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Temperature inversion

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Temperature inversion

Old 19th Nov 2006, 14:30
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Temperature inversion

Hello folks,

according to my company SOP's takeoff with reduced thrust is not permitted with temperature inversion. However I could not find a value for that in the books. I would obviously avoid a reduced take off thrust with a severe temperature inversion I guess... opinions ?
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Old 19th Nov 2006, 17:51
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Sounds a bit limiting and could help if the chief pilot clarified the rule.
Used to fly for an outfit which required the FE (if time permitted) to note the SAT at, I think, 1500', 1000' and 500' on the approach for the outbound crew.
We'd then apply a correction to the surface temp for the derate.
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Old 20th Nov 2006, 00:22
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I think that this one is a bit of over-kill on behalf of your company's SOPs I-2021.

For one thing, how can you be sure with any certainty that an inversion exists, and if so, how to quantify it? A reduced thrust takeoff with an existing inversion is the one time that I would not be so concerned, a takeoff at a weight requiring full Rated thrust would be much more a concern in inversion conditions.

Consider a takeoff on a nice temperate day, say 20 degrees, with an Assumed / Flex Temperature of 40 degrees. The performance of your aircraft is guaranteed right up to an actual temperature of 40 degrees, so, if you do encounter 'the mother of all inversions' you're still safe. (A 20 degree temperature rise from Takeoff to 1500 feet AGL is indeed the mother of all inversions). As a further safety margin, in the example scenario given, asuming that, more realistically, the temperature only rose 5 to 6 degrees, you can still increase to Rated Thrust to compensate for any performance loss.

Consider now a takeoff at a weight requiring Full Rated Thrust. If an inversion is encountered, you have no reserve of performance. This is still not too alarming, as in the worst case performance scenario (Engine failure at Vef/V1), there is the assumed performance degradation between Net and Gross performance which is provided for variations in pilot technique, and environmental conditions variation from that assumed (amongst other things). For the lowly 2 engined aircraft this degradation is 0.8% from 2.4%, a full 33.3% reserve of performance, still no need for great concern.

What a good SOP might include is a statement to the effect that if the Actual Takeoff Weight is at or close to the limit for the De-Rated thrust (beware the difference between De-Rated and Reduced Thrust), then the Full Thrust rating, with appropriate Reduced Thrust (Assumed / Flex), should be used. As an example, if the temperature is 40 degrees, and your weight is at or close to the TO-1 De-Rate limit, then Full TO with reduction (e.g. Full TO and Assumed / Flex 50 degrees) should be used.

Perhaps you could tell your Chief Pilot that Reduced Thrust Takeoffs are just fine, but he'd better consider banning Takeoffs at limiting weight for the Full Thrust Rating in inversion conditions.

As a Post-Script, there MAY be some degree of 'Inversion Protection' built in to your RTOWs, depending upon your performance provider's policy. Although the regulations don't require it, I programme isothermal atmospheric conditions up to 1500 feet (or MAA if higher) into the performance work that I do. It's not to be lazy, but to err slightly on the side of caution. Any other P/E comments? John? Mutt?

Safe? - Yes, Conservative? - Yes, but I still think that it's over-kill.

Regards,

Old Smokey
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Old 20th Nov 2006, 03:52
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Reduced/flex thrust with an inversion?
Not a problem, as Old Smokey indicates.

