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speed additive are cumulative?

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speed additive are cumulative?

Old 18th Mar 2010, 23:36
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speed additive are cumulative?

According AFM (Hawker 800XP) must add an extra 15 knots speed for approach and landing in icing condition.

Is this speed additive for icing cumulative with that calculated for head wind component?

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Old 19th Mar 2010, 01:56
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Could you give us the exact wording from the AFM?

With some aircraft an abnormal problem (example electrical bus failure, no wing anti ice available) can lead to a speed additive in icing conditions.

But I've never heard it replaces the "half headwind plus full gust to a maximum of 20 kts" additive some manufacturers require.
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Old 19th Mar 2010, 02:20
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Approach Speed additives in the Douglas/Boeing 717 are cumulative but only to a maximum additive of 20 kts.
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Old 19th Mar 2010, 03:15
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Approach Speed additives in the Douglas/Boeing 717 are cumulative but only to a maximum additive of 20 kts.
L1011, likewise.
B707, the same.
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Old 19th Mar 2010, 04:54
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Ditto B737
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Old 19th Mar 2010, 19:07
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The exact wording of the AFM would help, particularly as older 125s used to be designated either A or B variants depending on the regulatory authority. Thus the AFM may use grandfather rights from the older FAR or JAR for approach speed and/or icing certification.

An icing speed additive could be considered as resetting the safety margin from the stall, thus any small increment for wind/gusts etc would be additive, but this might (should) require that the landing distance is corrected. There should be a limiting speed increment as in the examples above.

See Landing Performance. Para 4.2. provides guidance on speed additives (15 kts max) and general information on landing distance.
Also see FAA AC 91-79 Runway Overrun Prevention, but here the speed increase could be interpreted as 20 kts (not all manufacturers would agree), but the document again stresses the importance of considering the effect of speed increase on landing distance.

If the approach is made in icing conditions then perhaps a contaminated or slippery runway would also have to be considered. A ‘fast’ landing in these circumstances may not be acceptable; similarly, any late speed reduction could reduce the stall margin with added risk.
Thus, if flying with icing additive speeds and the conditions require more speed, perhaps the better decision would be to ask ‘should we be doing this’, i.e. divert, and not trying to find a way of continuing.
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Old 20th Mar 2010, 04:09
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I have been involved with icing research and FAA rulemaking on the topic for many years, and I have to say that this question has never come up so far as I am aware. It is a very good question to inject into the discussions within the authorities. I will make some inquiries, but as has been pointed out, the exact wording of the AFM or FCOM is crucial.

In my experience with the 757, a landing with a 20 knot maximum speed additive results in a pretty shallow deck angle, with the resulting derotation being, shall we say, brief. I suspect that this limits the cumulative additive as much as anything.
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Old 20th Mar 2010, 06:30
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With airbus you add 5kts for icing + 1/3 of HW with a total max of 20kts.

The 5kts is to counteract the bleedpenalty for the anti icing rather than for the actual ice accretion.
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Old 20th Mar 2010, 13:45
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Could you please explain the reason behind this "bleed penalty" please? Using bleed air requires a faster approach speed?
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