mutt, I am confused
If the FM`S calculate the speeds for you, then surely there must be a way to adjust for an intersection departure.

If it`s not too much trouble, would you be able to explain (chronologically) the process you perform for calculating T/O data, from start to finish.

I understand how it could be done, but it seems as if it would be too limiting on TOW to cover all scenarios.

[This message has been edited by jtr (edited 28 September 2000).]

You can - I think I'm right in saying - always make a field a balanced field. You derate the takeoff. This makes the engines last longer and saves you having to pick one of the range of V1s we have been talking about. Because now you only have one.

Mutt,

Only re r/w length in the FMC; in our B744, the FMC does know the runway length. It's found on the APPROACH REF page for the departure/arrival runway in the active route.

However, as you correctly say, the FLL has to be obtained elsewhere.

G'day

As sad as it may seem, the FMC (744/777) is pretty limited. It doesn’t have an obstacle database built in to it as and Feather #3 has pointed out the Runway Length is part of the Navigation database and not the Performance database.

If ATC offers an intersection takeoff, the FMC has no way of calculating the takeoff weights , so unless the crew have predetermined takeoff weights from that intersection or are ACARS equipped, the intersection can’t be used.

JTR , the chronological is something like this….

• Enter a tailored takeoff chart for the airport/runway/surface condition/MEL/ Flap Setting/ Thrust Setting.
• Read the limiting takeoff weight at the actual temperature
• If the actual weight is equal or lower than the limiting takeoff weight, enter the Actual Weight in the FMC. The FMC will produce takeoff speeds. Over write the V-speeds for any necessary corrections, such as wet / contaminated or MEL reductions.
  (CAA certified aircraft has the option of Wet V-speeds on the FMC, FAA certified aircraft do not.):confused

The main place where this is limiting is for intersection takeoffs, we therefore add new runway’s (36* / 36# or similar) based on the takeoff distance from commonly used intersections. The end result is a lot of pages, therefore some are carried on the aircraft and others such as MEL Takeoff charts can be produced on demand in dispatch.

That’s our way of doing it, I’m sure that other airlines have totally different methods that suit them. If someone does know of a way of doing intersection takeoffs in these aircraft, I’d be interested in learning how.

Boeing has brought out a laptop computer program to allow crews to calculate takeoff weights in the cockpit. This was primarily designed to assist BBJ operators, but is now available for other aircraft types. (For a fee, of course..) ;)

Twistedenginestarter, if you are doing a derated takeoff, you still have the same range of V-speeds.

Mutt. http://www.pprune.org/ubb/NonCGI/cool.gif


[This message has been edited by mutt (edited 29 September 2000).]

Thanks mutt, just wanted to make sure, in my worried little head, that the T/O weight calculation was a different one for an intersection departure. Now it all makes sense.

Thanks for the info, seems that I was not far from the truth.

What's the next question ?

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Smooth Trimmer

Vmcg is demonstrated in Zero Wind conditions.
If setting V1 to Vmcg it is worth considering the crosswind. If operating close to the crosswind limits, you are now operating at 2 limiting conditions and probably on a contaminated runway.

One has to decide if you really want to do this.

Vmcg is tested according to the certification requirements. In the old days BCAR 25 (British) required 7kt of adverse crosswind and FAR 25 (US) did not. This meant that a jet certificated under British regs could have a much higher Vmcg than the identical jet certificated under US regs and could, on occasions, be limited by Vmcg. When JAR 25 (European) replaced BCARs they standardised to the American Way.

This doesn't change the problem, only the stated speeds. It means that big twins can legally operate from shorter runways but, if they lost an engine above V1 and above stated Vmcg but in an adverse crosswind, they might not be able to keep the aircraft straight. To my knowledge it hasn't happened yet......

Mutt

I have to disagree.

If you have a runway where by 140 kts you have reached a high enough speed to lose one engine then if you derate you will take longer to get to 140 kts and take longer afterwards. Most likely V1 (one engine out)will move towards Vr ie you need to get as close as possible to lift off to stand a chance of making the obstacle clearance.

In the meantime V1 (last chance to stop) reduces because, for any given speed, you are now further down the runway.

The very least this will do is to narrow the range of V1s.

My point is you are always able to narrow this band to zero by choosing a suitable de-rate. When you have done this you have - in effect - a balanced field ie you are seeing the same balanced field that would be seen by another aircraft fitted with less powerful engines - less powerful by the factor of the chosen de-rate.

In practice you may not want to de-rate to this figure because you would need to put power on after lift off in order to get a decent climb out, thus more-or-less defeating the point of the exercise (being kind to your RB211s). Maybe this is why you don't recognise the link between de-rates and affects on V1 range.

av8r76-

You got quite a discussion started here. Very simply, after the min speeds are met, V1 will increase as weight increases. But as you see, there are many other factors. Is "balanced field" clear now?

V1 MIN will decrease with higher temperatures, V1 BASIC will increase due to the lesser thrust to accelerate to Vr.

streamline-

V2 doesn't necessarily = 1.2 Vs. AT lighter weights, it is often driven by the Vmca requirement. V2 can't be any LESS than 1.2 Vs.

