Headwind additive for autothrottle use.
Join Date: Jun 2006
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Now, If we are manually man-handling the thrust levers we bug the whole headwind and half the gust and then aim for the bug. Later we bleed of some of that but that has already been discussed.
Join Date: Aug 2004
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Additive on a tailwind..
Guys,
Generally speaking...
Why are we adding speeds for HW and/or gust, but with Tailwind we had nothing if we have a steady wind (no gusts of course)?
Speaking on all kinds of planes, not A/T equipped necessarily.
Thanks!!!
Generally speaking...
Why are we adding speeds for HW and/or gust, but with Tailwind we had nothing if we have a steady wind (no gusts of course)?
Speaking on all kinds of planes, not A/T equipped necessarily.
Thanks!!!
Moderator
Why are we adding speeds for HW and/or gust, but with Tailwind we had nothing if we have a steady wind (no gusts of course)?
(a) the presumed friction layer used for certification is a power law (the 1/7th relationship) which describes a commonly observed reduction in steady wind velocity as the height above ground reduces in the absence of obstructions. It is presumed that the steady wind will be reasonably predictable in its behaviour. This relationship typically is the basis for low level wind profiles incorporated into Flight Manual data.
(b) for an headwind, the wind speed is expected to reduce somewhat as the aircraft approaches the runway resulting in an undershoot shear .. hence we carry a margin to give some fat for this. If a tailwind, we would be at double jeopardy if we were to carry a margin as the effect now becomes an overshoot shear situation.
(c) the usual Boeing margin (half the steady headwind) is an attempt to provide some protection against undershoot shear while not exposing the landing to an overly excessive speed entering the flare.
(d) for the gust situation, we presume a randomness about the gust time history so it is more appropriate to allow for all the gust value on the basis that this may appear at any time during the landing.
(e) one recalls that the landing factor is 1.67 dry and this provides a reasonable margin to accommodate a small variation in speeds in the flare. The 20kt limit reflects a reasonable maximum speed delta to fit in with the 1.67 if circumstances on the day conspire to find the aircraft entering the flare with the full additive still present. This has been a long used value both in US and UK certification practices.
An older thread looked at these things in some detail and is probably worth a read in the context of this present thread.
(a) the presumed friction layer used for certification is a power law (the 1/7th relationship) which describes a commonly observed reduction in steady wind velocity as the height above ground reduces in the absence of obstructions. It is presumed that the steady wind will be reasonably predictable in its behaviour. This relationship typically is the basis for low level wind profiles incorporated into Flight Manual data.
(b) for an headwind, the wind speed is expected to reduce somewhat as the aircraft approaches the runway resulting in an undershoot shear .. hence we carry a margin to give some fat for this. If a tailwind, we would be at double jeopardy if we were to carry a margin as the effect now becomes an overshoot shear situation.
(c) the usual Boeing margin (half the steady headwind) is an attempt to provide some protection against undershoot shear while not exposing the landing to an overly excessive speed entering the flare.
(d) for the gust situation, we presume a randomness about the gust time history so it is more appropriate to allow for all the gust value on the basis that this may appear at any time during the landing.
(e) one recalls that the landing factor is 1.67 dry and this provides a reasonable margin to accommodate a small variation in speeds in the flare. The 20kt limit reflects a reasonable maximum speed delta to fit in with the 1.67 if circumstances on the day conspire to find the aircraft entering the flare with the full additive still present. This has been a long used value both in US and UK certification practices.
An older thread looked at these things in some detail and is probably worth a read in the context of this present thread.
Speed additives during an approach with autothrottle engaged depend on aircraft type / system performance; hence refer to the manufacturer’s advice.
The additives may only be required in particular conditions or at certain stages of the approach. The additives provide additional safety protection, but also require consideration of the landing distances required, which if excessive, present an additional and potentially greater hazard to safe operations.
Speed may be bleed off before the threshold enabling ‘standard’ performance to be used, but any speed change may destabilize the A/T system or trim condition – invalidating autoland performance, or result in higher crew workload, or violate stabilized approach criteria.
Boeing provides guidance for increased landing distances due to speed excess (also see AC 91-79), but these data assume manual thrust reduction. Some A/T’s have a variable retard initiation height or rates-of-closure resulting in different flare distances whilst attempting to give a consistent speed loss (normally 7kts). However, Boeing’s landing performance assumes a constant flare distance of 1000ft for manual landing (1200ft for big jets), which with A/T may not be achieved, or 1500-2500 ft for autoland/HUD which may be more realistic for manual landing with A/T engaged.
