Bleed engine start - what is this?

Question

Sometimes on my VHF scanner I hear pilots requesting permission from ATC for a 'bleed engine start'

What does this mean and why do they have to ask for special permission?

Answer 1

It's a cross bleed start.  That just means they are using one running engine to start one that isn't running.  They usually use the APU, which is a small jet engine in the tail.  The reason they need permission is that the running engine has to be run up to a high power setting to get the air needed to start the other engine.  This is a hazard for other aircraft, or ground vehicles, people, ect.  The bleed reference is where the air is tapped on the engine compressors to start the other engine.  Many jets use air-driven starters.  Hope this clears it up for you.

Answer 2

I'm guessing you're talking about a "cross bleed start."  This means that air from the compressor of one engine will be ducted to the other engine to get it to start spinning.  This takes quite a bit of air to do this, so the running engine needs to be spooled up considerably above idle speed.  Since this creates a lot of jet blast, pilots notify ATC to make sure the area behind them is kept clear.