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View Full Version : Concorde: why not two different V1´s?


PifPaf
9th Mar 2003, 22:14
Since I left fighters to fly airliners, one question was unsolved for me: why don´t we have two different speeds to decide to reject or continue takeoff?
This issue bothered me much more after Concorde´s accident at Gonesse.
All of you know that we could have different “Vee-ones” for climb and field restrictions. On 2-eng fighters, this two speeds are taken into account. The first one is called “go/no-go” (or decision) speed, while the other is “refusal” speed. If you have an engine failure before “go/no-go”, you must RTO. If you experience the failure after “refusal”, you must proceed. But, if the engine fails between go/no-go and refusal, it´s at pilot´s discretion to reject or continue takeoff. You can either stop on the remaining RWY lenght or check 35ft at the end of the TO distance.
On conventional jets, its easy to understand that we have “slow” V1’s because it must be lower than VR. And what about a high-performance acft like Concorde?
I found out that at Gonesse´s accident, the calculated TO speeds were V1: 150 Kt, VR: 198 Kt, V2: 220 Kt.
We can see that there was a considerable interval between V1 and VR. It´s very easy to conclude that they couldn´t continue before 150 Kt. But what about abort some knots after it?!?
I presume that 150Kt was the minimum airspeed, due to climb restrictions – am I right? If so, and considering that CDG has a very long RWY, I still think that they could have another speed (between 150Kt and 198Kt) until which they could stop the big bird (respecting, of course, field and brakes limits).
Maybe I´m just wondering and Conc arleady has these two V1´s...
I´d appreciate any comments.
Best regards,

PP

(sorry for any offense to Shakespere´s language!):O

NW1
9th Mar 2003, 23:35
PP

In my company's operation we fly Concorde using performance conventions very similar to any other airliner. It was certified to achieve similar (and in most parameters better) numbers than conventional aircraft. Our performance calculation results in just one V1 speed (like any other jet transport I know of). The deeper logic behind it are buried in regulations and certification procedures and I for one cannot quote them without revisiting performance "A" books long shelved!!

What I can say is that you can take a single engine failure into the air from V1 or stop safely with such a failure from before V1, but no performance calulation could have protected any aircraft which suffered the sequence of events that occured that day in Paris - there is nothing they could have done.

(PS: Don't worry about Shakespeare - I never did understand his language .....)

fireflybob
10th Mar 2003, 08:29
I cannot speak for Concorde but it is often the case that a range of V1s are available with reference to take off performance. It is, of course, necessary to select one V1 to use on a particular take off.

There are many (airmanship) factors which might impinge on whether one would use a minimum or maximum V1. For example, if the end of the runway is a vertical drop down to the beach (aka Madeira) one might be inclined to use a min V1 since the last thing you want to do is go off the end but being 34 ft at the end of TORA/TODA would not worry you too much. Conversely, you might you a maximum V1 if, in the engine failure case, a significant emergency turn is involved after take off. On the other hand you might pick a speed between min and max V1 as a compromise.

All this assumes that a range of V1s is available. In the case of limiting perfomance this may not be so.

PifPaf, in some companies this concept is taught and practised - it's all a case of "gilding the lily" if you want to. The concept of V1 is, some might say, somewhat outdated but it is a simple system that works quite well.

Hotel Charlie
10th Mar 2003, 08:46
Having a failure after V1 only means GO UNLESS you have an unflyable condition. The concord guys did have an unflyable and had they known, probably would have stopped! One cannot make procedures that take every possible and impossible event into account. Public Jo does not like to know or hear about, but there are certain risks involved in aviation!

PifPaf
11th Mar 2003, 02:45
NW1, thanks for your reply! Just wan't to be clear that I do believe the crew had nothing else different to do.:(

fireflybob, I'm not sure if I correctly understood your explanation, but I think you're not 100% correct. You cannot choose min or max V1 at your discretion. Let's check your 2 examples:
1 - considering 34 at the end of TODA, you will be out of regulations.
2 - choosing a max V1 you could depart, but what if you had a failure before this speed? Until which velocity you could stop in the remaining lenght? You would need a "check point".
I agree that V1 computation, as you said, works quite well. But for (as I posted) "conventional" aircraft, not for the Concorde.

Hotel Charlie, my question has too much to do with the first thing you said: Having a failure after V1 only means GO UNLESS you have an unflyable conditionLet me try to explain in other words (please note that as I'm not a Concorde pilot - unfortunately -, I'm just supposing!):
Let's assume I found that for a given condition my VR is 198Kt. I checked my performance manual and found that 150Kt is the minimum airspeed from which I could experience an eng failure and still reach 35ft at the end of the TODA.
Using the same conditions, I found from the charts that 190Kt is the maximum airspeed from which I could initiate a RTO and stop my acft in the TORA.
Now, what happens if I had an eng failure at 170Kt? I would have two options:
1 - If I decided to continue, I would overfly the end of TODA more than 35ft;
2 - If I decided to reject, I would be able to stop before the end of the TORA.
Now, note that, in both options, I would be respecting all the regulations!!!:ok:
That's why I still didn't understand why in such acft we couldn't have two checking speeds!