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JammedStab
6th Apr 2015, 01:30
Our FMC appears to automatically calculate v-speeds for full takeoff thrust for an initial calculation. After using our computer to calculate v-speeds using TO2, I noticed that while V2 was within 1 knot of the FMC calculation, Vr was about 6 knots different between the FMC and computer. That is until TO2 was selected in the FMC, then the FMC Vr changed significantly to be within 1 knot of the computer calculation.

Why such a big difference in Vr based on thrust setting.

Intruder
6th Apr 2015, 03:00
Maybe because with more thrust you get more acceleration and less time to get from VR to V2...

FlyingStone
6th Apr 2015, 05:58
Was V1 close or equal to VR? Lower thrust rating means lower Vmcg, which may reduce V1/VR.

CCA
6th Apr 2015, 06:01
V2 is basically constant for flap setting and weight.

So think of it this way, if V2 for whatever weight and flap setting is say 150kts.

And using TO2 say for arguments sake VR is 140kts, after rotation you end up at 160kts (V2+10).

If you use TO1 and rotate at 140kts you may end up at say 170kts. (V2+20).

And Max TO at say 180kts (V2+30) after rotation therefore VR is reduced with the aim that you finish at V2+10 all the time.

Obviously minimum speeds must be respected so VR can only be reduced so far before you result in getting VRmin. (1.05 x VMCA)

main_dog
6th Apr 2015, 15:57
Jammedstab, I think you're on the B744? I'm not 100% sure I've understood your question but I'll give it a try. Keep in mind that Vr must always be in excess of 1.05 VMCA, among other things.

V2 speeds on the 744 are essentially independent of thrust rating (as they are always well in excess of the minimum required 1.1 VMCA) and only depend on flap setting and weight, so the FMS precalculated V2 will usually immediately agree within one knot or so of the company computer-calculated V2 as long as the weights and flap settings coincide.

Vr however is more limited by its minimum of 1.05 VMCA, and as the default setting in the FMS is full takeoff thrust, the FMS precalculated Vr may initially be several knots higher than the company provided one (since VMCA at full TO will of course be significantly higher than VMCA at TO2). When you then input into the FMS the actual rating you will use, especially if it's TO2, its precalculated Vr should drop significantly and may then closely match the company provided one.

Gysbreght
6th Apr 2015, 22:45
Main Dog,

If VR was limited by 1.05 Vmca, V2 would increase with it, because V2 cannot be less than VR plus the inevitable speed increment between initiation of rotation and 35 feet with one engine inoperative, which is greater at the higher thrust setting.

However if, as you say, "V2 speeds on the 744 are essentially independent of thrust rating (as they are always well in excess of the minimum required 1.1 VMCA)", then it is unlikely that VR is limited by 1.05 Vmca.

latetonite
7th Apr 2015, 07:46
Guys, please wake Up. This is basic performance.
And the type of aircraft is irrelevant in this matter.

wondering
7th Apr 2015, 09:41
The 'Derated Takeoff' section of 'Getting to Grips with Aircraft Performance' by Airbus explains it.

main_dog
7th Apr 2015, 09:58
Guys, please wake Up. This is basic performance.
And the type of aircraft is irrelevant in this matter.

Er, not a very constructive contribution? Also, type is relevant. While the same regulatory limitations to the definition of V1/Vr/V2 apply to all types, for most takeoffs there will be a range of possible speeds allowed.

Airbus and Boeing for example go about performance calculations very differently, and it also varies by type: on an A321 V2 may be the result of a complex optimization calculation involving V1/Vr and V2/Vs ratios, and take available runway length and obstacles into account, while on a B747-400 V2 is calculated simply as a function of weight and flap setting, nothing else.

In addition operators are free to modify performance numbers (within allowed parameters), such as using a lower (more "go-minded") V1, thus adding further complexity to the issue.

If VR was limited by 1.05 Vmca, V2 would increase with it

then it is unlikely that VR is limited by 1.05 Vmca

Not sure what you mean Gysbreght, but strictly speaking there is no direct relationship required between Vr and V2, except that the latter must be greater or equal to the former. Both however are limited by their relationship with VMCA.

The OP asked why FMS precalculated Vr changes significantly when a different thrust rating is selected in his FMS. Every thrust rating comes with its own VMCA, with the most de-rated having the lowest VMCA of course, thus allowing for a substantially lower Vr than the highest thrust rating.

Precalculated V2 instead will not change (on a B744) as it is always dependent on weight and flap setting only (although this V2 does of course always satisfy the 1.1 VMCA and 1.2 Vs requirements, at all possible thrust ratings).

Ergo, when you first look at FMS precalculated speeds on a B744 FMS (when the default full TO thrust rating is still in the system), V2 will be spot-on provided weight and flap settings are correct, while Vr could be several knots higher than the Vr you will actually use. As soon as you "tell" the FMS that the rating you will use that day will be lower, say the maximum de-rate or TO2 as it's called on the Boeing, Vr may drop accordingly.

DISCLAIMER, I am just a line guy with an interest in performance, so if anyone has a better explanation of why the OPs Vr is changing with a change in selected thrust rating then by all means let us know!

JeroenC
7th Apr 2015, 21:53
How can V2 be unrelated to thrust rating? More power = higher screen height speed?

JammedStab
19th Apr 2015, 03:11
Jammedstab, I think you're on the B744? I'm not 100% sure I've understood your question but I'll give it a try. Keep in mind that Vr must always be in excess of 1.05 VMCA, among other things.

