Vr > V2
 

Gents,

I had an interesting discussion today, and would like your take.

Our manual provides guidance when departing when windshear conditions may be present. One thing we do is to increase Vr, and it tells us exactly how to determine the increment. What it does not say is what to do about V2.

We found a scenario where the Vr increment would cause Vr > V2. That just doesn't seem right. Doing some homework, FAR25 doesn't address this issue.

Is this correct? Can Vr exceed V2?

Why not? What does one have to do with the other?

We increase Vr as well during windshear ops.

Increased Vr procedures are simply a matter of using the excess performance available when operating at less than Performance Limited TOW to mitigate against the possibility of Windshear.

The philosophy is that, if you can rotate at a certain speed and comply with the field length and climb gradient at a limiting weight, you will CERTAINLY also meet the requirements if you rotate at that speed at a REDUCED weight.

If you were to use all three v speeds and thrust setting for RTOW at a lower weight than they were calculated for, you would still achieve an excess of performance.

BUT your BEST performance for your current weight engine out will be achieved at v2-v2+15 for your ACTUAL weight.

SO- take the extra energy from the increased Vr into the air, as an extra buffer against windshear after lift-off.

HOWEVER if you encounter windshear on the runway OR have an engine-out, revert to the best performance for you ACTUAL weight- the V2 you have calculated

Our sim checks we know we will get a wind shear so increase VR and rotate slowly to make sure we don't get behind the power curve. In real life you may not know this is going to happen but in the sim you always will. It is a game so you have to play it.

Windshear is almost always associated with specific, reported, predictable atmospheric conditions.

If it takes you completely by suprise, chances are you weren't paying attention.

If it takes you, by surprise or not, you will hear: why did you take off in the first place?

Check Airman, what you are doing is very similar to "improved climb", usually Boeing manuals state that if you increase Vr, then increase V2 by the same amount, it defeats the purpose if you accelerate to a higher Vr and then lose that speed increase by slowing down to the original V2.

Mutt

Sorry, mutt, not true.

Improved climb is a MTOW Enhanching procedure.

Normal take-off charts are calculted using a "Standard" V2 os 1.2Vs. Improved Climb is a seperate set of charts allowing a longer takeoff roll to achieve a higher v2, and thus a better climb gradient- but it is not specifically related to windshear.

With the advent on computer based OPTs, EVERY takeoff is "Improved climb".

Increased Vr is a suplementary procedure specifically related to windshear mitigation, and you do NOT increase the V2 as part of it.

And Micro- I agree with you! If I were suspicious enough of Windshear to use an increased Vr, I'd probably decide to wait the conditions out instead.

Increased Vr is a suplementary procedure specifically related to windshear mitigation, and you do NOT increase the V2 as part of it.

My comments are generic.

(a) nil windshear, as it turns out, if you increase VR then, OEI, you will end up with a higher than scheduled V2 .. the two being tied up with the rotation rate. AEO, you will end up with a modestly significant increment above V2 anyway. Mutt's comment probably could have been clarified ..

(b) if you actually end up in a significant windshear situation, then V2 becomes a bit irrelevant ..

(i) OEI ... shouldn't have got out of bed that day. Crash, burn, die in all likelihood.

(ii) AEO ... you'll be going for the feathers or whatever is equivalent on your particular machine. V2 is of no interest at the time and has no relevance to survival.

John,

Specific to the 777 we have a supplementary procedure whereby you set all speeds normally, but then ALSO calculate the Vr for RTOW, to a maximum of normal Vr + 20 knots.

If you suspect windshear may be encountered, you may delay rotation until this higher speed, but V2 is calculated and set in the MCP normally.

If you suspect windshear may be encountered, you may delay rotation until this higher speed, but V2 is calculated and set in the MCP normally.

That sounds pretty sensible to me .. but, in the event of windshear, it follows that you disregard the bugged V2 and go for the indications on the ball ?

