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Dubai 39 deg TO and Flat Rated Engines

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Dubai 39 deg TO and Flat Rated Engines

Old 10th Sep 2018, 15:47
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Dubai 39 deg TO and Flat Rated Engines

Hi there,

I was reading about this article:

Temperature Inversions and Takeoff Performance Calculation | Flight

In the article above, it talked about GE-90's flat rated temp (Tref) as ISA+15. In Dubai's case, it;s 30 deg. With an OAT of 39 degrees, it will exceed the Tref (as mentioned in the article), thus leading to a decrease in thrust in order to maintain the EGT limits.

I was playing around with the calculations and with the following inputs:
OMDB
Rwy: 12R
Wind: 140.10
OAT: 40 deg
QNH: 1010
TOW: 350T (close to max)

Performance calculated TO takeoff with flaps 5. When I changed the OAT to 41 deg, no calculations allowed. Thus, if i'm faced with a low-level temperature inversion after liftoff, i'm screwed. Any thoughts on this?
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Old 10th Sep 2018, 16:18
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Not if runway limited, no?
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Old 10th Sep 2018, 17:02
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Your thought is right. At least once you're 2nd segment limited.
But you're only screwed theoretically, practically there's still the allowance between the 2.4% gross and 1.6% net climb gradient (actual obstacle clearance) which is hopefully going to save your day.
But it's true, industry doesn't account for this.
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Old 10th Sep 2018, 17:27
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You are only screwed with an engine failure. Isn't it basically flat there anyway?
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Old 10th Sep 2018, 17:43
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Originally Posted by wiedehopf View Post
You are only screwed with an engine failure. Isn't it basically flat there anyway?
You mean apart from the world’s tallest building?
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Old 10th Sep 2018, 18:06
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Assuming this is a B777-300ER-ULR.......with GE 90-115's...... At 351T - To the tonne rounded down MTOM

In general for MTOM operations, Flap 5 is limiting in this case. Flap 15 or 20 will be required depending on the runway and performance corridor. On 12L with QNH 1010, and 10kt headwind, Flap15/20 performance will allow a MTOM operation. As soon as the QNH goes below 1000 and the temperature above 40 degrees, Performance will not allow MTOM.

GE 90's have Takeoff Bump ( company option). In this case they can add circa 2-3t to the performance limited MTOM. Available -2000 to 3000 pressure alt and between 35 and 50 degrees.

However, the usual summer OMDB scenario is 30R, wind calm to 5kt headwind component - varying, 43 degrees, 998QNH. With these conditions, usually flap 15 from K15 intersection allows Max Structural Takeoff Mass.

The effects on aircraft performance of LLTI's are probably masked from the pilot noticing by the addition and subtraction of the Landing gear inner doors opening and closing during the gear up cycle. Two engines this is equivalent to roughly 2 degrees pitch.

The most noticeable inversion in these conditions is a 6-10degree inversion at around FL200, 10-15 kts gentle loss and a consequent loss of rate of climb at the aircraft noses down to regain commanded speed.
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Old 10th Sep 2018, 18:55
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So just being curious what makes 12L worse than 30R?
Also isn't 12R used for departures when operating runways 12, maybe that's what you meant Mr Levitator?
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Old 11th Sep 2018, 01:32
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Your airframe, whatever it is B or AB and the respective variant, is calculated by the load people. Be it Being with BCOP or the Airbus program,it calculates all of the parameters for TO, be it all engine, or EO.
This is all sorted out with the loading of the aircraft, and the temperatures at both DEP and ARR.

As for the questions on DBX, you take off over the Ocean or the Desert. In addition, the worlds tallest building is not an obstacle in the fligthpath.
OP., quite frequently , you will have a temperature inversion at DBX (this, in addition to the associated windhshear, are the reasons why there are so many GA)

The inversion level is very typical, especially the headwind/tailwind, (or vice versa) depending on the time of day. unfortunately, you will have no idea of this with the AWOS at 10M and a balloon reading twice a day.

Knowledge is power, why are there so many GA on final, and on second attempt, success......

Last edited by underfire; 11th Sep 2018 at 01:50.
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Old 12th Sep 2018, 17:00
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wiedehopf;
Correct - Typo, 12R usually in use for departure.
Despite the fact that DXB is percieved to be in a 'flat environment', there are in fact many manmade obstacles in close proximity to the airport on the dep and arr tracks. For instance there is an Engine Out procedure 30R to avoid a tall building.

underfire;
The load people are infact the pilots. Loading control simply release a weight and cg. Some companies have dispatch pre calculate a MTOM based on forecasted conditions at STD, but not all. It is the commander who will be part of and responsible for the performance calculations used for the actual takeoff.

Go Arounds are usually runway occupied in DXB. Previous wake trail trial extended and new 3Nm separation applied. On the second approach the crew will be acutely aware of the previous issue and mitigate accordingly and effectively during the second approach.
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Old 15th Sep 2018, 16:24
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Not sure if this helps -


​​​​​​​
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Old 15th Sep 2018, 17:15
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Originally Posted by extricate View Post
...
In the article above, it talked about GE-90's flat rated temp (Tref) as ISA+15. In Dubai's case, it;s 30 deg. With an OAT of 39 degrees, it will exceed the Tref (as mentioned in the article), thus leading to a decrease in thrust in order to maintain the EGT limits.
...
With an EGT margin of >9° C on both engines you will not exceed max EGT.
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Old 16th Sep 2018, 05:56
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With an OAT of 39 degrees, it will exceed the Tref (as mentioned in the article), thus leading to a decrease in thrust in order to maintain the EGT limits.
Ah, not really. Even if we're talking engines with zero EGT margin, thrust will not decrease to protect EGT limits unless the flight crew commands it by retarding the throttles/thrust levers. By design, the FADEC logic does not protect EGT limits except during starting. So, if things go bad and EGT is headed north redline, the flight crew gets to decide if the conditions dictate retarding the throttles to protect EGT redline (an economic decision), or exceeding EGT limits to protect the aircraft (a safety decision).
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