Reduced Thrust ("flex") takeoff
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Some airports like LCY, a flex take off is allowed by the ops manual, but I would never consider it in the 146 as the time to recognise and act on a subtle failure is too short, and there are a great many obstacles on the climbout to the west. The occupants of Canary Wharf must be worried enough normally, but to see an aircraft staggering towards them at mid tower level would give them the s---s. The captain must make the descision regardless of the bean counters. Are you familiar with the 146, Killer shark?
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Just like to point out that there are airplanes out there that v1/vr/v2 are all about 1 knot apart. And technically, even though the AFM guys are going to throw rocks at me. V1 can equal Vr and Vr can equal V2. Nobody that I know of does that, but if are flying a Heavy run your takeoff calculations at MTOW and MAXimize Vr to clear the 35 ft obstacle. In otherwords, leave V2 what it is now and push Vr as close as possible to V2. If the MAX Vr is more than V2 set V2 equal Vr. This will cause the pilot to track down to V2 +10 instead of trying to accelerate to V2 +10. I like to call it "Flare Recovery".
Dog Tired
john-tullamarine:. .slightly confused about your level of understanding here...or are you taking the mickey? you appear to have knowledge but... the basics of flex vs derate (without trying to write a thesis) are:. .flex defines a thrust level below max at which perf reqmts are satisfied.. .derate defines the max TO thrust rating LIMIT.. .broadly speaking,flex works for less than MTOW,on a half decent rwy and derate works for limiting rwys.. .If, for the same TOW, the max thrust is reduced (derated) the assoc vmcg is decreased therefore V1 is reduced; accel/stop distance is reduced and V2 can still be met.. .you knew all this already didn't you? your remark 'not much at all' was a wind-up wasn't it? <img src="tongue.gif" border="0">
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747focal,
Certainly no problem having V1 = VR if the distance geometry fits but would be surprised to see the usual sort of swept wing jet with VR = V2 due to the significant pitch attitude change required for takeoff. I would expect that the time taken for the rotation flare would usually require VR < V2 if the (normal) aim is to hit V2 at or about the screen OEI.
For prop aircraft, having a comparatively small pitch attitude change, it is not a problem to have VR at or near V2.
I take a contrary view regarding obstacles. Presuming adequate ASDA, far better to push V1 up to minimise the takeoff distance by minimising the period of acceleration (and hence distance) OEI between V1 and VR. In general, VR is chosen to suit the rotation speed increase so as to peg V2 at screen and bears a comparatively fixed relationship to V2.
fantom,
Sorry about that, mate ... yes, I have an appropriate background ... just my strange sense of humour .. I should just stick to being a boring old engineer.
As far as I am concerned, both as a pilot and ops engineer, the only difference of significance to the pilot is that, for a flex takeoff, the availability of a bit more thrust after the failure is there for the taking should the pilot have a reasonable need to do so. . .Of far more importance is adequate training of, and understanding by, the pilots within a particular operation so that they have sufficient knowledge and confidence to decide when it is, and is not, appropriate to use reduced thrust takeoff procedures (over and above the routine exceptions one normally sees in operations manuals).
The position you adopt regarding the V1/VR split is not quite so simple as just looking at Vmcg .. but we are all on the same side .. pass, friend.
Certainly no problem having V1 = VR if the distance geometry fits but would be surprised to see the usual sort of swept wing jet with VR = V2 due to the significant pitch attitude change required for takeoff. I would expect that the time taken for the rotation flare would usually require VR < V2 if the (normal) aim is to hit V2 at or about the screen OEI.
For prop aircraft, having a comparatively small pitch attitude change, it is not a problem to have VR at or near V2.
I take a contrary view regarding obstacles. Presuming adequate ASDA, far better to push V1 up to minimise the takeoff distance by minimising the period of acceleration (and hence distance) OEI between V1 and VR. In general, VR is chosen to suit the rotation speed increase so as to peg V2 at screen and bears a comparatively fixed relationship to V2.
fantom,
Sorry about that, mate ... yes, I have an appropriate background ... just my strange sense of humour .. I should just stick to being a boring old engineer.
As far as I am concerned, both as a pilot and ops engineer, the only difference of significance to the pilot is that, for a flex takeoff, the availability of a bit more thrust after the failure is there for the taking should the pilot have a reasonable need to do so. . .Of far more importance is adequate training of, and understanding by, the pilots within a particular operation so that they have sufficient knowledge and confidence to decide when it is, and is not, appropriate to use reduced thrust takeoff procedures (over and above the routine exceptions one normally sees in operations manuals).
The position you adopt regarding the V1/VR split is not quite so simple as just looking at Vmcg .. but we are all on the same side .. pass, friend.
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JT:. .No afternoon diversion or wind-up for me, just because I work with the stuff doesn't mean I know whether people like it or not (the use of flex, that is)!
Also, "Pedantic" is my middle name.
Croqueteer:. .I have certainly dabbled with 146 flex performance, and it's fair to say I am familiar with it. I would prefer to leave LCY by train or boat. I'm not sure whether I'd prefer to be in Canary Warf looking down at a 146, or in a 146 looking up at Canary Warf (although I'd rather be in a 146 than a Dash-8). Your call!
Also, "Pedantic" is my middle name.
