ERJ APU procedures

[Greg Gemelli]

Hey David,

I can't speak to Eagle's APU procedure but I have some thoughts based on our procedures at SkyWest on the CRJ200 (which is a lot like the ERJ is terms of performance and design). This may provide some food for thought.

Not knowing the exact architecture of the ERJ's pressurization and anti-icing systems I can't say with certainty but these systems are fairly straight forward. Basically they all work the same. This said I have strong doubts that the suggestion for running the APU on the ERJ during operations below 10K has anything to do with pressurization. It's more about "Bleed Air" resource management during high load operations.

During normal operations I can't see why you would operate the APU once you have at least 1 engine running. The KEY words here are "normal operations". The engine (s) can supply adequate bleed air pressure to the PACKs (provide cabin cooling/heat and pressurization) so long as the load "need for Bleed Air" is met. But again this is during NORMAL OPERATIONS.

So what defines the need for the APU after the engines are running? In most cases it's the need for the Anti-Icing system during taxi or during the departure/arrival phases of flight. Again we need to consider the draw on the available "Bleed Air" in a given configuration. In many cases the aircraft has protections in place to prevent a "Bleed Air" demand that exceeds the aircrafts capability. A perfect example would be operating in icing conditions which would put a high bleed air demand on the engines if they were the only source. Bleed Air is compressed hot air that is taken off the engine from the compressor stage of the turbine (prior to being mixed with fuel). The APU can also provide Bleed Air (I will get to this in a minute). Anyway if you take this air from the engines it reduces the engines ability to produce thrust (less air in equals less air out). So on an aircraft like the CRJ200 that I fly, we can't provide enough bleed air to satisfy the needs for pressurization, Wing anti-ice and Cowl (engine) anti-ice with the engines being the ONLY bleed air source. Actually the aircraft won't let us configure it that way. The reason is it puts to high a demand on the engines and reduces their performance to a point where safety, due to a lack of available thrust could be an issue. So in this situation we use the APU to provide Bleed Air to supply the needs of the Anti-Ice and Pressurization systems. After we are airborne we will transfer the Bleed Source back to the engines and shut down the APU.

The above is just one example of a non-standard operation requiring the use of an APU during taxi or flight.

However in Normal Ops, we use ground power at the gate, start the APU 20min prior to pushback, shut down the APU after engine start up (we typically taxi single engine and do a cross-bleed start of the other engine approx 3min before expected takeoff).

Finally regarding the suggestion that use of the APU for pressurization below 10K increases engine life by the instructions doesn't make sense. Well it makes sense as YES it would reduce engine load (as I explained above) but at the cost of APU use (wear n tear, fuel) so from an economical sense it doesn't add up.

Like I said I don't know Eagle procedure and I don't fly the ERJ but with the many similarities on the systems between the ERJ and the CRJ200 I think what I'm suggesting here should lead you in the right direction regarding proper APU use.

Hope this helps

Greg

[Pete Murray]

Wow Greg,
Thanks for the nuts and bolts...basically if you encounter an approach (high winds, icing, precip/freezing) you would want to start the APU as an auxiliary bleed air source. But if it's sunny, light winds, scattered clouds temp 66...you could do without APU until taxi in to gate?