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ISA Deviation On FMC Forecast Page  
User currently offlinesmartt1982 From United Kingdom, joined Nov 2007, 225 posts, RR: 0
Posted (4 years 7 months 6 days 13 hours ago) and read 5660 times:

Hi,
Regarding the Smiths FMC mainly used on 737s. On the decent forecast page there is an option to put the QNH of destination and ISA deviation. In relation to the ISA deviation, is this on the ground at your intended destination or as I have also been told, an average ISA deviation figure for were you currently are ie in the cruise at FL360etc (deviation from ISA atmosphere of -56.5)

Also, in relation to the Vref speed that airliners fly on approach, in the Boeing manuals for the 737 NG it says if the approach is planned to be flown manually at any stage add a 5 knot additive onto your vref, even if it is calm, not considering headwind/gust added factor here. As per company protocol we do this but a point that has come to light is when is this extra 5 knots bleed off? as recent training documentation has stated that any extra speed above vref crossing the threshold will result in using more runway than was planned or possibly available.

I am speaking about the 800 series here but would like to other views on all other types, is this a type specific thing and either way what is the reason being this extra 5 knots even when it is calm as we do this even when planning a CAT 3 autoland, is it anything to do with auto throttle not correcting quickly enough?. Is it a common thing for all airliner types/companies? since I have also heard some planes touch down at a speed below vref.

Many Thanks
steve

1 replies: All unread, jump to last
 
User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 1, posted (4 years 7 months 6 days 5 hours ago) and read 5626 times:

Quoting smartt1982 (Thread starter):
I am speaking about the 800 series here but would like to other views on all other types, is this a type specific thing and either way what is the reason being this extra 5 knots even when it is calm as we do this even when planning a CAT 3 autoland, is it anything to do with auto throttle not correcting quickly enough?.

FAR's required that the autothrottle never take you below Vref. The autothrottle on the 737 can be +/- 5 knots, so Vref+5 means that you won't drop below Vref. The procedure stays the same with a non-autothrottle takeoff so you have a consistent procedure. After all, approach speed shouldn't depend on who is controlling your speed.

Quoting smartt1982 (Thread starter):
Is it a common thing for all airliner types/companies? since I have also heard some planes touch down at a speed below vref.

Standard procedure on all Puget-Sound-heritage Boeing's that I'm aware of. Given that you cross the threshold at Vref+, it's normal to touchdown slower, it just depends how much speed you bleed off in the flare.

Tom.


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