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Philip Mew
13th Jun 2015, 16:30
Using VHHH / Hong Kong as an example, let's say the ATIS gives the following :
departures 25L
29 degrees Celsius temperature
1006 hectopascals QNH
3 knot headwind component

I use 30 degrees C, 1005 hP, and a 5 knot tailwind component in the OPT.
In other words, I plan for a slightly worse case scenario. I'm covered if any of those parameters change resulting in better takeoff performance, that is temperature goes down or QNH goes up or wind really does give a headwind component.

1.
Any reason to not continue this practice ?

2.
Haven't been able to find a reference as to when new numbers MUST be calculated with the exception if actual weight is higher than planned weight. In this example, at what temperature is recalculating a must do - 31 degrees ? 32 degrees ? How about QNH - if it goes down to 1004 hP ? 1003 hP ?

It's my understanding that the OPT has some built in buffers and calculates take off numbers in a very conservative way. I want to avoid having to recalculate numbers if ATIS or actual parameters change not in favour of better takeoff performance at a inconvenient time. In the queue for takeoff, for example.

Thanks all.

B737900er
13th Jun 2015, 17:21
1 degree or HPA won't make much difference. The wind can make a difference.

Put it this way, it comes to judgment. If you believe (from experience) it won't make any difference then no need to calculate it.

I personally would not do another Perf calculation for trivial changes but I will for significant changes for example dry to wet runway or anti-ice will be needed.

JammedStab
14th Jun 2015, 00:42
Different type but we usually do not use any of the headwind component. If it is the time of day where the temperature will be increasing, I add 1 or 2 degrees depending on how long to departure.

I only closely monitor wind, pressure, and temperature for calculation changes on the rare occasion we are at our max performance for the runway being used based on the thrust being used. This would be the case if headwind is minimal for that situation or they are being used to meet the performance requirements.

CCA
14th Jun 2015, 02:41
Safety is already built in, if we continue down the road of adding safety for safety sake next it'll be add 5C drop 5hpa and put 5T with a headwind and why not make it wet when the runway is dry which a seems popular :ugh:

If the padded condition didn't allow a take off yet the actual conditions did which one would you use?

JammedStab
14th Jun 2015, 04:12
You have a good point.

However, when 370 pax all weigh the standard weight and they all have only their standard carry-on baggage weight, I'll stop using zero wind.

zlin77
14th Jun 2015, 05:31
Don't have my 777 FCOM handy, but from memory it only uses half the headwind and increases the tailwind (+50%?), will try to give you improved climb V2 but defaults to balanced field if unable to do so, unless you are expecting the temp to increase prior to departure just use the ATIS temp...