Similarly, if you don't have windshear, you are not going to reduce to the bugged V2 but, rather, keep whatever you end up with up to a sensible overspeed margin ? With the OEI caveat relating to very close in obstacles, to reduce speed would be a tad silly, I think.

Best avoided in the first place but, we have the same procedure and I never thought much of it.



If there's windshear around and you encounter it I would much prefer to be in the air with altitude beneath executing the recovery technique than still on the runway, with too high a groundspeed to stop and insufficient IAS to lift off.

:eek:

Interesting. We don't have a procedure on our 777s for increased Vr for suspected windshear (we have the Improved Climb mentioned earlier) but we do go full length, full power and F15. Or not go at all, which is the preferred option!

IMHO, problems are caused by no differentiation between wind shear and wind gradient and the ambiguity present in most manuals about procedure.

I've come into LHR on a nice day with 15-20kts down the runway with "Windshear reported, loss of 5kts at 200 feet" on the ATIS. No sh*t Sherlock! What were we expecting here? All around, people with shiny new stripes are briefing for W/S G/A, lining up with full power on the brakes, etc.

You also get "Windshear advisories in effect" at US airports, which I take to mean the system is working, not that there are actual windshear conditions present. I haven't been able to find a reference - does anyone know whether this is a correct interpretation?

Agree,much confusion about the term of wind shear.
Windshear recovery techniques,increase of max VR are for thunderstorm produced windshear,not temperature or wind shears which may require extra handling work/go around but dont necessitate such drastic recovery techniques..

I think posters not taking off in windshear conditions here imply convective related WS.

That would be useless in terms of performance and contrary to the FCTM for take offs in windy conditions,,,,(engine surge...)

If you encounter windshear you fly the windshear escape maneouvre, which is based on attitude and stick-shaker,not IAS.

Tarket IAS for a normal take-off is V2+15-V2+25, so the extra "OOMPH" will probably just leave you at the upper end on that.

wake turbulence is windshear.....

Out of the FAA Controllers Handbook:

a. When low level wind shear/microburst is reported by pilots, Integrated Terminal Weather System (ITWS), or detected on wind shear detection systems such as LLWAS NE++, LLWAS-RS, WSP, or TDWR, controllers must issue the alert to all arriving and departing aircraft. Continue the alert to aircraft until it is broadcast on the ATIS and pilots indicate they have received the appropriate ATIS code. A statement must be included on the ATIS for 20 minutes following the last report or indication of the wind shear/microburst.
PHRASEOLOGY-
LOW LEVEL WIND SHEAR (or MICROBURST, as appropriate) ADVISORIES IN EFFECT.

whizofoz

and bubbers 44

whiz...your own statement says: almost always...not always.

It is entirely possible that you may observe meteorological clues that might lead you to believe windshear/microburst is possible...but not assured...so, do you sit on the ground for a couple of hours or do you add speed to Vr to help out?

IF windshear/microburst was totally avoidable all the time by a simple metric of temps, wind and so forth, we would do it. And doppler radar isn't available everywhere.

I was in CLT when a DC9 crashed during approach/go around and it was blamed on microburst among other things. I watched the smoke come up...I knew I wasn't going to takeoff in that crap.

I observed the weather personally as I was doing my walkaround. the sky was green, the storm had EVIL in it 10 miutes before the crash...but there is no requirement for ATC to advise you that if you continue you will die.

windshear is still in the infancy of predicting and avoiding, so Bubbers 44 is right...you never know and 99.44percent is not good enough

I used to fly an aircraft where VR was always greater than V2. We never calculated V2 as it was assumed we would never fly below VR!

I was waiting for my pax one day in Oklahoma flying a Falcon 20 as captain. Reports were a storm was approaching the airport with hail and a tornado warning. I had the plane put in a hanger until it passed and told my pax we had to wait until it passed. I watched the last take off, a twin engine Cessna take off with a 10 knot headwind. As he passed the end of the runway the wind switched to a 40 knot tailwind with heavy rain. He was very lucky. If we had taken off behind him we would not have made it. As SSR said don't count on luck with wind shear. It might not be your lucky day.