Engine relight testing
 

I am trying to learn what happens during altitude relight trials. When is a trial considered successful or a failure ? What is normal behaviour for a commercial engine ? Is there an international specification which says that an engine has to relight by a particluar altitude for safe operation etc
Thanks:ok:

I don't have an answer, but I can broaden the question;

During type training on the Bell 206 the other day, we were reviewing the Flight Manual instruction that an in flight relight/restart not be attempted above 12000'. So I asked the obvious, can you attempt a start on the ground at that altitude? If the engine is stopped, it does not know if it is stopped on the ground, or in flight!

The instructor did not have an answer, perhaps one will emerge here....

Pilot DAR

Thanks - that certainly has broadened the question as I hadn't thought of helicopters! I expect it depends on the temperature as well.

The reason for the question was that aviation fuel gets more viscosus the colder it gets, and so at low altitude it sprays nicely out of the injector, but as you go higher and it gets colder, the spray pattern slowly reduces until in the extreme condition it is just a jet rather than a fine mist. Then it is very hard to ignite.

I just wondered whether there is a measurement of how many times to try relighting, or how high or something ?

If it helps, the Airbus and Boeing flt manuals show a relight envelope; max alt's, speeds for windmill start and/or starter assisted.

thanks - that sounds like the sort of information which might help me. Are these manuals freely available on the internet ?

or maybe if not would you mind writing down some of the parameters for one of the plains you mention ?

What is your maximum elevation approved for take-off? I would guess it's below 12,000' MSL as are "most" aircraft.

I suspect the key factors involved in re-light envelopes are:
-Igniter type and power rating (joules)
-Effective fuel air ratio at the point of ignition

DBW

Another couple of important considerations are:

Fuel delivery pressure and fuel vapor pressure - usually it's the latter that dictates when the engine is going to cough and give up on you and conversely when it's likely to start again.

We did some interesting work a couple of months back to try and get some MMEL relief for dispatch with a boost pump inoperative. The FVP got a bit too low and the engine surged and died on us several thousand feet below the predicted altitude.:eek:

Each engine/airframe manufacturer will specify a different procedure and limitations. In our operation, we demonstrate crossbleed and windmill airstarts at the outer limit of the envelope in the AFM for certification purposes.

There is no requirement for a specific relight altitude for Part 27 helicopters anyway. There is only a requirement that a relight altitude be defined. Could be sea level and you'd meet the letter of the law.

I had this exact situation when preparing an OEM engine manual - and had quite a time convincing a ground-bound design engineer that his numbers were illogical! But I refused to issue a chart with contradictory airstart vs ground start altitudes.

I've always felt it important for any user finding such contradictions to squawk them to the OEM, and require a logical answer. :cool:

It may get more viscous, but not to the extent that you are going to have difficulties starting the engine. The engine driven fuel pumps on the types I am familiar with ( C20's, PT6's and T58's) all supply more than enough pressure to get a good spray pattern out of the nozzles, even at very low temps. I have operated C20 powered helicopters at temps of -40 and had good starts, as long as the battery was up to snuff....

Thread so far seems to be focussing on altitude to actually achieve engine start. This is obviously important, however with helicopters (singles anyway) could I suggest you have a far greater concern in how long does it take to start. First thing you want to know is how much height am I going to lose during the start, so is it worth trying or just concentrating on walking away from the landing.

Hot Relights at High Altitude.

Just happened to try to determine the hot relight ceiling for a Vulcan newly fitted with Bristol Olympus 104 engines circa 1958. Here is an extract from memoirs.

"The performance of the new Mk104 Bristol Olympus engines was remarkable. Acceleration characteristics at altitudes up to and above 50,000 ft were better than any engine in the world at that time. I even found that I could close down an engine, perform a hot relight out to ten seconds after close down and achieve a rapid acceleration to high power from an RPM much lower than normal idle at those altitudes. My report stated that those engines were a credit to the manufacturer."

In those days there was no continuos ignition as installed in some engines now and the concept of relighting an engine whilst still hot was emerging. I do not recall the design of the ignition system and presume that there were a few high energy igniters carefully positioned in the combustion chambers.

My tests for reliabilty of hot relights were limited to within 10 seconds of flame out from closed power lever/throttle and did not extend to a determination of the threshholds where a relight became uncertain.

Were hot relights well known in Vulcan Squadrons?

engine relight boundries reason
 

the only reason is lack of oxygen at higher levels leading to difficult combustion

Probably lots to do with the cleared envelope from certification flight test would be my guess.