B777 Flap 15 v/s Flap 20 departure
 

Dear all,

This pertains to the 'parameters' one must use to 'decide' whether for 'today's t/o conditions' what is the 'best' flap setting.

This is a comparison between flap 15 v/s 20 Dep for NEWARK/KEWR/22R (11000'/3353metres)
Acceleration ALT 1000' AGL in both cases.

ATOW 335.3T

RTOW Today was
-350T (F20/OLW)
-349T (F15/OLW)
-Structural (351T) if packs OFF

Here is the snapshot of the performance F15
http://i68.tinypic.com/zvddep.jpg
F20 performance -
http://i63.tinypic.com/dowcn4.jpg
F15 takeoff details
http://i66.tinypic.com/2iub51e.jpg
F20 takeoff details
http://i65.tinypic.com/2e2qk4x.jpg

Kindly ignore assume temperature calculations & disclaimer in the snapshot.

Q1) F15 ASD 11000' (that's actually the full runway length)
F20 ASD 10039'
(Gives me a 1000' margin at the end of a RTO with proper pilot technique)

Can I say F20 is a more desirable departure compared to F15?
(Let's ignore the given benefit margins in calculating ASD)

Q2) F20 MIN V1 (obstacle limited) is 149 kts
Actual V1 used as per company policy was 160 kts

Can I say if I had an engine failure at 155 kts and I don't REJECT I will still meet all my obstacle clearances from lift off end? (Assuming Single engine acceleration from 155kts to V1 & Then to V2)

Q3) what other benefits (if there are any) can be used if I use a F15 or F20 departure?

Q4) what does MAX V1/VR speed mean? Should this info be used to determine what flap setting would be better ?

1. Yes, F20 will be more desirable as you are off the ground sooner so in case of gusty wind or wind shear, you will have more margin to accelerate and still clear obstacles.

2. Yes, continuing takeoff after engine out at speeds higher than MIN V1 will meet all obstacle clearances, rejects can be made up to MAX V1 (runway margin will reduce as speed increase toward MAX V1, after MAX V1, an overrun is likely). At reduced thrusts, Max and Min V1 will become closer together as reduced acceleration means a higher speed is needed at engine failure to continue (higher Min V1) and reduced acceleration means more runway is used to reach Max V1, so Max V1 has to be reduced to have enough runway to stop.

3. If the turn-around was quick, F20 departure gives lower V1 which means less energy into brakes, so less brake cooling time is needed if the brakes were very hot from the previous arrival or from taxiing. There will be less chance of the fuse plugs on tires melting as the larger runway margin can allow autobrake to be disengaged and manual braking done at a less aggressive rate if enough runway remaining/deceleration is good enough. Advantage of F15 is higher rate of climb which can be useful when encounter downdraft, especially when engine out (of course if downdraft is expected before takeoff, not change to F15 entering downdraft, unless captain's judgement says otherwise - with enough altitude, caution and good technique, switching to F15 and accelerating while sacrificing a bit of altitude can gain higher climb rate afterwards, this should not be done until initial obstacles are cleared as distance taken to accelerate will reduce obstacle clearance). A strong downdraft and engine out, F15 may allow level flight while F20 may cause slight sink. F15 may also allow a slightly better assumed temperature, causing less wear on engines, but safety considerations should come before this. There may be a recommendation on flap to use when windshear or downdraft are possible (of course don't takeoff into a windshear if radar detects it or it is reported) in the FCTM, I've seen it for some Boeing models.

4. MAX V1/VR means that MAX V1 is limited by VR speed as V1 cannot be higher than VR. If VR is high enough such as in F15 case, MAX V1 is limited by stop distance available (field length) (higher V1 won't allow a stop to be made at speeds close to V1). But in F20 case, V1 cannot be increase more than VR so it does not reach the field length limit speed. (example: speed after VR in F20 case, you still have enough margin to stop if you reject, but you will have already rotated and decide to go). Seeing MAX V1/VR vs FIELD LENGTH for another flap setting in the MAX V1 line means the MAX V1/VR flap setting gives more stop distance margin as the plane rotates before reaching the reject speed where all the runway is used to stop.

Hopefully this helps and feel free to correct me anywhere if I'm wrong for the experienced folks out there.

I see the ATM note, but you show ATM set to 0...

Winds are set to 0 as well.

Without knowing the ambient conditions both are acceptable, as ever its a compromise.

Maybe select OPTIMUM on the flap line and let the OPT work it out for you..

Agg, mention the loss of an engine at 155kts and continuing the flight. That is 5kts below V1 with flaps 20 and 10kts below V1 with flaps 15. If the engine failure is recognized prior to V1, it become a rejected takeoff, end of story. If the non flying pilot states "Engine Failure" prior to calling "V1." It is an abort.


One thrust settings flaps 15 shows 102.8 and flaps 20, 104.1, not much difference. Both well below max thrust. Flip a coin!

I think there is one further point you should keep in consideration. Climb performance hence obstacle clearance single engine. Its easy to get focused on accel/stop distances and the margins they bring whilst being unable to see data on how much you are going to clear objects vertically at a very high speed.

Don't forget up to V1-6 kts the aircraft will flash ENGINE FAIL on the PFD and shout it at you too.

If you read the FCOM SP Adverse weather it states F20 provides the best performance in windshear conditions providing the departure is not obstacle or climb gradient limited. It also suggests departing with F15 as a compromise as performance is almost as good. Which the limited differences between the above analyses bears out.

RRtrentsymphony - very grateful to you for your kind explanation (exactly what I was looking for)

Underfire - ATM set to 0 yes but as long as 'show Full' is not showing in 2R Bezel button I think it's should be fine :)

Zlin77 - yes that's the SOP but this speculation is from academic point of view.

That would be better detailed in the original post for context in the responses.