V1 at high altitudes

[michal 44]

Yes, alchemy it is.

[FishermanIvan 110]

With flaps 15 you will have worse climb gradient, it could be OK at this airport but it would not work at many mountainous airports since you have to have sufficient ground clearance when flying with single engine.

Oh big time. You have to get the jet cleaned up quickly or it climbs like a pig.

[Martin Beauchemin 0]

I have a friend who works for westjet and when they are fully loaded with a 738 taking off on that 6000 foot runway in Maui, Hawaii they sometimes take off with flaps 25 and a depressurized cockpit as to not take power from the engines on the initial climb. Of course once flaps are retracted they pressurize the cabin again.

Edited January 16, 2012 by Martin Beauchemin

[michal 44]

taking off on that 6000 foot runway in Maui, Hawaii

It is a 7000 ft runway over there ...

[Martin Beauchemin 0]

It is a 7000 ft runway over there ...

My bad. Your right haha.

[davidz 26]

Remember, the FMC values are just QRH numbers. You need to run all your data (runway elevation/length, ambient conditions, etc.) through a takeoff performance calculator (e.g. TOPCAT) or use lookup tables (you'll have to work in the ops department at an airline to get these) to see if the takeoff is legal.

[bakaman 28]

can the NGX be operated at Quito airport (9200 ft) ?

[Paul_Smith 92]

And the thing that really matters when you're braking; runway remaining and groundspeed. At 140 kts at La Paz the GS is over 165 knots easily. There's no way to stop it. V1 would have to be like 90-95 knots to get it stopped on the runway.

Huh? Vspeeds are calulated on the ground. There is a posibility GS might be lower then IAS as a result of headwind, but V1 calculations include headwinds so that is already accounted for.

[adamant365 28]

can the NGX be operated at Quito airport (9200 ft) ?

Yes. Both Delta (with their CFM56-7B26 equipped 737-700 "rocket jets") and United 737-700's (I think they are B24's) operate from UIO.EDIT: I'm sure there are others...those are just the US operators. Edited January 16, 2012 by adamant365

[bakaman 28]

Yes. Both Delta (with their CFM56-7B26 equipped 737-700 "rocket jets") and United 737-700's (I think they are B24's) operate from UIO.EDIT: I'm sure there are others...those are just the US operators.

thanks for your reply! I know that in real life 700's and 800's version are operated at quito. I meant with the PMDG NGX 737. Anyway i did SEQU-TNCA this afternoon with 130 pax, around 9,5t of fuel and 2,6t of luggages. I took off without a pb with packs off. There was a 300m margin. thanks TOPCAT!

[japascoe 98]

Yeah the important thing to remember about V1 is that all it tells you is that: if you have an engine failure just below that speed and either (a) continue the take-off or ( b ) abort, taking the first action at V1, you will use the same amount of runway in both cases. It gives you no guarantees as to whether the amount of runway you actually have will be sufficient.

Edited January 16, 2012 by japascoe

[Fabo 213]

John-Alan: That might be the case with FMC V-speeds (not sure) but certainly not in TOPCAT.Paul: IAS vs TAS is dependent on pressure altitude, and so will be quite different in UIO, while almost the same at low-level airports. The difference would be the same as when flying over the sea at 9200ft altitude.TAS=GS in calm wind conditions.

[Paul_Smith 92]

Sorry, John-Alan, but that is not correct. V1 is the speed at which it is no longer possible for the pilot to abort the take-off on the runway remaining - this is why runway length and braking conditions are included in the calculation. You seem to be talking about the obstacle clearance take-off roll. If you look at the FAA definitions, you will see they refer to detecting engine failures and taking action. This is because V1 is the point where you are committed to take off whether you like it or not.Peter - You are sort of right. I should have said CAS instead of IAS.

[Fabo 213]

Paul,aircraft performance is an intriguing subject, and can be very confusing. For example now,John-Alan is right if we are discussing balanced-field speeds, which are defined for every weight at certain flap setting, and there are adjustments for wind, pressure altitude, slope and temperature. If you study PI section of the FCOM, you will see that the balanced speeds by themselves are not linked to runway length. That would be another requirement, to find out, if your corrected field length puts your weight below limiting weight. If it is so, then you can take-off using your computed speeds.Performance programs can then make use of other information to maximise the performance, for example on an upslope runway your accelerate-stop distance is lower, so it would look like you can take more weight, but not quite - higher weight means slower EO accelearation post -V1, and you can not reach the clearance height above the runway end, so... The performance program will take the clearway into account though, thus maybe allowing you to uplift the weight you wanted. However we are not talking simple balanced field anymore, and you can turn the QRH speeds off in the FMC. (this might, in the right conditions, counterintuitivly allow you to take more weight out on the same runway in calm wind when rolling up the runway, rather when rolling down).(and just to add to confusion, when you are not runway limited, but say obstacle limited, you can shove the speeds all around to get the most optimal climb performance rather than runway performance... for ex. you increase V1, your ASDR goes to hell, your TODR might actually go down, but with higher V2, you just climb better)About the CAS vs. IAS - CAS is just IAS corrected for instrument error, and probe position error, so my point is, if anything, more valid.

Edited January 18, 2012 by Fabo

[japascoe 98]

Paul I think we're both thinking more or less the same, but coming at it from different angles.The FAA definition (here) is as follows [emphasis mine]:"V 1 means the maximum speed in the takeoff at which the pilot must take the first action (e.g., apply brakes, reduce thrust, deploy speed brakes) to stop the airplane within the accelerate-stop distance. V₁also means the minimum speed in the takeoff, following a failure of the critical engine at V_EF, at which the pilot can continue the takeoff and achieve the required height above the takeoff surface within the takeoff distance."Note that there is no mention of runway remaining, only the accelerate-stop distance is mentioned. The accelerate-stop distance will depend on your weight, if your plane weighs more, then the accelerate-stop distance will be greater (as will the safe takeoff distance for that matter, since it will take longer to accelerate). The accelerate-stop distance also depends on stuff like runway condition (wet or dry), braking condition and derate.In a take-off performance calculation you will determine a weight limit that ensures the accelerate-stop distance is less than the amount of runway available, given the current braking conditions. You then work out what V1 speed to use to ensure you will stop within that distance.If your weight and thrust settings are such to produce an accelerate-stop appopriate to the runway, then indeed aborting at V1 will ensure you stop before (or, in the extreme case, right at) the end of the runway. Though I don't have it myself, I assume that this is what TOPCAT does.However, if, like the OP, you do not do a full performance calculation but only a V1 calculation, it is possible that your accelerate-stop distance is greater than the runway available. In that case you will run off the end of the runway, even if you abort at V1, since V1 is defined relative to the accelerate-stop distance, not relative to the actual runway available.As a caveat to that, in any proper take-off performance analysis, it will be ensured that the accelerate-stop distance is equal to, or less than, the runway available, so in practice V1 will indeed guarantee you can abort and safely stop the plane on the runway.