Vapp in gusty conditions
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Aside: yes, flying manually with autothrust engaged is calling for trouble.
Yes, thread drift, but what is the reasoning behind manual flying/manual throttles. It is a limitation, not the way we train or fly on the GLEX. The A/T move, so you know exactly what the throttles are doing, if you need to takeover, one click and you have control instantly. The Navy even flies in that configuration for shipboard traps.
GF
GF
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GF,
I believe that Boeing, at least, has a concern about manually responding to autothrust induced pitch changes in the case of under wing mounted engines. This may be why the 717 FCOM allows autothrottle use when in manual flight; on the other hand, it may be an artifact from Douglas that has not changed.
Boeing is pretty clear about their preference for manual throttle during manual flight on the 767/757. That said, none of the operators that I have flown the 767/757 for actually adhere to that; typically, the autothrottle remains engaged until quite a bit closer to the runway than the autopilot. The concept that I was trained with was simply that if your throttle hand moves forward, then your flying hand moves forward, and vice versa.
The Boeing explanation of eliminating speed additives during autothrottle use is that the autothrottle system is very aggressive when increasing thrust in response to a speed decay, while it is rather lazy about reducing thrust in response to a speed increase. Therefore, it will operate conservatively in dynamic situations, resulting in an average speed somewhat on the high side of bug.
I believe that Boeing, at least, has a concern about manually responding to autothrust induced pitch changes in the case of under wing mounted engines. This may be why the 717 FCOM allows autothrottle use when in manual flight; on the other hand, it may be an artifact from Douglas that has not changed.
Boeing is pretty clear about their preference for manual throttle during manual flight on the 767/757. That said, none of the operators that I have flown the 767/757 for actually adhere to that; typically, the autothrottle remains engaged until quite a bit closer to the runway than the autopilot. The concept that I was trained with was simply that if your throttle hand moves forward, then your flying hand moves forward, and vice versa.
The Boeing explanation of eliminating speed additives during autothrottle use is that the autothrottle system is very aggressive when increasing thrust in response to a speed decay, while it is rather lazy about reducing thrust in response to a speed increase. Therefore, it will operate conservatively in dynamic situations, resulting in an average speed somewhat on the high side of bug.
I only rode the jump on EAL B757s and they used the auto throttles most of the way down final, even at KDCA visual to 18 down the Potomac.
GF
GF
Certainly, when performing an autoland with autothrottles engaged of course
no speed additives are necessary on the 75/67
However the autothrottles are not approved for use to touchdown while flying manually. Unlike an Autoland they will not go to idle in the flare, they will stay in speed mode attempting to maintain the MCP speed.
During a manually flown approach, even with autothrottles engaged until close in, the usual, half the steady and all the gust additive must be added to target speed.
no speed additives are necessary on the 75/67
However the autothrottles are not approved for use to touchdown while flying manually. Unlike an Autoland they will not go to idle in the flare, they will stay in speed mode attempting to maintain the MCP speed.
During a manually flown approach, even with autothrottles engaged until close in, the usual, half the steady and all the gust additive must be added to target speed.
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Different corrections possible
Although not required by regulations, from a safety point of view it is wise to make corrections to the approach speed in case of strong gusty wind conditions to avoid reductions in stall margins. The speed correction given by 1/2 headwind + gusts with a maximum of 20 kt. is often advised by aircraft manufactures such as Boeing, Fokker, and McDonald Douglas. However, it should be realised that it is not an official requirement. It is not part of the aircraft certification (unlike stated in one thread). It is left to the operator to use this or not. Also there are another corrections used. For instance Airbus advises the following wind corrections depending on the aircraft model:
· A320/A330/A340 series: 1/3 of the headwind component (excluding the gust), limited to 15 kt;
· A310/A300-600: 1/3 of the tower average wind or the gust increment, whichever is higher, limited to 15 kt;
Another method is the one recommended by Bombardier and Cessna which only adds half of the reported gust to the approach speed.
· A320/A330/A340 series: 1/3 of the headwind component (excluding the gust), limited to 15 kt;
· A310/A300-600: 1/3 of the tower average wind or the gust increment, whichever is higher, limited to 15 kt;
Another method is the one recommended by Bombardier and Cessna which only adds half of the reported gust to the approach speed.
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These various techniques/recommended practices have, as their intent, assessment of three problems -
(a) the steady wind is expected to reduce as the aircraft gets nearer to the ground. The typical presumed profile is the one-seventh power law flight test/certification boundary layer equation
speed(1)/speed(2) = (height(1)/height(2))^(1/7)
or some other empirically derived relationship. Typically, the atmospheric boundary layer is considered to be appropriate below, say, 2000ft and provides a useful means of estimating speeds at heights different to the measuring station
(b) gusts are random and may bite you at any stage during the approach and landing
(c) come over the fence increasingly too fast for the runway and conditions and you progressively increase the risk (probability) of an overrun
What's the answer ? Good question ..
The recommended practices are intended to provide some guidance on how to manage the three considerations. None is precise, none is intrinsically more generally "correct" than another .. but all are usefully helpful in the real world.
(a) the steady wind is expected to reduce as the aircraft gets nearer to the ground. The typical presumed profile is the one-seventh power law flight test/certification boundary layer equation
speed(1)/speed(2) = (height(1)/height(2))^(1/7)
or some other empirically derived relationship. Typically, the atmospheric boundary layer is considered to be appropriate below, say, 2000ft and provides a useful means of estimating speeds at heights different to the measuring station
(b) gusts are random and may bite you at any stage during the approach and landing
(c) come over the fence increasingly too fast for the runway and conditions and you progressively increase the risk (probability) of an overrun
What's the answer ? Good question ..
The recommended practices are intended to provide some guidance on how to manage the three considerations. None is precise, none is intrinsically more generally "correct" than another .. but all are usefully helpful in the real world.
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Stilton is correct...I should have clarified that in my post.
For those who have not already seen them, NLR has done some pretty good work on this topic, and I recommend reading their crosswind papers:
http://www.nlr.nl/id~4382/l~en.pdf
http://www.nlr.nl/id~5114/lang~en.pdf
For those who have not already seen them, NLR has done some pretty good work on this topic, and I recommend reading their crosswind papers:
http://www.nlr.nl/id~4382/l~en.pdf
http://www.nlr.nl/id~5114/lang~en.pdf
