VR calculation

[stickyb]

This question arises from a comment made by someone in the EK tailstrike thread. I don't want to clutter up that thread, not being a professional pilot, and please excuse me if this is nonsense.

In the EK thread, it was said that with all flex takeoffs the calculation was such that the actual takeoff point was at or near the same physical point on a given runway.

So I take that to mean that VR occurs at the same point on any given runway.

Being a computer guy I can see the logic in programmimg like that - ie given the known runway length, work out the acceleration (thrust), flaps etc for a wieght and temp so that VR is acheived at a specific position.

Does that also mean that V1 will also occur at the same point on any given runway (not the same point obviously as VR, but xx feet before)

[Dit]

So I take that to mean that VR occurs at the same point on any given runway.If you took non-derated thrust everytime, Vr would occur at near enough the same point on every runway. (If for the purposes of illustration we ignore weight and atmospheric conditions)

However, the point of derating the thrust is to extend life of the engines by running the at reduced thrust, thus extending the takeoff run. (If the runway is long enough to allow this!)

Does that also mean that V1 will also occur at the same point on any given runwayNo, V1 also takes account of rejecting the takeoff, so is more influenced by runway length. Consider the extreme case of an aircraft that could make Vr at the end of the paved surface on a 5000' runway, V1 would be different for the 5000' runway and say a 10000' runway.

[mutt]

No, V1 also takes account of rejecting the takeoff, so is more influenced by runway length. Consider the extreme case of an aircraft that could make Vr at the end of the paved surface on a 5000' runway, V1 would be different for the 5000' runway and say a 10000' runway.

No its not! Non-optimized V1 is the same regardless of the runway length if you are using the same weight/flap/thrust settings. :) Hence the reason certain FMS's can provide V-speeds and does the aircraft QRH.

Mutt

[Dit]

What is a 'Non-optimized V1'?

How can V1 be the same regardless of runway lenth?

Doesn't the FMS know the runway length and thus use that to calculate the V1 for that runway?

I very much doubt the validity of your post.

[mutt]

Doesn't the FMS know the runway length and thus use that to calculate the V1 for that runway? Nope!

I very much doubt the validity of your post. Not a problem, but prove me wrong :)

Mutt

[Dit]

Have a look at pg32 of Boeing's 'Understand Range of V1'. (http://www.smartcockpit.com/pdf/flightops/aerodynamics/28) - V1 will not be the same regardless of runway length.

I can't prove that the FMS (I'm taling about the 737-800, as its the only one I'm familiar with) includes runway length in its V speed calculations as I don't have access to the FMS manuals. However, as it provides different speeds for a wet runway, this would suggest that it does, as if the runway length is considered infinite V1 would be the same wet and dry.

I agree that there is no way the QRH would know the runway length, but I've never used this for Vspeed calculations, as my company provides airport-specific performance booklets.

You still haven't explained what a 'Non-optimized V1' is and Google throws up no results, nor does a quick search here.

[mutt]

We should actually take this discussion across to the Tech Forum :):)

Anyway what do you understand from slide 32?

Look at slide 18, 170,000 kgs V1=140 runway required 9300 feet. If you are operating from a 12,000 ft runway, what impact do you think this will have on the V1 from the FMS?

If you look at the other slides, you will find Min V1 and Max V1, the concept of optimized V1 will select the best V1 for the actual runway, that way the V1 for a specific weight wont be constant. But if you arent using optimized V-speeds then the runway required for a specific weight/thrust/flap is constant.

As for the FMS, next time you are in the sim, enter a weight that is greater than the runway limit weight, check if the FMS stops you from doing so!

Finally, getting back to slide 32, if you look at 4/5/6/7, that is the relationship for optimized V-speeds.

Mutt

[stickyb]

V1 will not be the same regardless of runway length.



Gents - thanks for the replies so far. I can easily understand that V1 will not be the same regardless of runway length, but my question was really about the physical actual point on the runway that V1 is reached.

If VR always happens at the same point on a given runway, will the calulations that cause that also cause VR to happen at a given point?

[Old Smokey]

Unless you are using optimised V speeds (read Mutt's posts), for a Flex Thrust Takeoff, the V1 and Vr point on a given runway will for all practical purposes be the same. This is because Flex reduces the thrust in accordance with the actual weight, emulating the performance of the aircraft as though it was at performance limiting weight.

My reason for stating "for all practical purposes" is that, lower TAS and Ground Speed are not considered for the same V speeds at a lower temperature than that "assumed", leading to a small degree of conservatism. The greater the split between Flex Temperature and Actual Temperature, the greater the conservatism (the aircraft will reach V1 and Vr a little sooner).

Regards,

Old Smokey

[mutt]

I can easily understand that V1 will not be the same regardless of runway length As i explained, it doesnt have to change!

but my question was really about the physical actual point on the runway that V1 is reached. It will be a constant point!

We actually measured this for a B747, we wanted take photos of the rotation, it was quite easy to calculate the exact point where they would reach V1 and VR.

Mutt