Croqueteer:. .I have certainly dabbled with 146 flex performance, and it's fair to say I am familiar with it. I would prefer to leave LCY by train or boat. I'm not sure whether I'd prefer to be in Canary Warf looking down at a 146, or in a 146 looking up at Canary Warf (although I'd rather be in a 146 than a Dash-8). Your call!
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John T-
Interesting set up. Since I'm in the sim next month and if all parties agree, I'll see if I can set up that situation. The DC-9 is a rudder airplane when engine out and if you get the ailiron/spoilers rocking and rolling the airplane is just waving bye, bye to you.
Interesting set up. Since I'm in the sim next month and if all parties agree, I'll see if I can set up that situation. The DC-9 is a rudder airplane when engine out and if you get the ailiron/spoilers rocking and rolling the airplane is just waving bye, bye to you.
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Killer Shark,
Peace, brother ...
I think that the use of flex thrust has been with us for so long now that it sits reasonably well with the pilot fraternity; although I have a significant concern with the degree to which the thought process seems to have become monkey see, monkey do.
I can recall in the early days of flex ops, Wal Stack at Qantas gave his 707 pilots the carrot of a minimum 1000ft ASD pad .. seemed to quieten any dissent. I suspect that that sort of luxury probably is not the rule .... Then again, early in the piece, one crew observed that the FE pushed the throttles forward to set climb thrust .. so the procedure was amended to make the flex thrust limit not less than climb thrust.
So far as trying to maintain the best margins by using rated, rather than reduced thrust, for takeoff is, I suggest, a spurious logic. Why would we not then leave most of the payload behind ? .. that would increase the performance a whole heap more ... and as for aerodromes any higher than sea level ... and OAT above ISA - 20 .. well !!
The more important considerations are
(a) the boss needs to make a dollar or we are talking about aero club flying.
(b) the relevant limit performance cases are only reference situations ... if there is a sensible case for doing so, use rated thrust, offload some payload ... but be prepared to account for your actions at the Monday morning coffee session in the Chief Pilot's office ..
yxcapt,
Love to hear of your experiences in due course.
The nature of the normal aircraft response is that the serious problems don't really arise unless you are down in the vicinity of Vmca; then the immediate prime concern is controlling bank so that the aircraft doesn't slip into the dead engine... and, of course, it is all shake, rattle, and roll ...
One problem some of the guys experience is the new task of having to co-ordinate rudder and aileron inputs, rather than rudder with only a minor aileron balance for higher speed failures. I have found the best way to get around this problem is to emphasise
(a) control yaw-induced roll aggressively.
(b) the downgoing control horn is on the same side as the appropriate rudder
(c) have an initial guess as to the required rudder input, apply it, and lock the rudders
(d) final adjustment of control input magnitude is an iterative process .. if the rudder inputs are constrained to be step rather than a continuous modulation, then the roller coaster ride is avoided.
All good fun, the guys almost always have a ball, and the stick and rudder polish transfers quite impressively to the routine V1 failure case. Most of the guys are quite surprised at the handling sensitivity with CG ..
[ 06 February 2002: Message edited by: john_tullamarine ]</p>
Peace, brother ...
I think that the use of flex thrust has been with us for so long now that it sits reasonably well with the pilot fraternity; although I have a significant concern with the degree to which the thought process seems to have become monkey see, monkey do.
I can recall in the early days of flex ops, Wal Stack at Qantas gave his 707 pilots the carrot of a minimum 1000ft ASD pad .. seemed to quieten any dissent. I suspect that that sort of luxury probably is not the rule .... Then again, early in the piece, one crew observed that the FE pushed the throttles forward to set climb thrust .. so the procedure was amended to make the flex thrust limit not less than climb thrust.
So far as trying to maintain the best margins by using rated, rather than reduced thrust, for takeoff is, I suggest, a spurious logic. Why would we not then leave most of the payload behind ? .. that would increase the performance a whole heap more ... and as for aerodromes any higher than sea level ... and OAT above ISA - 20 .. well !!
The more important considerations are
(a) the boss needs to make a dollar or we are talking about aero club flying.
(b) the relevant limit performance cases are only reference situations ... if there is a sensible case for doing so, use rated thrust, offload some payload ... but be prepared to account for your actions at the Monday morning coffee session in the Chief Pilot's office ..
yxcapt,
Love to hear of your experiences in due course.
The nature of the normal aircraft response is that the serious problems don't really arise unless you are down in the vicinity of Vmca; then the immediate prime concern is controlling bank so that the aircraft doesn't slip into the dead engine... and, of course, it is all shake, rattle, and roll ...
One problem some of the guys experience is the new task of having to co-ordinate rudder and aileron inputs, rather than rudder with only a minor aileron balance for higher speed failures. I have found the best way to get around this problem is to emphasise
(a) control yaw-induced roll aggressively.
(b) the downgoing control horn is on the same side as the appropriate rudder
(c) have an initial guess as to the required rudder input, apply it, and lock the rudders
(d) final adjustment of control input magnitude is an iterative process .. if the rudder inputs are constrained to be step rather than a continuous modulation, then the roller coaster ride is avoided.
All good fun, the guys almost always have a ball, and the stick and rudder polish transfers quite impressively to the routine V1 failure case. Most of the guys are quite surprised at the handling sensitivity with CG ..
[ 06 February 2002: Message edited by: john_tullamarine ]</p>
Dog Tired
john-t. .having re-read my note to you, it is I who apologise. don't blame me though,blame the castlemaine xxxx I was drinking.......sorry. <img src="tongue.gif" border="0">
