear extension sensing provides a signal to relays controlling functions in the stall warning and nose gear steering systems.

Any explanation.

[dixi188]

Without access to my 747-200 notes, I think it's because without nos 1 & 4 system the outboard spoilers won't deploy and if the inboard ones came fully up and you selected reverse you would have quite a large pitch down moment, so it waits for the nose gear to be lowered.

[tcasblue]

Originally Posted by dixi188

Without access to my 747-200 notes, I think it's because without nos 1 & 4 system the outboard spoilers won't deploy and if the inboard ones came fully up and you selected reverse you would have quite a large pitch down moment, so it waits for the nose gear to be lowered.

Thanks,

The QRH makes it sound like it is not possible to get reverse thrust even if you try. 744 might be different than the 742 though.

[Capt Quentin McHale]

tcasblue,

According to my B744 notes, Hyd Sys 1 physically has nothing to do with the spoiler/speedbrake system whatsoever. Only Hyd Sys 2,3 and 4 power the spoilers. However, by design (thank you Mr Boeing) the PSEU must see a signal from both Hyd Sys 1 and 4 plus aircraft on ground and thrust levers at idle in order for the auto speed brakes to go to full deployment.

If the PSEU does not see a Hyd signal from Sys 1 and/or 4, or thrust levers not at idle, or landing gear tilted (body truck not down and locked) then the auto speed brake actuator will retract and the speed brake lever will remain in the Armed position until it is physically moved to the full UP position and all spoilers go up to 45 degrees.

What the nose gear has to do with anything I have no idea and no mention in my notes. Methinks the QRH has thrown up a curve ball without clear explanation.

Rgds McHale.

[Icelanta]

Controlability issues basically.

[CCA]

There's is s combination of things going on with a 1 & 4 failure.

Firstly though a single system 4 failure requires the speed brake to NOT be armed as without the inboard spoilers (5,6,7 & 8) the outboards will cause a pitch up upon touch down / activating.

Now a double failure 1 & 4 leaves a risk of the aircraft going into ground mode in flight as there's no hydraulics keeping the gear tilt in check.

This is where the nose gear gets involved, if it's compressed then it must be on the ground (there's no tilt to worry about) The speed brake is limited to flight mode because of the possible faulty main gear tilt indications, so that's why it won't go past half way on touch down but is free to go the rest of the way upon nose gear compression.

Reverses are linked to the nose as well to ensure they aren't unlocked because the main gear tilt attempts to say the aircraft is in the ground.

So to put it simply without 1 & 4 gear tilt is no longer used for safety critical ground sensing some other things may still go into ground mode in flight like flight idle may go to ground idle.

[tcasblue]

Originally Posted by CCA

Reverses are linked to the nose as well to ensure they aren't unlocked because the main gear tilt attempts to say the aircraft is in the ground.

Thanks,

There is no mention in the FCOM about reverses linked to nosegear. That is why I was wondering about this.

[CCA]

Yea the FCOM is fairly sparse these days, there's quite a lot missing or simply just not a in depth explanation. The maintenance manual description operation is where the full story lies.

The nose gear squat switch is used as part of the air ground sensing it just doesn't normally get used for items that use the main gear for air ground sensing.