seratonin7

Several of my last few flights have required this. Why are they needed...what equipment is not working to warrant it? Why do they keep the jet bridge attached during an air start? Just curious.

coachrowsey

The APU is not working.

LarryJ

Because of the size of the engines, an electric starter motor would have to be huge in order to turn it fast enough for start so they use pnuematic starters. High pressure air from the APU is used to spin the pnuematic starters which spin the engines.

If the APU is unusable, or the APU bleed air is unusable, you use external air pressure for the start. An air cart is hooked up to the pnuematic system and provides the air for the starts.

The jet bridge is unrelated to the air start.

BoeingBoy

While technically unrelated to the air start, many jet bridges have safety features built in to prevent moving them until the external electrical power is unplugged from the aircraft and the cord retracted. This is to prevent damage to the aircraft that could result if the jetway were moved back with the cord still connected.

When an air start is required because the APU is inoperative (not just the bleed air), the external power cord is left hooked up while at least one engine is started and provide electrical power. Therefore many jet bridges won't move until after starting the first engine, switching electrical power over to the engine-driven generator, and disconnecting/retracting the external electrical power cord.

Jim

coswellnc

Jim, can you tell me what a "cross bleed start" is? I hear it on LiveATC.net all the time. Pilots tell ground that they need to do this. I assume it is beacuse they need some extra room or extra time in order to do this.

PHLGovFlyer

I'll chime in for Jim (assuming he doesn't mind...).

Cross bleed start is where bleed air from one engine that is already running is used to start the other engine(s). Depending on the aircraft type it is easier when the APU is not working to start a single engine with a ground air start unit (sometimes called a "huffer") and then cross bleed start the other engine(s). This allows you to only use the huffer once and let the aircraft's systems do the rest.

BoeingBoy

It's when you use high pressure air ("bleed air" since it's bled off from the compressor section) from one engine to start the other engine(s).

Both - an engine at idle doesn't produce enough pressure/quantity of air to start the other engine(s) so the thrust lever on the running engine is advanced to a power setting that will provide sufficient pressure/quantity. Thus aircraft behind you need to allow extra space or risk getting buffeted about (or worse if it happens to be a small airplane).

The added thrust needed to produce sufficient pressure/quantity air is also enough to cause the airplane to accelerate, which would require riding the brakes to keep taxi speed down. Better to do it while parked with the brakes set. So that's the two reasons for contacting ATC.

The actual time to cross bleed start an engine is about the same as with the APU, through often longer than with an external air source. A bit of trivia is that the newer high bypass engines require more power on the operating engine than the older low bypass engines to crossbleed start the other engine(s).

Jim

PS - I type too slow...

fishintheobx

Jim: can't an APU be listed as INOP if it's past it's max hours?

BoeingBoy

Generally it can be anytime something is wrong with it. Likewise, either the generator or bleed air can be inop. It's a matter of then complying with the Minimum Equipment List, which may have specific operating limitations if certain equipment is completely or partially inop. It's definitely an exaggeration, but at times it seems at though as long as the wings, engines, and wheels are all present you can operate the plane.

I suspect that the ETOPS flights are an exception in regards to the APU, however.

Jim

PropWasher

Another reason for cross-bleeding rather than air-start. Proximity of engine intake to ground support staff (eg A320).

BoeingBoy

Depending on the aircraft - where the plugin for the high pressure air is - it's all about ground personnel/equipment and foreign object damage to the engines. On the 737 the plugnin is on the bottom of the fuselage and pretty much on a line between the front of the engines, so the air hose has to pass in front of one engine or the other. Thus it's normal to only use ground air to start one engine.

Jim

PropWasher

This is a pet peeve of mine. The amount of time it takes for an arriving aircraft with u/s APU to shutdown engines because of bridge GPU.