runallnight
Posts: 2
Joined: Mon Mar 20, 2006 1:19 am

757 Altitude Limit With Downed Computer

Mon Mar 20, 2006 1:45 am

Hello, new member here... March 17 I rode NW #780, LAS-MSP on a 757.
We turned back to the gate while taxiing towards takeoff; the pilots said
that one of the computers had failed to boot up correctly. After several
minutes of repair work, they said the (new?) computer seemed OK but
that the APU would not start again and thus we would wait a bit more to
bring out the external starter equipment. It got a bit warm in the cabin
since the air conditioning was not going. Finally the A/C came on (APU?)
but the crew announced that the computer problem had returned.

Here is the crux of my question - at this point the crew said that we could
proceed despite the downed computer, but at reduced altitude. This cut
fuel efficiency, so they had the fueling folks come by to top us up. Once
in the air, the only altitude announcement was 27,000 feet. We had taken
off late by somewhere around 1:15 to 1:30, and arrived late by about 1:15,
so we seemed to have made good airspeed or had a good tailwind or did
well with air traffic.

Anyway, why would loss of one (out of multiple?) computer prior to takeoff
cause a restriction of altitude on a 757-200?
 
EMBQA
Posts: 7798
Joined: Sat Oct 25, 2003 3:52 am

RE: 757 Altitude Limit With Downed Computer

Mon Mar 20, 2006 2:39 am

Quoting Runallnight (Thread starter):
We had taken
off late by somewhere around 1:15 to 1:30, and arrived late by about 1:15,
so we seemed to have made good airspeed or had a good tailwind or did
well with air traffic.

We'll I'm not a pilot, but a friend that is said the winds aloft have been screaming lately. He had one east-west leg that was 1.0... his return trip west-east was .5 !!!!

Your problem sounds some what RVSM related, but how I don't know which computer was acting up.

[Edited 2006-03-19 18:41:19]
"It's not the size of the dog in the fight, but the size of the fight in the dog"
 
fr8mech
Posts: 6719
Joined: Mon Sep 26, 2005 9:00 am

RE: 757 Altitude Limit With Downed Computer

Mon Mar 20, 2006 3:02 am

Well, it all depends on the computer. The B757 has dozens of them. We call them black-boxes but they're computers.

Why would it cause an altitude restriction? The only computer that comes to mind, without looking at the MEL, is the Pac Controller. But it would require a total failure of the controller, rare.

I don't think a failed FMC (flight management computer) imposes a flight level restriction, but it may.

Hard to say what the problem was without knowing more specifics.
When seconds count...the police are minutes away.
 
Grbld
Posts: 349
Joined: Tue Dec 06, 2005 9:25 pm

RE: 757 Altitude Limit With Downed Computer

Mon Mar 20, 2006 7:13 pm

No, a failed FMC doesn't impose an altitude restriction. Heck, half our 737 fleet flies on one only. There probably was something going on with the pressurizatioin system.

The thing is, that you say that you were able to proceed, despite the inop computer, but at a reduced altitude. Reading your message literaly, it doesn't say that the inop computer CAUSED the flight at reduced altitude.

It could've well been that due to busy airspace, the flight plan had to be re-filed at a lower level in order to avoid further delays. Or it could've been another thing totally unrelated to the inop computer that caused the lower level to be filed.

Grbld
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: 757 Altitude Limit With Downed Computer

Tue Mar 21, 2006 4:48 am

Or it could be a flight crew taking the easy way out, saying something everyone is familiar with broke down rather than open up for endless questions about what a PACK is, fear among pax about flying with something broken in the system without which, surely, they would suffocate... "right?"... while everyone has seen computers break down with few ill effects on life in general.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Goldenshield
Posts: 5015
Joined: Sun Jan 14, 2001 3:45 pm

RE: 757 Altitude Limit With Downed Computer

Tue Mar 21, 2006 6:04 am

Most aircraft are certified up to FL250 with a pack deferred. FL270 would be the top of the lower portion of non-RVSM airspace. The upper portion would begin above 41,000 feet, but, in most cases, you would still need RVSM capability to get up there.

Part 91 appendix G lists RVSM equipment as:

-2 independantly operating altitude measurement systems (altimeters.)
-at least 1 automatic altitude control system (autopilot with altitude hold.)
-an altitude alerter system.
-TCAS (can be waived.)

Also needed is an RVSM qualified crew.
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