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B737 LAND ALT

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B737 LAND ALT

Old 9th Oct 2019, 13:28
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B737 LAND ALT

Dear colleagues,

question regarding B737 correct procedure for setting LAND LAT during normal prefight preparation.

I am looking what "Field Altitude" should be selected?
For example, that in case of destination airfield elevation is 768ft, what LAND ALT should be correctly selected? 750ft or 800ft. I have experience whit different opinions regarding this, rounded to nearest number or round to first higher alt. on the knob.

I would appreciate link to the manual regarding correct number or in which manual I can find correct answer.

Many thanks, regards guys
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Old 9th Oct 2019, 14:41
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Nearest altitude works as high precision is not necessary for the system to work properly. Since there can be substantial elevation differences across a particular airfield, the published field elevation may actually be different from the touchdown elevation. Still works fine.
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Old 9th Oct 2019, 14:57
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The descent mode is activated when the airplane descends 0.25 psi below the
selected FLT ALT. The cabin begins a proportional descent to slightly below the
selected LAND ALT. The controller programs the cabin to land slightly
pressurized so that rapid changes in altitude during approach result in minimum
cabin pressure changes.
While taxiing in, the controller drives the outflow valve slowly to the full open
position depressurizing the cabin.
However my (old) FCOM says also:
LAND ALT (landing or destination airport altitude).
With regard to the tech explanation it makes more sense IMHO to dial in the rwy elevation.... but +/- 25 ft, who cares?
Rgds

Ah, Tomaski was faster when I was typing

Last edited by gearlever; 9th Oct 2019 at 15:23. Reason: Typing too late
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Old 9th Oct 2019, 16:07
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Originally Posted by gearlever
With regard to the tech explanation it makes more sense IMHO to dial in the rwy elevation.... but +/- 25 ft, who cares?
Rgds
The procedure is to set it during preflight, so deciding on a runway in use so early in the process is not practical.

My airline's procedure is to "set field elevation" but does not specify whether to round up or just closest.
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