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Takeoff Speeds/Other Question  
User currently offlineVikkyvik From United States of America, joined Jul 2003, 10130 posts, RR: 26
Posted (9 years 2 months 1 week 4 days 19 hours ago) and read 3709 times:
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Preface: My understanding of V1 is that it is the maximum speed at which a rejected takeoff can be performed. Corrections welcome.

Alright, I did some simple calculations to express my question (all numbers are a product of my imagination, but hopefully somewhat reasonable):

Imagine both aircraft are the same type, and both have the same loading and weight distribution. Both aircraft in this example accelerate at 10 fps and decelerate at 15 fps (I hate working in English units, but it is just easier right now).

Aircraft A:
V1 = 140 mph (205.3 fps) for runway
Accelerate distance to V1 = 4202 ft
Decelerate distance from V1 = 2815 ft

Aircraft B:
V1 = 120 mph (176 fps) for same runway
Accelerate distance to V1 = 3098 ft
Decelerate distance from V1 = 2053 ft

So, aircraft A would require a 7017 ft. runway, whereas aircraft B would require a 5151 ft. runway. So if both aircraft were loaded equally, and had the same Vr and V2, etc., how is the V1 speed determined? In my current understanding, Aircraft B could simply choose a shorter runway by reducing the V1 speed. I really hope that my current understanding is wrong.

Onto the other question...I don't really expect any replies, nor would I realistically hope for any. But I've been reading the NW strike threads in CivAv (against my better judgment), and the discussion there is EXTREMELY.....um....two-sided? There are two different viewpoints, and neither gives a sh*t about the other. I'm TENTATIVELY wondering if I can get any opinions/experiences about working around/with unions in general from both sides which are not extreme....doesn't have to be about the current situation at all.

Thanks a bunch guys/girls. Tech/Ops is seriously the often overlooked bastion of reasonable rapport on this site.

~Vik


"Two and a Half Men" was filmed in front of a live ostrich.
13 replies: All unread, jump to last
 
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 1, posted (9 years 2 months 1 week 4 days 15 hours ago) and read 3681 times:

I'm not sure I follow why two identical a/c that as you state accel/decel at identical rates would have diff. V1 in the first place. Unless it's too early in the morn. I would say they would have the same V1. Remember V1 says you can accel to V1 and stop AND also accel and t/o on the remaining rnwy. This is assuming eng. fail. at V1. That's how it's computed to put it simply. V1 can be lowered in the case of "clutter" (a wet, icy, etc. rnwy) because you can see under those conditions it would take a longer dist. to stop than to continue.

User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 2, posted (9 years 2 months 1 week 4 days 13 hours ago) and read 3667 times:

The establishment of V1 for an airplane is done (at various weights etc.) by the manufacturer under the provisions of FAR 25.107

It is established in relation to VEF which is a theoretical engine failure speed and VMCG which is the minimum speed at which you can maintain directional control the critical engine failed and the other producing takeoff thrust.

It does not have anything directly to do with runway length. Rather, runway length required is limited by the distance needed to accelerate to VEF, lose an engine, accelerate an additional two seconds on one engine (recognition time) then reject the takeoff using any means that is "safe and reliable." which normally means brakes only.

If you have FAR AIM For Flight Crew published by ASA, Part 25 is included therein. Otherwise you can get the FARs online.

You will find the descriptions of all the takeoff speeds. Somewhat technical.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 3, posted (9 years 2 months 1 week 4 days 13 hours ago) and read 3663 times:

Quoting CosmicCruiser (Reply 1):
Remember V1 says you can accel to V1 and stop AND also accel and t/o on the remaining rnwy

At first, I thought you meant, a rejected takeoff, meaning accelerate, then come to a stop, and THEN accelerate again and takeoff. I see now you are referring to something else.

If I remember correctly, V1 has nothing to do with terrain avoidance or obstacle clearance. It's calculated based entirely upon aircraft performance, weight, and runway length.

V2, the takeoff safety speed, is that which accounts for obstacle clearance and terrain avoidance. At takeoff thrust, or a computed lower thrust when required for noise abatement, a certain maximum rate of climb is available, and is dependent on airspeed. It's easy to imagine that at a higher airspeed, a higher rate of climb is available. V2 is the minimum speed at which the takeoff procedure can be safely executed. Most procedures dictate flying at V2 + some margin, 10-15 knots, for increased safety and performance.

Please correct me if I'm mistaken.



Position and hold
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 4, posted (9 years 2 months 1 week 4 days 13 hours ago) and read 3656 times:

Quoting Bri2k1 (Reply 3):
If I remember correctly, V1 has nothing to do with terrain avoidance or obstacle clearance. It's calculated based entirely upon aircraft performance, weight, and runway length.

You are correct and I didn't say V1 did. I put it simply that at V1 you must be able to stop or go with the remaining rnwy and an eng. failure. As I also pointed out V1 is normally reduced with a "cluttered" rwy for the obvious reasons I posted earlier.

Quoting Bri2k1 (Reply 3):
At first, I thought you meant, a rejected takeoff, meaning accelerate, then come to a stop, and THEN accelerate again and takeoff. I see now you are referring to something else.

