Maximum allowable OAT
Thread Starter
Maximum allowable OAT
Sorry if this has been done before, but....
I've done searches trying to find the MAXIMUM allowable OAT for operating aircraft. Lots to do with minimum temps and fuel freezing etc, but not a lot about maximum. I've now flown several aircraft where the max allowable OAT is +49C. Having chatted to some airline mates I find they too have a published 49/50degC nominal limit!
I find it difficult to believe that aircraft just haven't been tested above this temperature, so any ideas what gives? Our best guess was that it was a fuel limit as well.
Thanks in advance - sorry if I don't reply immediately to responses but I will check back!
I've done searches trying to find the MAXIMUM allowable OAT for operating aircraft. Lots to do with minimum temps and fuel freezing etc, but not a lot about maximum. I've now flown several aircraft where the max allowable OAT is +49C. Having chatted to some airline mates I find they too have a published 49/50degC nominal limit!
I find it difficult to believe that aircraft just haven't been tested above this temperature, so any ideas what gives? Our best guess was that it was a fuel limit as well.
Thanks in advance - sorry if I don't reply immediately to responses but I will check back!
Depends upon the aircraft, in the GA world the most common reasons for maximum OAT values are either structural limits on primary composite structure, or risk of vapour locking in AVGAS/MOGAS fuel systems.
I believe that some older types have experimentally determined maximum OAT values due to engine cooling limitations, but I'm not sure that I could name one so could be wrong.
G
I believe that some older types have experimentally determined maximum OAT values due to engine cooling limitations, but I'm not sure that I could name one so could be wrong.
G
Thread Starter
Thanks - the aircraft I'm thinking of are all AVTUR (JET-A / JETA-1) powered turbo-prop or jet, from 40yrs old to 5 yrs old!
The reason for asking is that I now occasionally have to operate above the max specified allowable OAT. The aircraft seem to perform well, but it's always nice to know what limit it is that you're exceeding before disregarding it! Engine cooling isn't a factor I don't believe and the same 49/50degC limit is spread across different types. The only common thing we could come up with is fuel (which does define the minimum temp we can use). The Shell website quotes a flashpoint of 38degC for JetA and JetA1but not a 'boiling' point.
Perhaps I'm just looking for an answer that isn't there!
The reason for asking is that I now occasionally have to operate above the max specified allowable OAT. The aircraft seem to perform well, but it's always nice to know what limit it is that you're exceeding before disregarding it! Engine cooling isn't a factor I don't believe and the same 49/50degC limit is spread across different types. The only common thing we could come up with is fuel (which does define the minimum temp we can use). The Shell website quotes a flashpoint of 38degC for JetA and JetA1but not a 'boiling' point.
Perhaps I'm just looking for an answer that isn't there!
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For some aircraft (F70/100, F50) ISA plus 35C, ie 50C for take-off. However, not so long ago the F100 I was flying could not depart when the OAT was greater than 40C as our performance tables didn't go above this. The tables now carried go to 50C.
However, if you operate outside the maximum allowable temperatures, you are on you own mate. You have just endangered your passengers, those on the ground, invalidated your insurance and quite rightly, put your licence in jeopardy. Just because the damn thing flies it doesn't mean that it's safe. Don't do it - you'll give us (pilots) a bad name for no good reason!
However, if you operate outside the maximum allowable temperatures, you are on you own mate. You have just endangered your passengers, those on the ground, invalidated your insurance and quite rightly, put your licence in jeopardy. Just because the damn thing flies it doesn't mean that it's safe. Don't do it - you'll give us (pilots) a bad name for no good reason!
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Most early Boeing designs (B707 for example) had a maximim operating temperature for departure of +49C, however if one is prepared to pay the price, Boeing offers additional operating data for higher temps.
The Lockheed TriStar was provided to operators with operating data to +54C, right out of the box.
I suspect newer designs are similar.
The Lockheed TriStar was provided to operators with operating data to +54C, right out of the box.
I suspect newer designs are similar.
Thread Starter
Piltdown Man,
Hang fire tiger! I'm not operating outside the quoted temps just because I feel like it! It's auth'd and approved, it's just that I don't know what the limitation is that I'm being allowed to override, so I want to find out. As for passengers and those on the ground being in danger, we don't have passengers very often and those on the ground are the main cause for concern
Is the max temp limit just a payment thing? Pay more and the manufacturer will approve a greater C of A envelope if the aircraft can do it?
Hang fire tiger! I'm not operating outside the quoted temps just because I feel like it! It's auth'd and approved, it's just that I don't know what the limitation is that I'm being allowed to override, so I want to find out. As for passengers and those on the ground being in danger, we don't have passengers very often and those on the ground are the main cause for concern
Is the max temp limit just a payment thing? Pay more and the manufacturer will approve a greater C of A envelope if the aircraft can do it?