Thus, when using A/T during a manual landing consider carefully the implications on landing distance and the actual runway conditions, particularly where gusting / cross winds can also add to the actual stopping distance.
JT respectfully disagree with your para (e).
Whilst landing distance factors provide a safety margin for small variations in normal operations, they are not intended to accommodate ‘deliberate’ speed increments.
See AIC 91_06. Note that the ‘15kt margin’ for higher speeds at the threshold refers to the ‘reference’ method of certification which is no longer used; and AFAIK no modern commercial aircraft uses this datum for landing performance.
Also, it should be remembered that the actual safety margin on a wet or contaminated runway is not the same as that for a dry runway even though the certification distance factors are greater. Recent Canadian research suggests distance factors of 2.2 – 2.4 may be required in contaminated conditions to provide the equivalent level of safety as on a dry runway.
“The margin of error for the threshold crossing airspeed is +5/-0 knots. If the pilot has planned to carry additional airspeed beyond the threshold due to gusty surface wind conditions, or other factors, then the effect of this additional airspeed must be included in the actual landing distance”. ( AC 91-71 Runway Overrun Prevention.)
The additives may only be required in particular conditions or at certain stages of the approach. The additives provide additional safety protection, but also require consideration of the landing distances required, which if excessive, present an additional and potentially greater hazard to safe operations.
Speed may be bleed off before the threshold enabling ‘standard’ performance to be used, but any speed change may destabilize the A/T system or trim condition – invalidating autoland performance, or result in higher crew workload, or violate stabilized approach criteria.
Boeing provides guidance for increased landing distances due to speed excess (also see AC 91-79), but these data assume manual thrust reduction. Some A/T’s have a variable retard initiation height or rates-of-closure resulting in different flare distances whilst attempting to give a consistent speed loss (normally 7kts). However, Boeing’s landing performance assumes a constant flare distance of 1000ft for manual landing (1200ft for big jets), which with A/T may not be achieved, or 1500-2500 ft for autoland/HUD which may be more realistic for manual landing with A/T engaged.
Thus, when using A/T during a manual landing consider carefully the implications on landing distance and the actual runway conditions, particularly where gusting / cross winds can also add to the actual stopping distance.
JT respectfully disagree with your para (e).
Whilst landing distance factors provide a safety margin for small variations in normal operations, they are not intended to accommodate ‘deliberate’ speed increments.
See AIC 91_06. Note that the ‘15kt margin’ for higher speeds at the threshold refers to the ‘reference’ method of certification which is no longer used; and AFAIK no modern commercial aircraft uses this datum for landing performance.
Also, it should be remembered that the actual safety margin on a wet or contaminated runway is not the same as that for a dry runway even though the certification distance factors are greater. Recent Canadian research suggests distance factors of 2.2 – 2.4 may be required in contaminated conditions to provide the equivalent level of safety as on a dry runway.
“The margin of error for the threshold crossing airspeed is +5/-0 knots. If the pilot has planned to carry additional airspeed beyond the threshold due to gusty surface wind conditions, or other factors, then the effect of this additional airspeed must be included in the actual landing distance”. ( AC 91-71 Runway Overrun Prevention.)
Moderator
I am quite comfortable that my comments are compatible with both circulars.
Consider that
(a) the Boeing 20kt max additive is for the approach and is not intended to be carried into the flare
(b) the factored W/V built into the AFM numbers provides a significant margin
(c) entry into the flare significantly outside "normal" parameters is adequate justification for a missed approach unless the excess runway is significant.
(d) there is implicit an understanding that the pilot will consider all factors in determining just how critically he/she views the maintenance of profile etc. and, in particular, touchdown zone and retardation effort.
We can, of course, continue to agree to differ.
Consider that
(a) the Boeing 20kt max additive is for the approach and is not intended to be carried into the flare
(b) the factored W/V built into the AFM numbers provides a significant margin
(c) entry into the flare significantly outside "normal" parameters is adequate justification for a missed approach unless the excess runway is significant.
(d) there is implicit an understanding that the pilot will consider all factors in determining just how critically he/she views the maintenance of profile etc. and, in particular, touchdown zone and retardation effort.
We can, of course, continue to agree to differ.
