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Boeing 737 VR speed question
Hi All,
Say I have the example (demo) base line data; B737-800, speeds V1 137 VR 138 V2 142 Based on the windshear from FCOM SP section and FCTM gusting crosswind. "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" "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. Avoid rotation during a gust. If a gust is experienced near VR, as indicated by stagnant airspeed or rapid airspeed acceleration, momentarily delay rotation." Can people please explain from a practical element based on the speeds I have given for the example how VR would change if you were going to conduct operations on the aircraft if needed (I know from a practical point of view no one would take off from known windshear!!) What does it mean to say "Consider increasing Vr speed to the performance limited gross weight rotation speed, not to exceed actual gross weight Vr + 20 knots" and, "Rotate at the adjusted (higher) rotation speed." Does this mean your VR based on this example would be 158 kts IE at planning from the RTOW chart??? for both of the above OR you use perf data for VR 138 kts and if you encounter the event you use the new speed of 158 kts? Also "consider increasing VR speed to perf limited gross weight"? What does this mean in plain english???? Thanks. |
Let’s say your Vr at your actual weight was 138, but the runway could support a higher TOW of 174k with a resulting Vr of 160. But, max rotation would be 138+20 or 158. So rotate at 158 unless you get wind shear before that speed. |
Originally Posted by ImbracableCrunk
(Post 10266355)
Let’s say your Vr at your actual weight was 138, but the runway could support a higher TOW of 174k with a resulting Vr of 160. But, max rotation would be 138+20 or 158. So rotate at 158 unless you get wind shear before that speed. ImbracableCrunk is furthermore correct. Simply said: - determine maximum TOW and Vr speed for ambient conditions - determine actual TOW and associated speeds for ambient conditions - use no more than. +20kts above actual Vr for adjusted Vr but no more than the Vr for maximum weight |
A few thoughts ...
Consider increasing Vr speed to the performance limited gross weight rotation speed, not to exceed actual gross weight Vr + 20 knots. In shear/gusting conditions, a bit extra is for pilot comfort, improved climb gradient capability, and increased stall margins. What you are doing is trading the longer runway weight capability to achieve a modified overspeed takeoff. Overspeed increase generally is limited to around 20 kts to get the benefit of the improved climb gradient without too much distance increase .. above this, the gradient improvement is likely to taper off progressively while the distance increase becomes comparatively dramatic. Keep in mind that the climb gradient graph is a bit like an upturned teacup. Set V speeds for the actual gross weight. You're looking at a dynamic met situation with a goodly chance that things might cause you to need to rotate earlier than at the planned overspeed Vr. Bugging the base speeds means you have the minimum speed schedule for the weights right there in front of you should things get untidy. Rotate at the adjusted (higher) rotation speed. ie, if things go fine, you will do something akin to an overspeed takeoff 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 To do otherwise puts you in the lap of the Gods as you have no quantitative idea of what the real acceleration is ... Avoid rotation during a gust. If a gust is experienced near VR, as indicated by stagnant airspeed or rapid airspeed acceleration, momentarily delay rotation Again, you don't have any idea quite what the weather gods might be doing for the short term. On the other hand, if you have pushed it up to a limiting overspeed case, one wouldn't want to delay to the extent where you might go off the end of the runway. However, keep in mind that, if you don't lose a motor, then you have a distance factor on top of the expected certification AEO distance so, probably, it's not all bad. ... and if you encounter the event you use the new speed of 158 kts? Not quite. You're planning and intending on using the higher speed .. UNLESS things get messy. ... consider increasing VR speed to perf limited gross weight"? What does this mean in plain english???? As above, in effect you are now running with an overspeed schedule for the continued case while maintaining the lower scheduled V1 for the reject case. |
In practical terms this method is a horses rear end... too much confusion about what should happen and when, I’m my opinion. If if you are concerned about the shear, either don’t depart, or a much more simple method, without all the flightdeck ambiguity at a critical time, is to just use full rated thrust and improved climb data. It will give you all of the extra performance, without the extra brain ache.... Just my 2 cents |
Always thought this was a dubious practice So you stay on the ground beyond the computed VR building speed to assist with any wind shear encounter But what if you encounter a major wind shear while you’re still trundling down the runway ? If it’s significant enough you could be in a situation where you don’t have enough airspeed to lift off but your groundspeed is too high to stop if you unwisely chose to attempt to I’d rather be in the air (where aircraft are meant to be) with speed and some altitude and go straight into the windshear recovery maneuver, your odds are better |
Stilton,
I thought JT covered your para 2 & 3. Q: But what if you encounter a major wind shear while you’re still trundling down the runway ? A: you reject the t/o and that shouldn't be a problem as your new "W/Shear" V1 is still within the performance limits of the a/c. "If it’s significant enough you could be in a situation where you don’t have enough airspeed to lift off but your groundspeed is too high to stop if you unwisely chose to attempt to" How so? If you encounter w/shear on the t/o prior to V1 your rejection should solve the problem, unless the shear has affected acceleration and you've not picked this up thus resulting in an overrun. "I’d rather be in the air (where aircraft are meant to be) with speed and some altitude and go straight into the windshear recovery maneuver, your odds are better" Not sure about that! Plenty of w/shear accident history shows you may not be able to outclimb it. If you rejected close to V1 and overrun the runway I'd say there'd be less loss of life than getting airborne and hitting the ground some kms from the airport. But, as someone earlier said if there's already been reports of w/shear best choice is to delay departure. Let others be the pioneers if they wish! |
Originally Posted by stilton
(Post 10267095)
Always thought this was a dubious practice So you stay on the ground beyond the computed VR building speed to assist with any wind shear encounter But what if you encounter a major wind shear while you’re still trundling down the runway ? If it’s significant enough you could be in a situation where you don’t have enough airspeed to lift off but your groundspeed is too high to stop if you unwisely chose to attempt to I’d rather be in the air (where aircraft are meant to be) with speed and some altitude and go straight into the windshear recovery maneuver, your odds are better |
