Vr > V2

[Dan Winterland]

Here's a tip. If there's a big black cloud over the airfield and you're worried about windshear or microbursts, keep an eye on the OAT guage. If it drops about 5 degrees all of a sudden, don't go. It's the cold air from a downdraught hitting the ground and spreading out. Not conduscive to safe aircraft operating!

[stilton]

Seriously good tip Dan.


'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! '



Come on, you have to let us in on the type now..

[sevenstrokeroll]

watching OAT is a vital part of assesing the microburst threat.

I personally have spoken with the FAA administrator and deputy administrator on the subject of wx and flying...they still have not executed the advice I gave them.

[Check Airman]

Thanks for all the helpful replies. I suppose once you're airborne, you fly your normal V2+15 (if no windshear), but it just felt wrong on so many different levels to see the speed bugs arranged that way. Live and learn.

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?

You're pretty much right. Any major US airport almost always has windshear advisories in effect- a lot like the warning like birds in the vicinity of the airport.

Here's a tip. If there's a big black cloud over the airfield and you're worried about windshear or microbursts, keep an eye on the OAT guage. If it drops about 5 degrees all of a sudden, don't go. It's the cold air from a downdraught hitting the ground and spreading out. Not conduscive to safe aircraft operating!

Thanks for that tip Dan. While waiting out some wx the other day, the ATIS temp fell 3-4 degrees with an associated wind shift. I'll certainly keep that information handy.

[bubbers44]

After the ORD DC10 incident we didn't have to slow to V2 any more. If we were doing fine at V2 plus 15 we could maintain it. I would have done it anyway but the corporate guys had to come up with it. Now the stabilized approach is the big thing. ACARS transmits descent rate and airspeed so any deviation from standard is sent to dispatch. Lots of go arounds for no reason I am sure. Retirement is wonderful.

[9.G]

interesting to see once again completely different philosophies entertained by Boeing and Airbus. On the bus it's straight forward either delay the whole thing or TOGA but no such thing as changing the speeds etc. That kinda makes sense coz we have both predictive WS and active WS warnings.

[Dan Winterland]

"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! "


The Victor MK2. With inboard engines, there wasn't a huge assymetric effect, although VMCA was a factor in a double assymetric go around as the rudder was quite small. The four speeds we used to calculate before take off were V1, VR, VPAT which was the heavyweight circuit pattern speed and VEPS (V Emergergeny Parachute Stream) - the speed we had to get below after touchdown at takeoff weight before we could deploy the brake chute to prevent shredding it.

The MK1 Victor was operated to military performance which being calculated in the early 1950s was essentially "as is" and was gross performance. The MK2 was the first aircraft to be powered by the RR Conway and as part of the Conway's cetifiction process, the aircraft worked to BCARs net performance, hence the slightly complicated calculations.

[john_tullamarine]

That kinda makes sense coz we have both predictive WS and active WS warnings

.. and limited, finite energy.

If the available energy on the day the procedure is put to the test is not-quite-up-to-it then the alternative procedure with an extra 20kt or so ... might just make the difference between success and failure.

Of course, the particular met circumstances may defeat the Boeing procedural approach as well .. but the latter, for the intermediate circumstance .. offers the better overall risk plan, I suggest.

[9.G]

J.T. we all agree that energy is the key to survival. It well depends what kinda WS hits and at what moment. Too many uncertainties. IAS is meaningsless as speed fluctuations are too big. That's the reason why airbus created the magic tool SRS which keep a min level of energy in case of WS encounter. No need to increase VR Alpha Prot simply doesn't allow airbus to stall in case of full stcik back and that's what we do in case needed. That's pretty much one can do with full thrust. That makes the difference in level of sophistication between airbus and boeing.

[john_tullamarine]

That makes the difference in level of sophistication between airbus and boeing

That may be quite true - I have a background on Boeing only.

However, in the case where the energy balance isn't quite going your way then your statement

That's pretty much all one can do with full thrust (my correction of the presumed typo)

clearly is not correct. The use of an overspeed margin provides more energy to trade and greater energy overall ... with a better probability of survival.

No need to increase VR Alpha Prot simply doesn't allow airbus to stall in case of full stcik back and that's what we do in case needed

No doubt in my mind that that is a wonderful tool. However, the aim of the game is to miss the ground so not stalling is only part of the equation. Presuming that the critical obstacle were not very close in, the extra knots still provide the better option in my mind.

It comes down to a case of risk assessment and management. Nothing wrong with the basic philosophy but the use of a few more knots, especially if the runway is not critically close in obstacle limited .. just might make the difference between a few beers more than usual in the bar that night .. or not needing the beers at all.

[aterpster]

j.t.

It comes down to a case of risk assessment and management. Nothing wrong with the basic philosophy but the use of a few more knots, especially if the runway is not critically close in obstacle limited .. just might make the difference between a few beers more than usual in the bar that night .. or not needing the beers at all.

Such as a takeoff in an L1011 on KJFK's Runway 13R (14,500) going to the other coast. It was raining hard (not TRW, rather a March 'noreasterly) with surface winds gusting to 50 knots and 100 knots reported at 1,500.

A very slight climb until V2 +50 then up into the muck.

[Denti]

@9.G, on the Boeings I've flown it is a mandatory full thrust take off as well if windshear is suspected or reported (which usually means bigger thrust increase as normal take offs usually use thrust reductions greater than 30%), and of course they have the full suite of predictive and active windshear warnings as well. Some of those pws warnings are inhibited during take off though, new caution alerts between 80kts and 400ft AGL, new warning alerts between 100kts and 50ft AGL, existing alerts remain active. Nothing wrong with using that 30% extra thrust on the ground to gather some energy in the form of a few knots more speed before attempting rotation, it just provides a bigger energy margin during the event.