Yes, I probably should have written "accel and stop OR accel and go..." that may have been clearer.


User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 5, posted (9 years 2 months 1 week 4 days 12 hours ago) and read 3647 times:

Quoting SlamClick (Reply 2):

It does not have anything directly to do with runway length.

Interesting! So, to CosmicCruiser's point, how does runway contamination, or "clutter," affect the speed? Do the manufacturers provide more numbers to account for a slew of various possible conditions? Or, as I believe you allude to, is the runway selection limited instead by the computed stopping distance, after taking into account runway length AND conditions?



Position and hold
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 6, posted (9 years 2 months 1 week 4 days 9 hours ago) and read 3635 times:

Quoting Bri2k1 (Reply 5):
how does runway contamination, or "clutter," affect the speed? Do the manufacturers provide more numbers to account for a slew of various possible conditions? Or, as I believe you allude to, is the runway selection limited instead by the computed stopping distance, after taking into account runway length AND conditions?

I said "directly" meaning that the V1 speed for a given weight does not depend on the runway. On the other hand, where a runway is short, a lower V1 speed will be used.

This is an "equivalent level of safety" in that the decision to GO is being made earlier. The ability to climb out with an engine failed is not in question. The only thing that is compromised is your ability to stop on the shortened runway.

Another case that occurs often is a very long runway at high density altitude. In this case, we know we could accelerate to tire speed limit and stop the plane in the remaining runway. Because of the higher density altitude however, climbing out with an engine inoperative may be a problem. Here we can use an "improved climb" takeoff where we add some knots to the charted V1 and VR speeds. This way we use the extra runway length to accelerate on the ground to a point nearer V2 speed.

This does a couple of things for us: It has us lifting off at a speed nearer to L/DMAX and the plane will accelerate to V2 faster with the wheels on the ground than it will wallowing through the air.

Bottom line, to understand the concept of V1 you have to think in terms of (a.) Charted V1 speed for your weight and (b.) Limiting V1 speed based on runway or equipment considerations (length, slope etc for the former and anti-skid inop, etc, for the latter) Balanced field length is when they are one and the same. Most takeoffs are not "balanced" but optimized for some factor.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 7, posted (9 years 2 months 1 week 4 days 7 hours ago) and read 3606 times:

[

Quoting SlamClick (Reply 6):
On the other hand, where a runway is short, a lower V1 speed will be used.

I'm not sure I follow you on this one. You must be able to stop or go at V1 therefore the rnwy length must be such that you can do either. I could take an MD-11 to a 4000' strip and make V1 80 kts. and I could certainly stop but I'll bet I can't fly off that length rny. You're right that V1 & Vr are a function of wgt,etc but the runway must be long enough to satisfy both parameters.
Other than that I agree with what you've said. In reality at high gross wgts and most runways under 12,000' long I don't see V1 get higher nor do I see Vr optimized , just higher for the wgt. ..good post
As a side note( thinking of the throttle post earlier) get a V1 cut (in the sim I hope) and see how long you continue to roll to Vr at high t/o wgts. It can be a while..LOL


User currently offlineBri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 8, posted (9 years 2 months 1 week 3 days 16 hours ago) and read 3531 times:

I think it makes sense to me. If a runway is short, a greater percentage of it will be used up making a stop (it might only take 20% of a 12,000 foot runway, but half of a 4800 footer), so the decision to stop has to be made sooner. If there's not enough room left to stop, you're going for it, period. Right?


Position and hold
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 9, posted (9 years 2 months 1 week 3 days 14 hours ago) and read 3514 times:

Quoting CosmicCruiser (Reply 7):
I could take an MD-11 to a 4000' strip and make V1 80 kts. and I could certainly stop but I'll bet I can't fly off that length rny. You're right that V1 & Vr are a function of wgt,etc but the runway must be long enough to satisfy both parameters.

Okay, you are right, the accelerate-stop part is clear enough. If you could stop when rejecting from 130 knots you could certainly stop from a lesser speed for two reasons. (1) The speed is lower therefore the energy is less and (2) You would reach the lower speed sooner after brake release thus having more runway left in front of you for stopping purposes.

That leaves accelerate-go.

First, as I said:

Quoting SlamClick (Reply 2):
It is established in relation to VEF which is a theoretical engine failure speed and VMCG which is the minimum speed at which you can maintain directional control the critical engine failed and the other producing takeoff thrust.

That means that V1 will never be as low as 80 knots. It will never even be as low as VMCG which is normally only a few knots (10-20 or so) below V1 speed.

There is a procedure for making takeoffs at very low weights that involves finding the lowest usable V1 speed as derived from the VMCG chart and the runway analysis.

Problem is, I don't recall all the steps. I used to teach it. Before I started teaching it I took the airline's decision tree for this situation, reduced it to its simplest form, put it on MS PowerPoint and made up a ten minute presentation. You have just caught me at a moment when I don't work for that airline anymore and I don't have a personal copy of the PowerPoint.