Originally Posted by exfocx
(Post 10267756)
Stilton,
I thought JT covered your para 2 & 3. Q: But what if you encounter a major wind shear while you’re still trundling down the runway ? A: you reject the t/o and that shouldn't be a problem as your new "W/Shear" V1 is still within the performance limits of the a/c. "If it’s significant enough you could be in a situation where you don’t have enough airspeed to lift off but your groundspeed is too high to stop if you unwisely chose to attempt to" How so? If you encounter w/shear on the t/o prior to V1 your rejection should solve the problem, unless the shear has affected acceleration and you've not picked this up thus resulting in an overrun. "I’d rather be in the air (where aircraft are meant to be) with speed and some altitude and go straight into the windshear recovery maneuver, your odds are better" Not sure about that! Plenty of w/shear accident history shows you may not be able to outclimb it. If you rejected close to V1 and overrun the runway I'd say there'd be less loss of life than getting airborne and hitting the ground some kms from the airport. But, as someone earlier said if there's already been reports of w/shear best choice is to delay departure. Let others be the pioneers if they wish! Point missed If you encounter a significant windshear at your increased VR you may well not be able to stop So you are still on the ground, past this ’artificial VR’ with a big loss of airspeed potentially enough to prevent you from lifting off but your groundspeed is too high to stop Thats the problem, and why I would never use this procedure If it’s too bad, delay departure, otherwise use normal speeds and fly the windshear escape maneuver if necessary |
Originally Posted by stilton
(Post 10267956)
Point missed If you encounter a significant windshear at your increased VR you may well not be able to stop So you are still on the ground, past this ’artificial VR’ with a big loss of airspeed potentially enough to prevent you from lifting off but your groundspeed is too high to stop Thats the problem, and why I would never use this procedure If it’s too bad, delay departure, otherwise use normal speeds and fly the windshear escape maneuver if necessary |
My point is I’d rather be airborne with speed in hand and altitude beneath me This gives me two energy options to trade if I encounter windshear I have some altitude to trade in case of sink and some speed that can be bled off in the escape maneuver Building up excess speed past normal VR on the runway only gives you one of these options and if the windshear is strong enough you are still on the runway and have lost any additional energy gained Youve lost any advantage, in fact you’re worse off than adhering to your normal VR |
Originally Posted by stilton
(Post 10268385)
My point is I’d rather be airborne with speed in hand and altitude beneath me This gives me two energy options to trade if I encounter windshear I have some altitude to trade in case of sink and some speed that can be bled off in the escape maneuver Building up excess speed past normal VR on the runway only gives you one of these options and if the windshear is strong enough you are still on the runway and have lost any additional energy gained Youve lost any advantage, in fact you’re worse off than adhering to your normal VR |
Stilton,
I think you missed the point. If you abort at V1 after you decide you have encountered w/shear, why would you overrun? You have used a higher performance gross wt V1 which is still valid for the conditions i.e. you can still depart at that wt. If you abort at that V1 you should still stop in the available distance and if you say no to that, please explain your reasoning. You suggest that getting airborne in known w/shear conditions is a better option, yet history says otherwise. "Youve lost any advantage, in fact you’re worse off than adhering to your normal VR" Really, so you know more than Boeing and Airbus on this? " So you are still on the ground, past this ’artificial VR’ with a big loss of airspeed potentially enough to prevent you from lifting off but your groundspeed is too high to stop Thats the problem, and why I would never use this procedure " Again you go back to this without a skerrick of evidence to support it. An abort at V1 will result in you stopping within the remaining distance, unless you stuff it up or, as I said previously, you have suffered an acceleration deficit, which you have not recognised and, thus your abort, while at V1, has been delayed resulting in more runway being used. But, if you can screw up an abort I'm guessing that the w/shear escape procedure is not going to be optimal either and where will you end up there? 3-5 km down the takeoff path and in amongst buildings etc compared to a less than optimal abort and a slow speed overrun! I know which I will result in the lower fatality. Your belief in your piloting skill to overcome w/shear is nothing short of amazing. |
Originally Posted by sonicbum
(Post 10268526)
If You rotate at Your RTOW Vr rather than at Your actual GW Vr You will simply delay the point in space at which You get airborne but You will do so with some extra speed that will potentially be useful if, in the first few hundred feet You encounter WS such that You are likely to lose 15/20 kt, let's imagine a strong tailwind component kicking in at 300-400 ft as a result of local wind phenomena (katabatic winds or so). Anyway as You well know any WS that is likely to be induced by TS activity around the field should be avoided in first place by sitting on the ground till conditions improve.
I agree with your specific example but that’s not the scenario I described In other words, what if that useful extra speed is gone ? If you are still on the runway AT this RTOW VR when you encounter a significant airspeed loss due to windshear, you could well lose all that energy you have gained by staying on the ground longer So you’re past the point you can reject and still on the ground with no choice but to lift off into a windshear At this point you have no advantage, no additional airspeed and of course no altitude |
Originally Posted by TangoAlphad
(Post 10268757)
You could argue you hit that same fairly severe windsheer while airbourne, probably pretty low down at a lower airspeed.
You are past V1 in both examples so you are going to end up flying. In the increased Vr example yes you have less potential energy by being lower but in the early rotate you have less kinetic energy to play with now airbourne. |
Originally Posted by exfocx
(Post 10268571)
You have used a higher performance gross wt V1 which is still valid for the conditions i.e. you can still depart at that wt. If you abort at that V1 you should still stop in the available distance and if you say no to that, please explain your reasoning.
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