[9.G]

J.T & Denti, I have no problems with extra speed. Any extra energy will be helpful. In practise however, how will you determine that you're now 20 Kts plus if your speed indication all over the place. I presume we're talking bout a real WS here +- 20 kts, at least. I'm not talking bout LHR WS warning as per usual, those're beer shakers. I'm talking bout the once in the far east NRT, TPE, HKG or US east coast. Being within a 50 kts waring zone of a TD, let alone a typhoon you can already pretty much forget your IAS indication. How will you know you're At the point of VR+20 with speeds fluctuations of at least +- 20 kts unless you got so much runway that one can just sit it out and rotate as per arse feeling. J.T. SRS will provide a min climb rate of 300ft/m for WS conditions, if followed the FD. If the WS is so severe, we're talking downburst now, then Aplha Protection will at least keep the a/c alfoat so to speak. Not changing the config is supposed to soften the blow apart from not disturbing the air flow for better lift initialy. In thoery it sounds nice, in practise all depends.

[john_tullamarine]

A very slight climb until V2 +50 then up into the muck.

I'm with you, sir.

In thoery it sounds nice, in practise all depends.

I'm sorry to be a tad obtuse ... olde phart disease at play again, you see ..

If conditions are such that one can't conduct an overspeed takeoff with even a remote degree of safety, then what permits the safe conduct of an on min speed takeoff under the same conditions ?

Something doesn't quite gel. Perhaps you could re-read your post and contemplate the apparent illogic ?

Alternatively, one might perhaps be better served going back to the pub in such conditions and toasting the brave folk who took off ?

[9.G]

Alternatively, one might perhaps be better served going back to the pub in such conditions and toasting the brave folk who took off ?

Well. that's always the best solution, no doubts. I'm not against that extra 20 Kts at all I'm just trying to comprehend how it's supposed to work in reality. I've never seen it or heard of it before.

If conditions are such that one can't conduct an overspeed takeoff with even a remote degree of safety, then what permits the safe conduct of an on min speed takeoff under the same conditions ?

Well, the answer is simple- dynamic of the situation and knowledge of local particularites. NRT for instance can have severe LVL WS on a clear sunny day with anticyclone weather. It's on the ATIS with announced +- 20 Kts speed gain and drop most of the time associated with cross winds and yet the show goes on. So it all depends on when it hits you with what magnitude. Aviation had to be seized in the far east for at least 3 months following the beer is better than WS philosophy yet it goes on.

P.S. I should probably clarify the reasons of being perplexed. While airbus is basing it's entire philosophy on the concept of energy installing sophisticating tools to keep that min level of enegry recognizing the fact that IAS is meaningless in case of WS with the range of total 40 Kts. Boeing solved the problem of kinetic energy by adding 20 Kts to VR. When VR+20 is indicated and rotation is inititated and in the very next instant it's -40 Kts where does it leave one?

[9.G]

just checked 777 manuals:

Consider increasing Vr speed to the performance limited gross weight rotation speed, not to exceed actual gross weight Vr+20 knots. Set V speeds for the actual gross weight. Rotate at the adjusted (higher) rotation speed. This increased rotation speed results in an increased stall margin, and meets takeoff performance requirements. If windshear is encountered at or beyond the actual gross weight Vr, do not attempt to accelerate to the increased Vr, but rotate without hesitation.

That's quite a different statement.

[john_tullamarine]

I'm just trying to comprehend how it's supposed to work in reality

It's all to do with energy (presuming that one has some spare runway in the first place). Think of a bucket of water progressively emptying .. when it's gone, it's gone. A bigger bucket lasts a tad longer .. perhaps just enough to get over that knoll off the end of the runway ?

Well, the answer is simple- dynamic of the situation and knowledge of local particularites

Actually, I think that doesn't mean much at all, does it ?

where does it leave one

Much the same place as with an Airbus, I suggest.

That's quite a different statement

... but quite reasonable and consistent with the general story.


.. methinks perhaps you have been seduced by the razzle dazzle ?

[9.G]

Actually, I think that doesn't mean much at all, does it ?

It does to me coz dependable on what's the origin of the WS and how it's been determined, terrestrial or purely weather induced and how it's been determined based on Pireps or measured, do influence the decision.

but quite reasonable and consistent with the general story.

not if you're taking off at performance limited TOW. Whatcha gonna do
then?

Much the same place as with an Airbus, I suggest.

Perhaps, but on the bus, if at all, V2 is increased as well. One has the option of keeping lower V1 and appying higher VR,V2 but again they're both increased. Though I agree that in case of WS the goal is to stay airborne.

[Wizofoz]

Did you miss this part?

not to exceed actual gross weight Vr+20 knots.

So, if RTOW Vr is more than 20kts greater than ATOW Vr, you may use up to Vr+20- as John has been explaining.

not if you're taking off at performance limited TOW. Whatcha gonna do then?

If you're performance limited, you're performance limited. whether you're in an Airbus or a Boeing.

But if you HAVE extra performance at hand, why not use it?

[9.G]

Wizofoz, whatcha gonna do if your ATOW is equal to RTOW?

If you're performance limited, you're performance limited. whether you're in an Airbus or a Boeing.

which leaves one with the option of full thrust and pitch play to remain airborne.