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landing Perfomance B737NG

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landing Perfomance B737NG

Old 31st Oct 2008, 17:46
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landing Perfomance B737NG

Recently started flying the jet, the QRH says ldg distances are unfactored what exactly does it signify and factors of 1.67/1.43 have to be done or they have been considered? Perfomance on autobrakes show that the aircraft comes to a halt within the laid down distances.

Also can somebody tell me how to command a constant ROD say 2000fpm descent with VNAV path?
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Old 31st Oct 2008, 17:57
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The QRH unfactored distances of the 737 are pure distances from 50 feet above threshold to a complete stop (+/- corrections in case of tail/head wind, reversers inop. etc...). They are in the Performance inflight section and are advisory.The VNAV Path keeps the path between waypoint, rod coming with speeds, if you want a particular rod just use VS, (or Level change with manual throttles;-))FB
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Old 31st Oct 2008, 18:18
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Technically no you don't have to apply the factors to the QRH because the performance is intended to be used in flight, ie after dispatch. The despatch rules do not apply in flight.

The point of VNAV is to give an idle descent from T/D. Not sure why you would want to use VNAV path to descend at a specific ROD, the only way I can think of doing it is to create 2 waypoints with hard altitudes and a set distance apart.

For example, say ground speed of 300kts or 5nm a min, so for 2000fpm RoD, it would take 5 min to loose 10 000ft, in which time you cover 25 nm over the ground. So 2 waypoints seperated vertically by 10 000ft and laterally 25nm would do it. Much easier to use V/S or LVL CHG.
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Old 31st Oct 2008, 20:35
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The disadvantage of V/S and LVL CHG is that any altitude constraints in the route will not be protected. This is particularly important for someone new to type or unfamiliar with company routes etc. An autopilot coupled to VNAV PATH or VNAV SPD will assure compliance. If you are descending in VNAV path and you want to fly a constant ROD for a while, switch to VNAV SPD and over-ride the autothrottles from flight idle into ARM mode. You can then adjust thrust to achieve the required ROD. This is similar to using LVL CHG, except that when a waypoint is sequenced that has an altitude constraint; LVL CHG will bust straight through it. VNAV SPD will revert to VNAV PATH and comply with the restriction.
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Old 31st Oct 2008, 22:52
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See Takeoff / Landing on Wet, Contaminated and Slippery Runways,
Page 118 onwards, which may help.

Knowing the Distance.
Managing Threats and Errors During Approach and Landing.
AC 91-71 Runway Overrun Prevention.
Landing Performance of Large Transport Aeroplane.
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Old 1st Nov 2008, 00:12
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Technically no you don't have to apply the factors to the QRH because the performance is intended to be used in flight, ie after dispatch. The despatch rules do not apply in flight.

Perhaps a very simplistically dangerous philosophy ?

In the situation of an abnormal or emergency, the pilot appropriately needs to have a reasonable set of data available for his/her assessment in resolving the recovery. Indeed, it may be necessary (and appropriate, on occasion) for the recovery to involve reduced margins when compared to the normal certification approach.

Keeping in mind that the distance factors are built in to provide a reasonable probability of successfully completing the flight, it is more appropriate for the pilot to

(a) assess all reasonable options

(b) rank the options according to whatever criteria are pertinent to the particular situation

(c) adopt the most appropriate solution for recovery

In respect of landing distance factors, this might come down to comparing multiple available runways .. one should have a good risk-mitigating reason for deciding to use a runway with a significantly lesser factor. If, of course, there is only one runway available then that part of the solution risk assessment can be bypassed ...

The philosophy can be carried across to considerations of any post launch defect situation .. assess and rank the options prior to making the call. One of the resources you have is the MEL .. if you have time to review the MEL, and you choose not to, then I would prefer NOT to be in your shoes at the eventual enquiry if the recovery turns pear-shaped.

The 1.67 factor applies to heavy aircraft scheduled performance while 1.43 normally is for alternate aerodromes if the particular rules permit the distinction.
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