Question for 737 CL or NG Pilots

[Jim_12]

Recently had this discussion with a colleague the other day in a 737CL. While on a LNAV/VNAV APP approx 15nm or so out generally in the fmc des page it shows 210/flaps and the command bug starts to scale back to 210 knots I have always started to configure the aircraft at this point and it seems to work great rarely with any excess energy to bleed off or an unstable approach. However the person I was flying with insisted otherwise and chose to start extending flaps approx 10 nm from the runway like he had always done in the 200 series nearly every approach he was struggling to stay on profile and bleed off excess airspeed. My question to the 737 guru's when the fmc starts to command deceleration to maintain 210 knots and shows 210/flaps is the logic there to tell you that is when you should begin configuring to the next flap setting in order to remain on the predetermined VNAV path?

[RVF750]

Trying to stay to fast too late is bad news on the bigger NGs. You simply cannot get away with it comfortably on an -800!!


The FMC is quite conservative in slowing and will gradually work it's way back to the UP speed, not 210 itself. Once you pop Flap 1 in it reduces to the 1 speed and continues this all the way to REF +5 or more if you've put a wind margin in the FMC APP REF page.


Rushing is just unnecessary stress though.....

[biggles61]

We start selecting flap at the next flap manoeuvring speed plus 20 knots or lower to reduce wear on the flap tracks e.g. Flaps up 210, select flap 1 at <230
Flap 1 190, select flap 5 at < 210. The NG is more slippery than the classic especially at high weights so speedbrake is sometimes required, and if all else fails use the gear.

[BOAC]

What on earth is the point of selecting an LNAV/VNAV profile if you are not going to follow the profile? Why not just select LNAV/ILS/VOR/NDB or, heaven forbid, fly the **** aeroplane? Has the world gone mad?

[c100driver]

Check the LEGS page and you will see that the FMC actually wants a speed lower that 210, it will probably be 170 or 150 knots.

The descent page will have 210/FLAPS because it cannot bring the speed back below the Flaps UP speed. The FMC descent plan has a slow down for the next flap setting. If you hold onto 210 and delay the Flap you keep excess energy above the FMC calculated PATH.

In Approach mode the FMC has calculated a descent path built from the runway threshold backwards. It is a glide path. The aircraft will drive to follow the GP, With any excess energy carried into the approach plus putting out the flap "late" the aircraft is now fast and trying to follow the GP so will be a struggle to make it work.

The options are select Flaps as soon as the Command Airspeed Bug gets to 210 and accept a slower approach. Or just disconnect, ignore the PATH and look out the window and fly her like a Legacy B737.

Either way works.

[pudoc]

If you are following the VNAV path and if the FMC as calculated to have Flaps 1 at that point and your on path with flaps up, you are actually not on profile and are most like high/fast.

If you want to delay flaps as late as possible (10nm in this case), then don't use VNAV for the approach.