Maybe someone can step up with the procedure.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineSlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 68
Reply 10, posted (9 years 2 months 1 week 3 days 14 hours ago) and read 3514 times:

Quoting Bri2k1 (Reply 8):
so the decision to stop has to be made sooner. If there's not enough room left to stop, you're going for it, period. Right?

You are correct. It also applies to some other factors and they are logical if you give them some thought:

Runway Slope Harder to stop a plane going downhill. Reduce V1 to allow more stopping distance.

Headwind / Tailwind Easy to stop plane into wind, increase V1. Harder to stop with tailwind so decrease V1.

Anti-skid inop Less brake effectiveness, make decision to stop sooner, reduce V1.

Thrust reverser on MEL Your takeoff performance was not based on use of reverse thrust, so no change to V1.

Now V2 is an in-flight, indicated airspeed so there is not much that will change it, other than gross weight, configuration (flaps) and density altitude. It is very rare to find adjustments to V2 in aircraft performance charts.



Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineCosmicCruiser From United States of America, joined Feb 2005, 2255 posts, RR: 15
Reply 11, posted (9 years 2 months 1 week 3 days 13 hours ago) and read 3499 times:

Quoting SlamClick (Reply 9):
You have just caught me at a moment when I don't work for that airline anymore and I don't have a personal copy of the PowerPoint.

Maybe someone can step up with the procedure

Not necessary, I really haven't disagreed with you. I thought there may have been a misunderstanding at first about confirming accel/go dist too.


User currently offlineVikkyvik From United States of America, joined Jul 2003, 10130 posts, RR: 26
Reply 12, posted (9 years 1 month 3 weeks 2 days 21 hours ago) and read 3279 times:
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Hey I never said thanks for the replies in this thread. Helped my understanding of V1 a lot.

BUT....

of course, I have more questions now. Over Labor Day weekend I flew LAX-DFW-BOS and the reverse (take a wild guess what airline I flew). Anyway, the pilots on this roundtrip were more informative than a lot of the pilots I've had. For instance, on the DFW-BOS leg, as we were taxiing for takeoff, the pilot mentioned that our takeoff speed would be about 165 mph (143 kts). The runway was 17R (13,401 ft). I was watching the distance remaining signs on takeoff, and we lifted off around 5,500-6,000 feet. I was actually quite surprised at first, but then realized that even with a full passenger load, the fuel load must have been less than half of capacity, due to the relatively short flight.

Either way, so with this ~6,000 foot takeoff, leaving 7,401 feet of runway ahead of us, would I be correct in assuming that this was a flex takeoff down to the minimums mandated by the FAA? Meaning, if the minimum was 80% thrust, would we have been down to that thrust? Just curious due to the fairly normal takeoff roll on a very long runway. I may be wrong but I doubt it takes over 7,000 feet for a 757 to slow from 143 kts. to zero at maximum braking.

Thanks guys, I always appreciate the responses.

~Vik



"Two and a Half Men" was filmed in front of a live ostrich.
User currently offlineVuelingAirbus From Spain, joined Aug 2005, 113 posts, RR: 1
Reply 13, posted (9 years 1 month 3 weeks 2 days 15 hours ago) and read 3262 times:

Considering the runway requirements (TOR/TORA, TOD/TODA, and ASD/ASDA), a Maximum Takeoff Weight (MTOW) can be obtained for each runway limitation. As an example, when for a given takeoff weight the TOD is equal to the TODA, this takeoff weight is maximum regarding the Takeoff Distance limitation.

For a given takeoff weight, any increase of V1 leads to shortening the TOD(N-1) and TOR(N-1), and increasing the ASD, but has no influence on TOD(N) and TOR(N). Therefore, for a given runway (i.e. given TORA, TODA and ASDA), any increase in V1 leads to an increase in the MTOW[TOD(N-1)] and MTOW[TOR(N-1)], and to a reduction in MTOW[ASD], but has no influence on MTOW[TOD(N)] and MTOW[TOR(N)]. {N assumes no engine failure versus N-1 assumes an engine failure one second before V1}

This was very technical so i will try to explain in plain language. And there are a lot of other factors to be looked at like mentioned before if it comes to determening the actual V-speeds for a particular take off. Without other influence and if you are not field lenght limited it can be said that an increase in V1 will reduce your take off distance (TOD) and roll (TOR) and at the same time increase your accelerate stop distance (ASD). In the case of an engine failure if you get the same distance for continuing the take off after V1 OR stopping the aircraft than you have a balanced V1/balanced field lenght. However - on shorter runways you will be limited with your MTOW and ASDA. This means your ASDA is the limiting factor, and the aircraft will still be able to lift off with a speed and distance margin (rember that the runway lenght was limiting). In that case you can reduce your V1 to a value where you achive the same distance in case of engine failure with the same TOW. So this gives you a V1 range and as a side note - many airlines use laptops with performance programs and this will always give the pilots the choice to select their desired V1 speed out of a specific range (V1 min to V1 max). On long runwways without obstacles this V1 range can be from V(mcg) to V2...

Hope that helps...

RGDS


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