Question on flaps

[sensfan2014]

When I am descending I am using the flap and speed numbers on the plane, but when I watch the videos , most of  the people are using the flaps at much lower speeds:

and I have a second question , when using vnav to descend on many of the lower altitude descents ; the plane speeds up very high to make it to the next leg altitude but can’t get near the speed on the fmc. Is there a trick when using vnav 

[mfor]

Well those numbers refer to the maximum flap speeds - you can deploy the flaps at those speeds if you need the drag, but in most cases you'd deploy them later.

As long as you do not bust any speed restriction (ATC or Vspeed) I think it's fine. The FMC target speed on descend is kind of a lower limit, i.e. the AT will increase the throttle if the speed falls  below that speed minus 10 knots.

You can use the AP in speed/vs mode though to plan your descend manually - which is not unheard of in real life as well, especially since ATC will give you vectors and possibly altitudes that deviate from a preplanned STAR approach every now and then.

[Iain]

Vspeeds, Vref and Flap speeds are based on the weight of the aircraft.  Flap limit speeds are based on Vref (landing speed for your current weight + xx kts).

The reference table is in the cockpit, above your head to the left, the yellow stamp.  Something like this

Landing Speeds for the 737-300/400/500
Landing Weight		737-300 20K				737-400 23.5K				737-500 18.5K
		Flaps				Flaps				Flaps
/1000kg		40	30	15		40	30	15		40	30	15
70						155	159	177
65		152	153	165		149	154	171
60		145	147	158		143	147	164		140	144	154
55		138	141	151		137	141	156		134	139	148
50		131	134	144		130	134	149		128	133	141
45		123	127	136		124	127	141		122	125	135
40		115	119	128		116	119	132		114	117	125
35		107	111	119		109	111	123		107	109	116

 

 

It is generally not good practice to to use flaps near their max speeds to help with slowing the aircraft down, that is putting undue stress on the flaps and the flap mechanics, although flaps 10 is a setting not used for much apart from being able to keep your speed down without flying around with the gear down.

VNAV has to be used with caution, especially at low altitude.  You statement is a bit confusing "speeds up high, but can't get near the speed on the fmc".  Do you mean your IAS will not reach the speed stated on the LEGS page of the FMC?  Or you have excessive vertical speeds to maintain the MCP speed (a more common problem).  VNAV with follow the vertical profile in the FMC, but if you have no height restrictions, and have an alt set in mcp, VNAV simply sets idle thrust and pitches to hold speed, which can result in a high vertical speed. In this case, I would recommend using vertical speed mode and monitor the green arc in the ND to determine when you will reach you target altitude, adjust your vertical speed accordingly, if you can't descend without your speed increasing, then best to slow down more before starting descent, or use some speed brake.  Personally, I stop using VNAV once below about 6000 - 8000ft due to the possible high vertical speeds.

[sensfan2014]

Thanks , that helps 

[Hullu-poika]

Hello!

I have also a question regarding flaps. When I set flaps 1 from 0 I have a pitch down tendency in opposite to other flap settings. I could not see this on other 737 models and could not recognize real behaviour wathcing videos from real flights. How is it going in a real life? Shall flap 1 setting to produce pitch down and sink the aircraft a little? It feels strange for me...

And second question is about systems. We have a feature of high negative battery current indication during APU start. But shall it work the same way if ground power supply is connected?

[Litjan]

1.)Extending the flaps to 1 is generally enlarging the wing area to the rear with very little curvature added. That is why the center of lift is moving backwards - producing a nose-down momentum. It is actually quite pronounced in the real aircraft. If you don´t counteract the nose-down trim shift the nose will actually go down and the aircraft will loose lift as a consequence.

2.) When you start the APU, the generator buses (which are powered by the ground power) are not used - only the battery can produce enough current to start the APU. Thats why you see the high discharge and also why all the lights connected to the battery bus will dim during the APU start - even with ground power connected.

Cheers, Jan

Edited December 25, 2019 by Litjan

[Hullu-poika]

Thanks Litjan! Answers are very precise and clear as usual!