B737 performance question

[Tapshi]

why does V1 and Vr increase with temperature whereas V2 decreases?
the FCOM says so...

[The90sAME]

I can only answer with this, it's not going to answer your question fully, but i hope it helps, (Unless you already knew this)

As temperature increases, the air becomes less dense, thus needing a higher speed to get of the ground.

It works the other way around in cold temperatures.

Temperature decreases, Air Density increases, Lower T/O Speed.

[Tapshi]

that is right but isnt indicated airspeed independent of temperature/density?
and all speeds listed in the FCOM are IAS!

[The90sAME]

Im not 100% Sure, you'd need to wait for some a little more clued up than i am on this, im a 'Heavy' engineer, if it involves avionics or working these things out, count me out aha.

But racking my brains, not 100% if this is entirely correct, but your V speeds, even though shown on your IAS speed tape, theyre still calculated with; weights, temperature and flap settings.

[VarkDriver]

With credit to M. Hulshof Boeing 737 Performance Reference Handbook (FAA Edition) Page A-6

V1...A lower density altitude(higher OAT or lower PA) results in less life force which must be counteracted by a higher speed.For the continued takeoff this implies a higher V1, but this will be restricted by the ability to stop in the aborted takeoff case.
VR...A lower density altitude(higher OAT or lower PA) results in less lift force which must be counteracted by a higher speed, therefore VR needs to be increased.
V2...A higher OAT or higher PA result in LESS thrust, hence less acceleration capability. To keep the T/O Distance Required within the T/O Distance Available, V2 must be lowered. In the combined effect in ISA(higher PA with lower OAT and vice versa), the influence of PA is much more significant than the influence of OAT.

Again, not pitching this book, but more information than a body should know(both FAA and ICAO versions)
Cheers
Vark

[Tapshi]

thanks varkdriver

[de facto]

Just to confirm, when you go in the N1 limit page during the setup on the ground, the values of N1 are adjusted according to temperature right?

Correct among other things such as derate selected or not,packs On or OfF...

Does the N1 increase with PA and/or temp increasing to compensate the thrust decrease at a given N1?

Your engine is flat rated at seal level and isa+15c,all that matters for the engine and therefore the N1 limit is TEMP and Pressure Altitude...have a look in the qrh under GA N1 .

On a standard day, 0 ft AMSL, 15°C, how much thrust would one get in pushing the thrust lever until the far forward end

If you are flying 800s,it would be 27300 lbs of thrust,but you would over thrust the engine.Only if ground contact imminent AND on two engines may you add thrust over derate if used.(vmca issue).
.

Finally, why is the temp entered by the crew and not measured by the aircraft?

Because we need to earn the money no?
Seriously the latest fmcs do have the function of oat auto selection.

[212man]

A lower density altitude(higher OAT or lower PA)

Last time I checked, a higher OAT resulted in a higher DA

[Sue Ridgepipe]

While we're on the subject, can anyone tell me why when the actual OAT goes up the FMC calculated N1 for take-off goes up, even when using assumed temp for take-off? Isn't the N1 then based on the assumed temperature, so why does a change in the actual OAT have any effect?

[de facto]

Quote:
A lower density altitude(higher OAT or lower PA)
Last time I checked, a higher OAT resulted in a higher DA

Only 'cold' temperatures at or below 0c result in increased DA/MDA.
Higher than 0c ,DA/MDA are not changed(not increased).

[HazelNuts39]

Sue Ridgepipe,

The N1 for constant thrust (for any given altitude and airspeed) varies as the square root of absolute (°R or °K) ambient temperature.

[212man]

De facto,
I meant Density Altitude, not Decision Altitude, as referred to in the quote.

[de facto]

I meant Density Altitude, not Decision Altitude, as referred to in the quote.

Yep you're right,my bad

Now, just to get back to my second question. When you push forward the thrust levers to the far end, it will give you 27,300 at MSL...what's the commanding/limiting parameter by the EEC in that case: N1 max, N2 max, EGT? Would it be the same in EEC alternate mode?

Please use the search option, i already wrote about this in detail here before..