can any aircraft geeks here provide interesting information on this situation that happened last night?

The captain comes on PA shortly after door closes last night, A319 SFO-EWR, and says that we need to wait a few minutes for an engine start unit to arrive, and apologizes for the lack of aircon. My question -- does this mean the APU is inop? And are they relying on the other engine to restart in case anything goes wrong in the air? And what exactly is the ground cart supplying to start the engine? Air? Hydraulic? I saw the cart outside my window, like a large generator truck -- it must be pretty powerful!

thanks!

Yes on all counts. APU is most likely inop. 2nd engine can be started from the 1st engine. The truck provides air (commonly refered to as an "air start".)
Big Diesel "fan" on wheels can be moved to different aircraft and runed on to procide the required compression to start the engine. Used to be very common when the last few years of the DC-10 as to buy/install a new APU for only a few months of use vs use an airstart must not have been a good trade off fog the APU sales people.

The air start unit is commonly used when the APU in inop. However, you can have a perfectly good running APU that is not supplying electrical which could require an air start as well. Think of it like a jump start for your car. There are two kinds of air starts: neumatic air (continuous airflow) and bottled air, where it looks like a big propane storage tank that is pressurized in advance. Either way, the process must be taken seriously as the crew is working with highly pressurized air hoses. They force air through the engine manually, while the enine sprays fuel into the air/fuel mix. Once ignited, that engine can supply enough power to start the other engine and power the entire a/c. While this airstart occurs, there is nothing supplying air conditioning to the a/c, so it needs to be done quickly in hot temps.

Here is the at-the-gate process:


AS unit is running at idle next to a/c, personel at controls and on headset in push tractor, communication through hand signals amongst ground crew
Door of a/c is closed
(bridge cannot be pulled yet because ground power from bridge is still connected)
All clear is given to flight crew that ground crew is "off" the a/c
Captain gives go ahead to give engine 2 (starboard) air...really loud sound from AS unit as ground crew runs up air pressure on AS unit
Air is applied to #2 and the flight crew starts that engine
Engine runs to power level
Captain gives okay to pull ground power
GPU pulled and bridge comes off
Captain gives okay to disconnect air
Ground crew releases pressure in AS unit and disconnects it from a/c
(this is critical since the connection point is directly under the a/c between the two engines)
Aircraft is pushed out from gate
Once tractor is disconnected, the second engine is started. This could happen earlier, but the #2 engine will need to run up to almost 70% power to "cross-bleed" enough air over to the #1 engine.



Hope this helps!

fellow airplane geek

Probably not in this case.

I don't know the specifics on the A319 series, but there are usually several options for engine restart or aux. power in the air that are not the APU. For one, the airflow through the engines will be enough to turn the turbine blades, though I'm not sure if that alone is enough for a restart.

Many aircraft also have a ram-air turbine, or RAT, that can be deployed in flight. This is essentially a small "windmill" that can be deployed and uses the airflow across the moving plane to generate enough power for emergency systems.

If it's not supplying electrical power, and you require an air start, what exactly is the APU doing??? It's either broken, supplying electrical but no pneumatics, supplying pneumatic but no electrical, or supplying electrical and pneumatic (a perfectly good APU)

Since this is not directly related to United or Mileage Plus, I'm going to send it over to TravelBuzz.

letiole
UA co-mod

I experienced this last week and the pilot's explanation to the pax included a statement to the effect that the broken device was not needed to restart an engine while airborne. ^

Restarts in flight are normally windmilling starts -- letting the air blow through the engine to spin it.

The only time the APU would be needed in flight would be a double engine failure, or at least a failure of all the electrical generators and their backups.

I was once on a UA flight from DEN to YVR (737, I think) and about 45 minutes out the pilot turned around and flew back to DEN.

He said the APU had thrown up a warning light and although it wasn't a safety issue he was required to return to DEN and have it fixed. Presumably they didn't want to fix it in Vancouver.

Luckily DEN's a major hub and they were able to come up with another aircraft to put us on.

Cheers,
Geoff Glave
Vancouver, Canada