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The other day I operated in/out of Basrah, Iraq which was 49 degrees C when we tried to start up again. It was like a sim session. Hung starts, hot starts, eventually I got the left one going and turned into the wind which helped a bit and finally got the right one going. Aircraft don't like these extremes at all...
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I'd suggest that anybody who can authorise you to operate outside the aircraft limitations will have the all the answers, and more. Also, please, tell us all where you can get a "Get out of Jail Free Card". But, IMHO, the limitations are there because the manufacturer didn't pay for the certification of aircraft to operate at extremely high temperates, only because they thought nobody would operate in them. If no answers are forthcoming, try drawing out a few graphs, extrapolating outside (like what we are NOT meant to do) the performance data you actually do have and see if that gives you a few clues.
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You may be exceeding allowable/design temperatures in various avionics and equipment bays. While I doubt you'll find smoke billowing out of the boxes, you may well be frying some components, causing increased deterioration. That's eventually going to show up in higher maintenance costs AND higher than nominal failure rates.
And if those latter get too high you'll end up with uncertifiable rates of failure compared to the effects.
All those hot and hung starts described earlier probably didn't do much good to the engines concerned. Do it often enough and your IFSD rate is going to go up.
And if those latter get too high you'll end up with uncertifiable rates of failure compared to the effects.
All those hot and hung starts described earlier probably didn't do much good to the engines concerned. Do it often enough and your IFSD rate is going to go up.
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piltdown - there are some people out there that are crazy/lucky enough to not have to worry about licenses (sp?) and aircraft fatigue etc. you get told to go somewhere and so you just go. when you get there and find that it is 10deg hotter than is allowed you certainly dont sit around and wait for it to cool down.
propulike - isnt the temp limit different between the two types? if so then does that mean that it isnt a fuel temp problem? maybe the cooling issues on the older one plays more of a part?
propulike - isnt the temp limit different between the two types? if so then does that mean that it isnt a fuel temp problem? maybe the cooling issues on the older one plays more of a part?
Thread Starter
Juliet, As far as I remember form the old aircraft the temperature limit quoted was the same, only it was referenced via sea level ISA +35C instead of being quoted as an OAT.
Piltdown Man, I would also hope that anybody who can authorise me to operate outside the aircraft limitations will have the all the answers, and more - but I don't think they have and if they do they're not letting on. Hence more than an idle curiosity on my part.
Mad (Flt) Scientist, Yup, increased failure rates are expected, but this is due to a number of other factors as well such as sand/dust ingress. The temp limits of individual 'boxes' and displays range upwards from 70degC - which does require flight deck/cabin cooling before power up occasionally as it gets far hotter in than out! (Normaly just a case of opening doors/hatches/escapes etc)
The performance of the aircraft can be conservatively predicted by using more restrictive comparables - Kabul at 6000ft amsl in +49degC is 'legal' and has valid perf data, whereas Iraq at sea level and +54degC is 'illegal' as it breaks that temperature limit of +49C although I would expect better performance at the latter.
Which brings me back to - does anyone know why the +49/50degC limit? Perhaps it is just that no-one expected the aircraft to fly in temps above this, an assumption being proven wrong now!
Piltdown Man, I would also hope that anybody who can authorise me to operate outside the aircraft limitations will have the all the answers, and more - but I don't think they have and if they do they're not letting on. Hence more than an idle curiosity on my part.
Mad (Flt) Scientist, Yup, increased failure rates are expected, but this is due to a number of other factors as well such as sand/dust ingress. The temp limits of individual 'boxes' and displays range upwards from 70degC - which does require flight deck/cabin cooling before power up occasionally as it gets far hotter in than out! (Normaly just a case of opening doors/hatches/escapes etc)
The performance of the aircraft can be conservatively predicted by using more restrictive comparables - Kabul at 6000ft amsl in +49degC is 'legal' and has valid perf data, whereas Iraq at sea level and +54degC is 'illegal' as it breaks that temperature limit of +49C although I would expect better performance at the latter.
Which brings me back to - does anyone know why the +49/50degC limit? Perhaps it is just that no-one expected the aircraft to fly in temps above this, an assumption being proven wrong now!
The BAC 1-11s that we operated in the middle east had a max operating temperature of 50 deg.C and this was with water injection. This came from BAC. After this temp expect to go by road irrespective of runway length. I can remember several aborted take-offs because pilots tried to take-off outside temp limits instead of waiting for it to cool down in the late afternoon or early evening.
I know that these are old technology aircraft but I would assume that the same still holds true for some of the more modern technology aircraft.
I seem to remember that the Gulf Air Tristars were also limited to 50 deg. C. Also British Airways always flew out of Seeb at night during the summer months for the same reason.
Hope this helps.
CC
I know that these are old technology aircraft but I would assume that the same still holds true for some of the more modern technology aircraft.
I seem to remember that the Gulf Air Tristars were also limited to 50 deg. C. Also British Airways always flew out of Seeb at night during the summer months for the same reason.
Hope this helps.
CC