Vr however is more limited by its minimum of 1.05 VMCA, and as the default setting in the FMS is full takeoff thrust, the FMS precalculated Vr may initially be several knots higher than the company provided one (since VMCA at full TO will of course be significantly higher than VMCA at TO2). When you then input into the FMS the actual rating you will use, especially if it's TO2, its precalculated Vr should drop significantly and may then closely match the company provided one.

To follow up....

I did some FMC calculations in Chicago the other day. For a takeoff weight of approximately 327 tons this is what the FMC showed as default speeds for runway 10L with no ATM:

FMC Flap 20 TO thrust: V1:136 Vr:150 V2:165
FMC Flap 20 TO2 Thrust: V1:143 Vr:154 V2:165

According to the performance computer for TO thrust Min Vr: 125
According to the performance computer for TO2 thrust Min VR: 117

So the min Vr decreases by 8 knots by reducing the thrust according to the performance computer which makes sense according the VMCA reasoning stated by some. However the FMC speeds are way above VMCA so I would assume that this has nothing to do with it. And instead of decreasing VR for derated thrust, the FMC increased it. So the first question is why does the FMC VR speed change with a thrust rating change when Vr is well above VMCA and why is Vr increasing as thrust rating is decreasing.

I think Lateonenite knows the answer based on his earlier response. Would appreciate you letting us know.

Thanks

latetonite
19th Apr 2015, 05:07
When I state that the aircraft is irrelevant, I am referring to regulations, not what the FMC spits out.
The FMC takes into account the runway length, like your QRH speeds. The important issue here is the ASDA. Speeds are solely based on stopping distance an screen height at the end of your runway.
The WAT charts, or runway analasis takes into account your second segment climb requirements, further augmented by wind and slope.
A max allowed trust reduction of 25% might again change the picture as now your screen height could result in being much more than 35 feet, in case of a long runway. However the V1 for being able to stop remains the same.
Then the operator has some leeway in choosing the speed ratios, as long as he stays within regulations.
Al those parameters are fed into a computer now, and no wonder you end up with different speeds for different trust settings.
Vr is not by definition based on optimum V1.

A simple question, not a simple answer.

RAT 5
19th Apr 2015, 10:37
"The FMC takes into account the runway length, like your QRH speeds."

Is it not the case that FMC (& QRH) are simply balanced field speeds. It does/might not know the actual length of the runway.

latetonite
19th Apr 2015, 13:30
To Rat5: Correct, the QRH also does not require the field length.
I presume the speeds are derived from when ASD equals TOD for given conditions.

VNAV PATH
19th Apr 2015, 17:41
Think RAT 5 (post 13) is correct: FMC figures are computed with balanced runway. OPT figures are not.


Try same computation on a long, very long runway and different thrust ratings.


V2/VS optimum ratio is a factor.

JammedStab
19th Apr 2015, 22:15
The computations I did were for a 13,000' long runway. They don't often get much longer than that.

Intruder
20th Apr 2015, 03:35
Once more (see post #2)...

All other conditions remaining the same, and no limiting factors involved, reducing thrust will increase the time required to accelerate. Since rotation rate is assumed to be constant regardless of thrust setting, a higher Vr will be required to reach V2 at the same point in the rotation/initial climb.

If you tell the FMC you are using full TO thrust, but you tell the performance computer (or it tells you) to use TO2, the Vr values will likely be different. Once the parameters are consistent in the performance computer and the FMC, the displayed speeds should match

hans brinker
20th Apr 2015, 03:45
By definition VR is the speed where you initiate rotation as to arrive at V2 at 35ft. Regardless of all the other numbers/weights/heights/regulations, you are supposed to get to V2 at 35ft so if you have selected a derated takeoff power setting you will rotate at a higher speed because you will have less power to accelerate to V2 while climbing to 35 feet (as far as I know your climb angle should be the same for a derated takeoff per 1st segment climb requirements, but I could be wrong). I think this is what CCA said in post #4.

I don't think VMCA is a factor here, according to the numbers below provided by Jammed (I assume TO2 is the derated number for Boeing, I am stuck on brand A) a TO thrust/MAX T/O has a lower V1/VR than TO2/derated T/O. If it was a VMCA issue you would want more speed before taking it in the air at MAX T/O.
FMC Flap 20 TO thrust: V1:136 Vr:150 V2:165
FMC Flap 20 TO2 Thrust: V1:143 Vr:154 V2:165

As far as
"Guys, please wake Up. This is basic performance. And the type of aircraft is irrelevant in this matter."
is concerned, I really don't think your second post added any more information to the discussion at hand than the first............

I think Intruder types faster than I do!

FGD135
24th Apr 2015, 13:52
And the prize goes to Intruder and hans brinker for the correct answer. Congratulations to these two gentlemen!


The answer to the OP's question lies in the relationship between Vr and V2.


When considering that relationship, we must consider what, exactly, Vr actually is.


So, what is Vr? We all tend to think of it as "the airspeed at which the body is rotated for takeoff", but there is a much better answer to this question, and that is the answer given on the first line of hans brinker's post above (#18).


Intruder explained things in terms of the effect excess thrust has on the acceleration that has to occur between Vr and V2.


I'll throw in my two bob's worth by continuing along the line laid down by Intruder:


Imagine a Vr of 0 (zero) and a V2 of 150 knots. Theoretically possible, but achievable only when there is CONSIDERABLE excess thrust. If the excess thrust is not CONSIDERABLE then the starting speed (Vr) will have to be some value closer to V2. The less CONSIDERABLE, the greater the Vr (and the smaller the gap between Vr and V2).

JammedStab
25th Apr 2015, 03:27
Awesome. Special thanks to Intruder, Hans Brinker, and FGD 135 for their knowledge and professionalism in their answers.

PPrune at its best as shown by these gentlemen.