the significance of the relight envelope
 

With regard to the relight envelope as it is depicted i.e in the airbus QRH can someone point out whether the upper height limit ( i.e on my company’s airbuses the upper limit is 27500 ) only relates to the likelihood of a successful engine start or to the likelihood of perhaps causing harm to the engine should you attempt a relight higher ?

The relight envelope is where it's been demonstrated a very high likelihood of a successful engine start, as determined by an extensive flight test program (assuming, of course, an otherwise healthy engine). Changes/improvements to the engine after EIS that might affect the restart envelope are few and far between simply because no one wants to do the flight testing needed to re-establish the restart envelope.
Assuming you protect normal engine start limits (principally EGT), the likelihood of engine damage during any in-flight start attempt is minimal.
In other words, if you want to try a restart during drift down to the restart envelope, there is little reason not to.

C.M

Methinks the lack of sufficient O2 above 27500ft would be a major factor in engine relight. Different altitudes for different engines/start parameters perhaps.

Rgds McHale.

As correctly said the envelope is where possibility of successful start is demonstrated. So if care is taken to keep parameters (egt) within limits it can be attempted even outside the envelope. But let us see why would a pilot attempt that. Engine doesn't fail without a reason and when FADEC detects a sub idle condition it attempts an auto relight for 30 seconds varying fuel flow etc. If the engine was good enough to start it should have relit. Since it hasn't there is no point wasting time to relight outside the envelope because there are other things to do, some decisions need to be taken. By the time you do that you would be closer to the envelope.

Guys I am all with you in what you said . Even though i was certain that the envelope is related to the likelihood of a successful relight , I went a step further in trying to establish whether the level above the upper limit is a prohibitive one ( in general in such cases the manufacturer tends to be direct by either having a note saying so ) .

Hi,

no, it is just a matter of certification based on the engine manufacturer's data.
More info here : Turbine Engine Relighting In Flight

Like others have said the envelope is defined by the liklihood of success. However, air speed, pressure altitude and time gets into this. Remember you are probably getting closer to the cows or even worse, mountain tops. So don't squander your air speed and altitude by stretching out unlikely attempts at restarts.

I have seen some analysis (at lower altitude) where the only chance was to push the nose over and dive. Of course all engine restart envelopes are not the same if only because there are other trades to consider.

BA 009 Jakarta incident springs to mind where the crew had no idea why all 4 engines had failed.
"Owing to the high Indonesian mountains on the south coast of the island of Java, an altitude of at least 11,500 feet (3,500 m) was required to cross the coast safely. The crew decided that if the aircraft was unable to maintain altitude by the time they reached 12,000 feet (3,700 m) they would turn back out to sea and attempt to ditch into the Indian Ocean. The crew began engine restart drills, despite being well outside the recommended maximum engine in-flight start envelope altitude of 28,000 feet (8,500 m). The restart attempts failed."

"At 13,500 feet (4,100 m), the crew was approaching the altitude at which they would have to turn over the ocean and attempt a risky ditching. Although there were guidelines for the water landing procedure, no one had ever tried it in a Boeing 747. As they performed the engine restart procedure, engine number four finally started,"

BA 009

During the 747-8 flight testing, we had a lot of issues with in-flight starting of the GEnx-2B engine. The GEnx engine simply didn't want to light off a lot of the time (although once it lit, it would start fine - it was getting the fire going that was a problem). Lots of tweaks to the FADEC light-off software eventually resolved the issues satisfactorily, but my buddy that was responsible for certifying the in-flight start envelope remained nervous that we'd get an in-flight non start for some reason and it would re-open that can of worms.
I was on a flight test shortly before we got type cert - they were doing some engine-out autothrottle testing and we'd been cruising around at 37,000 for about an hour with an engine shutdown. After they'd completed the test condition, the pilot asked me if he could go ahead and try a windmill restart of that engine. Not only were we several thousand feet above the restart envelope (30k IIRC), the engine was cold soaked which is pretty much a worst case for getting a light-off. I responded 'sure - worst that can happen is it won't start'. Well it started right up, first try - no problem.
Next day I was in the office and stopped by my buddies desk and told him about it - he was aghast that we'd even tried - what if it had failed?:eek: 'Who cares, we were well outside the start envelope - I figured it didn't hurt to try'. But he was paranoid that if the FAA heard we'd had any in-flight start failure the FAA would want a complete investigation and want to re-do some of the in-flight start testing...:=

I'm pretty sure the all engine out procedure on all the Boeing airplanes has the crew begin attempting relights immediately regardless of altitude.