Tg 747-300 From Norway, joined Nov 1999, 1318 posts, RR: 0 Posted (11 years 3 weeks 5 days ago) and read 4720 times:
As I'm currently with the Royal Norwegian Air Force at a Norwegian AFB, I regularly am in contact with different equipment used on fighters while they are on the ground.
One such equipment is the GTC (Gas Turbine Compressor) which is a small jet engine used to provide air to fighters that don't have a internal APU. (So as you probably have understood, its a mobile APU).
since our airport also is a civil airport, 734,5,7,8 and MD80 are present every day together with prop aircrafts and GA aircrafts.
Sometimes we are called out to provide start air to the big jets. One weekend several jets, needed our support. Since the GTC "replace" the APU I'll guess that those a/c had inop. APU's.
My question actually regards the safety in operating aircrafts with inop. APU.
As long as the airport have the required ground facilities it would not be any problem starting the engines. But what if the aircrafts loose all engines while inflight. I know its not common, but it do have happened. (BA 747 that flew into volcano ash)
If only one of the engines stop, I assume that the other engine will provide the air needed for a restart, or is the velocity of the outside air enough to provide the required N speeds to ignite a engine?
If not, it will be troublesome to restart a plane with no running engines.
As I'm writing this the Air Ontario F28 disaster back in the early 1990s also comes to my mind. Off course I'm not an expert on that tragic accident, but after reading Air Disaster Vol.3 you got some ideas why the accident occurred.
It was actually several independent actions or lack of them in both the airline management and on that particular day.
Anyway the F28's APU was inop. and after landing at Dryden, an airport without the required facilities to provide air ) the a/c had to leave one engine running during the turn around.
That day it was snowing, but both Fokker and Air Ontario forbid de icing of aircraft with running engines.
I don't know why, but the captain did decide to take off, but ice had accumulated on the wings and the aircraft went down less than a minute after take off ,killing several of the passengers.
I'm not saying this accident wouldn't have occurred if the APU had been operative, but I guess it was one factor that lead to the accident.
So do it exist any rules about flying with inop. APUs ?
LMP737 From , joined Dec 1969, posts, RR:
Reply 1, posted (11 years 3 weeks 4 days 23 hours ago) and read 4678 times:
Having an inoperative APU is a deferrable item for an commercial airliner to have. In other words it's allowable to have it inop. Of course it will be a priority for an airlines maintenance department to fix it as sone as possible. There are exceptions however. If an twin engine aircraft is making an ETOPS flight the APU will have to be operational.
I guess one could say that having an inop APU might have been a contributing factor in the F28 accident. If the APU was operational they would have de-iced the airplane. However it seems to me that the main cause of that accident was more of a case of wanting to get somewhere so badly that safety rules were over looked.
To answer your question on engine relight, yes the velocity of the air going through the engine is sufficient for light off. Also the reason you don't want to de-ice with engines running is that you don't want the fluid getting sucked into the engine.
Slamclick From United States of America, joined Nov 2003, 10062 posts, RR: 67
Reply 2, posted (11 years 3 weeks 4 days 23 hours ago) and read 4685 times:
Here in the USA, the APU is really a layer of redundancy to our systems and is treated as such.
In the event of loss of all engines, many older planes (DC-9 for example) would not permit you to attempt to start the APU off the battery while gliding down. Big reason. The battery is part of the required "standby electrical power" which gives minimal flight instruments, one navigation and one communication radio, lighting adequate to use these things. Using it to attempt to start the APU would be a bad gamble and probably just draw down the amperage so that the standby electrics would not work the required 30 minutes.
We'd have to crash in the dark!
Some newer, bigger airplanes have a dedicated APU battery, along with ram air turbines for power and so forth.
So from our point of view, the lack of an APU would not cause an accident since our procedures were not based on its use in the first place. I'll agree I'd rather have APU power while gliding down, trying to get engines lit off, but our layers of redundancy normally don't go that deep.
Happiness is not seeing another trite Ste. Maarten photo all week long.
Avt007 From Canada, joined Jul 2000, 2132 posts, RR: 5
Reply 4, posted (11 years 3 weeks 4 days 22 hours ago) and read 4659 times:
I recently had a big argument with a guy regarding the safety of operating an F28 without an APU. Basically there is no safety of flight issue. As Slamclick said, trying to start it for electrical power could be a mistake (especially with the notorious F28 APU) and it cannot supply bleed air in flight. This is disabled through the squat switches. Airflow through the engines is sufficient for a relight.
411A From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 6, posted (11 years 3 weeks 4 days 16 hours ago) and read 4629 times:
APU's and jet aircraft, an interesting subject.
As L-188 pointed out aboue, early jet aircraft did not have APU's, and indeed did not need same as airports had ground air available for engine starts.
In the event that ground air was not available, special conditions needed to be met, specifically on the B707 (for example), high pressure (3000psi) air bottles were installed in the right wheel well to enable ONE engine to be started, and once that engine was running, sufficient cross bleed air from the operating engine (trubocompressor) was available to start the remaining engines...provided the Flight Engineer remembered to be SURE that the pack valves were CLOSED on the last engine shutdown the night before...if not SOL, except IF the ground engineer knew how to close them manually...and NOT many did.
All these procedures were noted in the AFM and if followed, success resulted.
With more modern aircraft, the APU can be used in flight, to provide both electrical power as well as pneumatics...and if you have pneumatics, thru ADP's, hydraulic power..
In addition, if the APU is allowed to be used in flight, the aircraft can be dispatched with one IDG inoperative, a useful alternative.
ETOPS aircraft have more restrictive procedures however..
VC-10 From United Kingdom, joined Oct 1999, 3717 posts, RR: 33
Reply 7, posted (11 years 3 weeks 4 days 13 hours ago) and read 4625 times:
on the B707 (for example), high pressure (3000psi) air bottles were installed in the right wheel well to enable ONE engine to be started
The VC10 had the same. The 3 bottles were in the tail cone immediately below the rudder and supplied combustor (just a small device to ignite kerosene) the exhaust gases of which drove #3 Eng starter motor.