However, a WAT limited takeoff with full thrust might get your attention darn quick when that inversion layer is entered, as it many times is at say (for example) AUH...and your rate of climb drops off drastically.
I have personally seen no more than 300 ft/min at V2+10, with all engines operating at full thrust.
Good thing no obstacles were present in the takeoff flight path.
Could be interesting otherwise.
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Old 20th Nov 2006, 07:57
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Ah yes, OS, woolly wording on my part.
411A - Just the area which sprang to mind when I read the Q. It's a physics lab out there
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Old 20th Nov 2006, 11:35
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[QUOTE=Old Smokey;2975534]I think that this one is a bit of over-kill on behalf of your company's SOPs I-2021.
What a good SOP might include is a statement to the effect that if the Actual Takeoff Weight is at or close to the limit for the De-Rated thrust (beware the difference between De-Rated and Reduced Thrust), then the Full Thrust rating, with appropriate Reduced Thrust (Assumed / Flex), should be used. As an example, if the temperature is 40 degrees, and your weight is at or close to the TO-1 De-Rate limit, then Full TO with reduction (e.g. Full TO and Assumed / Flex 50 degrees) should be used.

Please can you explain why is it better to T/O with full TO assumed 50, then with TO-1 with no assumed temp reduction? How about normal conditions (no temp inversion expected)?
Thnx
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Old 20th Nov 2006, 15:14
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Many thanks for your replies! I will try to look further on that !
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Old 20th Nov 2006, 15:17
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[QUOTE=littlejet;2976192]
Originally Posted by Old Smokey
I think that this one is a bit of over-kill on behalf of your company's SOPs I-2021.
What a good SOP might include is a statement to the effect that if the Actual Takeoff Weight is at or close to the limit for the De-Rated thrust (beware the difference between De-Rated and Reduced Thrust), then the Full Thrust rating, with appropriate Reduced Thrust (Assumed / Flex), should be used. As an example, if the temperature is 40 degrees, and your weight is at or close to the TO-1 De-Rate limit, then Full TO with reduction (e.g. Full TO and Assumed / Flex 50 degrees) should be used.
Please can you explain why is it better to T/O with full TO assumed 50, then with TO-1 with no assumed temp reduction? How about normal conditions (no temp inversion expected)?
Thnx
If the N1 value that you get by reducing to TO-1 full thrust is higher than the one you get by using TO with ATM, then I think you choose the second one to have a lower N1.
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Old 20th Nov 2006, 16:41
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I don’t think that this was a figment of your CP’s imagination as I had the same limitation at a previous airline. The wording was from memory that assumed temp is prohibited with a “Marked temperature inversion” ie a large inversion. I always thought that the reason was because of the windshear as you fly through the inversion layer, so this is just an extension of the “prohibited with windshear” limitation.

Like many other things in company manuals, this limitation is probably something which was present in an earlier version of the FCOMs, (like TR voltage range or max generator rise, etc). It was probably removed by Boeing either because it was found to be no longer necessary or was downgraded to a caution or info rather than a limitation and your airline has left it in.

If a limitation is there in print in your company manuals it is a bold (or bored!) manager that takes it out without checking the references, it might have been left in because it was too difficult or time consuming to investigate or maybe just had not been noticed because everybody there assumed it was still valid. I apologise if I am doing your airline a disservice but this is often how things work in practice. I bet this is still in many other 737 ops manuals around the world.

S&L
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Old 21st Nov 2006, 09:08
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Thanks a lot CaptainSandl !
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Old 22nd Nov 2006, 03:35
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Old Smokey,

We use manufacturers data that is based on the ISA, so it’s a standard lapse rate. Nothing additional is built in for Temperature Inversions.

Our policy is to account for ALL of the reported the inversion starting with fixed derate plus assumed, increasing to maximum thrust and different flap setting if required. Failing that, remove payload.

Nighttime inversions of 20°C are common during the summer months. We started accounting for the complete inversion following a crew report that they had a -50ft DESCENT rate after takeoff with all 4 engines running

Mutt
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Old 24th Nov 2006, 13:40
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HiMutt,

Thanks for the good reply, you have me thinking in a few areas that may indicate the need for improvement in our operations.

I'm interested in the application of the known inversion conditions to RTOW. My thoughts are that it would be (1) Impossible to affect Runway performance (Stop and Go) as the inversion is somewhat above the runway, and (2) Rare that the inversion would reach down to the average MAA (if there is such a thing as average MAA), thus rarely would Obstacle Climb be affected. That only leaves the WAT limit and/or 4th segment performance.

