747-400 Park Brake

[NSEU]

Just a (strange) thought.... Can the park brake be set using only one pedal?

It's not clear in the manuals if both (L/R) brake pedals need to be pushed to park the brake. They tell you to do this, but is this simply to ensure that both left AND right brakes are parked (and to stop the pedal moving away from you through rudder action).

The park brake lever locks both pedals (if both pedals are pushed) through a dual pawl mechanism and also activates the electric park brake valve (to trap hydraulic fluid in the system). Will the park brake mechanism allow only one of the pawls to latch?

I don't have a good schematic of the brake hydraulics. Perhaps this scenario is not possible due to fluid escaping somewhere.

Thanks for any insight.

Cheers.
NSEU

[Spanner Turner]

No, the brakes cannot be parked using one pedal. The act of pulling up the park brake lever pulls a cable which drives a mechanism that "catches" the pedals to hold them depressed. At the same time a switch at the pedals (adjacent Capt's left pedal) sends a signal to close the park brake valve in the R/H body gear wheel well. If only one pedal is pushed the mechanism cannot catch the pedals and they return to the normal position which also brakes the switch and the park brake valve re-opens. Next time you are in the cockpit just give it a go and you'll see.

From the manual.

The parking brake lever is pulled to set the brakes,
after the brakes have been applied. When the parking brake lever is
pulled, it operates a control linkage, causing two pawls to swing toward the brake pedal bellcrank. The pawls engage the
on the bellcranks and set the parking brake, irrespective of
the hydraulic pressure available.
The parking brake will not set unless both pedals are depressed fully,
from either the pilot’s or co-pilot’s side.
To release the parking brakes, fully depress both brake pedals, then
release them.

.

[NSEU]

The pawls engage the
on the bellcranks and set the parking brake, irrespective of
the hydraulic pressure available.

I'm trying to figure out why 1 pawl won't engage. Aren't the 2 pawls on a common shaft?

The parking brake will not set unless both pedals are depressed fully,
from either the pilot’s or co-pilot’s side.

Yes, I read this, too, but can't figure out why.

I assume you can push the Captain's left pedal and the F/O's right pedal to produce the same result because of the bus bars.

I guess I'll just have to try it for myself

Thanks,
Cheers.
NSEU

[Spanner Turner]

I assume you can push the Captain's left pedal and the F/O's right pedal to produce the same result because of the bus bars.

It would seem an odd way to set the parking brake, but YES, if the Capt depressed the left pedal and the F/O depressed the right pedal and then the hostie from the rear galley pulled the park brake handle, then Yes, the park brakes would be SET.

Aren't the 2 pawls on a common shaft?

They most certainly are. With the pedals in the NORM position the shaft cannot be rotated as the pawls hit against the pedals. For the pawls to be able to be rotated to the engage position, BOTH the pedals need to be depressed, which moves them out of the arc of the pawls. Once the pawls are fully rotated (lever held up) the pedals are then released onto the pawls which "catches" the pedals and holds them.

The pictures in the manual aren't great but it really is a very, very simple mechanical interlock system. Probably best explained by removing the left hand mid ceiling panel in 'A' zone and having a look with your Mach 1 eyeball if you can't visualise it.



.

[NSEU]

Thanks, Spanner Turner... Makes perfect sense.

I'm helping some guys out with a desktop PC simulator which will eventually have electronic links to full scale mockpits (of various degrees of reality), so all logic has to considered (Hence the strange question).

Unfortunately, I'm out on the line, so I don't get the chance to pull down those panels too often.... and my Mach 1 eyeballs are slightly subsonic these days

Cheers.
NSEU