B 747-400 hydraulic system 4

[ZAGORFLY]

Just to keep all of you awake and worried:
Can somebody explain me why if y loose the no4 Hyd you loose the TCAS too?
Then if y still are worried if I m a terrorist :
Can y tell me why this plane goes in ground mode when you loose Hyd 4?
I will sleep better BOAC thanks.

[TopBunk]

Can somebody explain me why if y loose the no4 Hyd you loose the TCAS too?.
...
Can y tell me why this plane goes in ground mode when you loose Hyd 4?

You don't, afaik.

My (admittedly 5 years since an update) QRH says with loss of sys 4:

It doesn't lose TCAS, afaik (QRH I have is 5 years old). My copy says:

Loss of Hyd Sys 4:

Cat 2/3 autoland not approved

Inop items:

right outboard elevator
outboard trailing edge flap hydraulic operation
two inboard spoilerpanels on each wing
wing gear hydraulic extension and retraction
Sys 4 primary brake source
autobrakes


Neither does it go into ground mode when you lose Hyd Sys 4 - why would anyone design it to do that?

[ZAGORFLY]

it is just a curiosity, please don't be alarmed...

There have been two reports by operator flight crews that after depressurizing
hydraulic system 4 in flight, the airplane entered a “ground mode” of operation. If
system 4 is depressurized, hydraulic pressure is no longer available to the wing
gear truck tilt actuators. With loss of system 4 pressure, over time, both wing gear
trucks may relax and move from the “tilted” position satisfying proximity sensor
ground mode logic. There are few obvious flight deck indications which allow the
crew to quickly or easily determine the airplane has entered the ground mode.
However, several airplane systems operate differently in the “ground mode” than
in the air mode”.

If the airplane enters the ground mode in flight, the following systems conditions
will exist:
- Stabilizer fuel will not transfer
- Fuel in reserve tanks 2 & 3 will not transfer
- Wing anti-ice is inoperative
- Reverse thrust is available in flight if reverse thrust levers are raised
- Approach idle minimum thrust setting is inoperative
- Autothrottle is inoperative (hydraulic system 4 loss only)
- Cabin Pressurization system
Below 15,000 feet outflow valves will gradually open to depressurize
airplane. No effect above 15,000 feet.
- Transponder is disabled.
- TCAS is operative in TA ONLY mode
- Minimum maneuvering speed indication is inoperative
- Upper deck doors may be unlocked in flight
- Yaw dampers are inoperative
- Speedbrake lever flight detent automatic stop is inoperative

[spannersatcx]

you'd need at least one wing to sense being on the ground before the a/c logic would think it's on the ground.

[NSEU]

For some time now, the air/ground logic in the Proximity Switch Electronics Unit (PSEU) has been modified to factor in low hydraulic pressure. Loss of 1 or 4 hydraulics should NOT put the aircraft into ground mode.

If either of these hydraulics fail, the aircraft uses nose gear compression to determine air/ground.

Sorry, I'm not at liberty to post the schematics for this.

Can you recall the circumstances under which the crew reported this? Sim sessions? Recent real life incidents?

Sounds like something hasn't been updated (software/hardware/humanware) :}

P.S. Please note that there are some systems which do not use the main gear sensors anyway for air/ground.

P.P.S. verb: to lose

[torghabe]

There are one tilt sensor on the each main LG truck, all of them has been combined in PRIM loop. And there are additional four tilt sensors (one on the each main LG truck too), which combined in ALT loop. If all of them (8) says: trucks not tilted (that means A/C on the GRND), you can use A/C different systems in GRND mode. Some systems use info from nose gear pressure sw sure, but HYD sys #4 only failure do not affect for TCAS, WAI, revers thrust...
TopBunk: you are absolutely right, nothing changes.

[NSEU]

If all of them (8) says: trucks not tilted (that means A/C on the GRND),

Agreement between PRI and ALTN air/ground is not always necessary to put some systems into ground mode or air mode. Some systems may only use the PRI system. Some systems may only use the ALTN (e.g. Speedbrake lever FLT lock). Some systems do use both. However, disagreement between PRI and ALTN will generate a disagreement message.

you'd need at least one wing to sense being on the ground before the a/c logic would think it's on the ground.

It depends on the system. Some systems need un-tilt of at least one bogey on both sides of the aircraft to put the aircraft on the ground.

The PSEU receives an input from the Hydim cards (regarding hydraulic pressure): Reference Wiring Schematic 32-61-04.