So, if my second guessing is correct, the Inversion environmental criteria would be applied to WAT and/or 4th Segment performance, seems reasonable to me, would (as always), appreciate your comments.

It seems that I'm a little lonely in applying isothermal atmosphere through to Final Takeoff Altitude (no others seem to), I'm comfortable with this degree of in-built inversion protection for normal operations, but, as you've indicated the 20 degree 'mother of all inversions' does really exist.

Littlejet, you asked -"Please can you explain why is it better to T/O with full TO assumed 50, then with TO-1 with no assumed temp reduction? How about normal conditions (no temp inversion expected)?"

The point that I was making relates to safety optimisation, call it 'Informed Good Airmanship' if you like. If an aircraft is right at it's operating limit, then so be it, you've little choice but to accept the regulatory safety margins (which are adequate) but with no extra. If the aircraft in the example was right at, or close to, Maximum Weight for TO-1, then the better choice is to go AT THE SAME THRUST, but at the Full TO rating with an assumed temperature thrust reduction. For the example, TO with Assumed/Flex 50 degrees provided the same thrust as Full TO-1. Performance would be identical, but by using TO with a thrust reduction, a significant reserve is available if needed. V1, Vr, and V2 speeds are related to Vmcg and Vmca, and, typically, the TO-1 speeds will be lower. In the worst case, thrust cannot be increased from TO-1 to Full TO without compromising directional control, thus, the reserve is not available. If the same Takeoff was planned using TO with a thrust reduction (to equal TO-1 thrust), the V speeds will be higher, and FULL TO thrust can now be applied in an emergency situation.

Granted, in the all engines case, thrust could be advanced from TO-1 to TO without any control problems.

In short, why go at the limiting weight for the Thrust and configuration, when an alternative thrust and/or configuration conferred increased safety margins? You asked what would be the choice for normal operations. All Non-Normal and Emergency operations commence as Normal Operations, so application of the 'do not fly at the limit if there is another choice' policy applies to Normal Operations.

As always, common sense comes into play as to how close to TO-1 would you need to be before advancing to the next option. There are commercial advantages (engine guarantee) in basing operations upon De-Rated Thrust.

Regards,

Old Smokey
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Old 24th Nov 2006, 14:11
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Once encountered an inversion while carrying out a 3-eng ferry flight. It was only the fact that we crossed the coast and hit cooler air, that allowed us to climb and accelerate! Concentrated the mind somewhat!
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Old 29th Nov 2006, 15:09
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I'm interested in the application of the known inversion conditions to RTOW. My thoughts are that it would be (1) Impossible to affect Runway performance (Stop and Go) as the inversion is somewhat above the runway, and (2) Rare that the inversion would reach down to the average MAA (if there is such a thing as average MAA), thus rarely would Obstacle Climb be affected. That only leaves the WAT limit and/or 4th segment performance.
O_S... sorry for the slow response....

I agree with #1, a temperature inversion will not affect the accelerate stop. However we operate with a MAA of 800ft and this height can have an inversion, we also use a standard Boeing formatted takeoff chart where the zero wind limit weight accounts for field, obstacle, brakes etc. Its therefore impossible to just apply the inversion penalty to only the climb limited weights.

I guess that we are more conservative than we need to be, but around here no one complains about that

Mutt

Last edited by mutt; 29th Nov 2006 at 15:45.
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Old 30th Nov 2006, 00:24
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Thank you Mutt, indeed, I, for one will never complain of conservatism. The only problem is keeping information about the embedded conservatism away from the accountants. So far, so good.......

Timely information in fact, I'm soon to embark on a tour of duty in the lands of the "mothers of inversions" - AUH / DXB / JED. The Cyclonic influence at DXB ought to moderate things somewhat.

Regards,

Old Smokey
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