The spread between V1 and Vr will vary signifigantly with density altitude. For example, in my a/c with a t/o weight of 300,000 lbs, at sea level the spread is 14 kts. At 7000 feet the spread is only 8 kts.

All the t/o performance charts I've dealt with use IAS and CAS. What charts are you referring to in mentioning EAS?

twistedenginestarter-

For purposes of this discussion, can we agree that derate and assumed temperature procedures accomplish the same thing? It's the takeoff weight that is reduced by the runway requirement if using assumed/derate. The V1 speed won't change all that much.

If using reduce/derate and I lose one on t/o, I know that all my obstacle and climb requirements have been met. I also have the added performance of the engines really eating colder air than the assumed, so my performance will be better than planned. I'm not an advocate of pushing up the power to get a more "decent" climbout. Why stress the remaining engine(s) at red-line if I've already cleared the obstacles? Once I'm past the obstacles, I'd rather have speed than altitude anyway.

(I know that will take some flak).

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[This message has been edited by quid (edited 30 September 2000).]

twistedenginestarter

I should have clarified my statement, when you are doing a Derate takeoff you still have the choice of Min V1, Bal V1, Max V1. The actual speeds are different to the maximum rated speeds.

It is possible to have all three of these at the same speed as shown below.

Min V1 = 142.3 KIAS
Max V1 = 142.3 KIAS
Bal V1 = 142.3 KIAS V1MCG = 124.6 KIAS at 15.0 DEG C
VR = 149.6 KIAS VMCA = 116.9 KIAS at 15.0 DEG C
V2 = 157.9 KIAS
VS = 133.6 KCAS

These are for a B777-200, 7000 feet runway and 239872 kgs, GE90-90B.

However, if we start dealing with a runway length of 10,000 feet and a weight of 200,000 kgs which allows us to use a 20% Derate, the figures are a lot different

The lower that you derate the engine the lower the VMCG becomes, the V2 will still remain around the same at 1.2 Vs.

Takeoff Speeds B777-200, 200,000 kgs, 10,000 feet runway.

No Derate.

Min V1 = 125.3 KIAS V1MCG = 125.3 KIAS at 15.0 DEG C
Max V1 = 134.3 KIAS
Bal V1 = 126.1 KIAS
VR = 134.3 KIAS
V2 = 145.3 KIAS
VS = 121.1 KCAS

20% Derate.

Min V1 = 113.3 KIAS V1MCG = 113.3 KIAS at 15.0 DEG
Max V1 = 137.5 KIAS
Bal V1 = 134.0 KIAS
VR = 137.5 KIAS
V2 = 143.9 KIAS
VS = 121.1 KCAS

Look at the Bal V1 in relation to V1MCG and V2.

So getting back to your original point, yes it is possible to have the Min V1 / Bal V1 / Max V1 speeds the same, either at a Maximum Rated Thrust or a Derated Thrust.

That also applies if you use a maximum assumed temperature reduction (25%) on top of the 20% Derate.


Mutt


[This message has been edited by mutt (edited 30 September 2000).]

[This message has been edited by mutt (edited 03 October 2000).]

Quid

I know my statement about v2= 1.2 Vs is not always 100% correct and I have stated in my answer that factors like Vmca,Vmcg, Vmbe etc play a role as well.

I just wanted to keep things simple and concentrated on the original question from av8r76 “How does weight influence V1”.
Once that is sorted out you can go into details.

As far as the spread between Vr and V1 is concerned and the influence of density altitude, I think I addressed that issue in my answer.

If you want to go into details too early while answering the original question the guy is going to get lost.

The way we calculate out RTOW charts is company secret and we exist for over 80 years now so we are one of the oldest Airlines on the globe.


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Smooth Trimmer

Mutt

Once again you are wrong.

However your example neatly shows what this thread has been about ie that V1 is a zone of decision rather than a single figure.

Your example shows V1s for 0% and 20% de-rate.

I said V1min will rise with increasing de-rate; and V1max will fall and thus eventually meet at the balanced field V1. Yet yours both do the reverse. Why?

The reason is both V1 figures are nothing to do with V1. The lower figure is following down Vmcg until it meets the rising 'V1min'. Similarly the higher figure is tracking Vr until again it meets the falling V1max.

If you had carried on to 40% 60% 80% derate, this pattern would have reversed to conform to what I proposed.

A good real life example.

I guess also that strictly speaking you were correct since in practice you can't always de-rate enough to get a balanced field because of operational limitations.

twistedenginestarter

I guess that as I haven’t worked on an aircraft that is capable of 60 – 80% Derate, I’ve failed to see your point in real life.

For clarification, V1min tracks VMCG and not the other way around. From the above you can see that the V1 for the accelerate-go is below V1MCG, it is therefore getting forced up to meet the V1MCG.

Mutt http://www.pprune.org/ubb/NonCGI/cool.gif