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View Full Version : B737 Series Flap/Speed selection policy different to FCTM procedure


Centaurus
22nd Aug 2018, 08:58
Re flap/speed selection for the B737 Series. Boeing FCTM discusses extension and retraction of flap policy. Assume you are flying at current airspeed 210 knots clean with AP and AT engaged. Time has come to configure for landing..
FCTM states depending on weight, select flap 1 and immediately wind MCP speed back to 190 knots. Approaching 190 knots select flap 5 and wind MCP speed back to 170 knots which depending on weight is manoeuvring speed for flap 5. Then gear down and select Flap 15 and wind MCP back to 150 knots. Finally, select flap 30/40 and wind back MCP to Vref + 5.+ additives as appropriate. Similar process for the 737NG I presume?

One particular Asian operator has introduced a different procedure for the Classic and the 737-800 and 900. In each case, when selecting flap in stages as described above, their SOP is not to select the manoeuvring airspeed for the new flap setting until the flaps reach the new setting. For example if the aircraft is at Flap 15 and 150 knots and landing flap is called for, the PM selects 30 or 40 and waits until the flaps have stopped moving. Once the flaps have stopped at the required setting the SOP is to then adjust the MCP speed at pilots leisure to VEF 30 or 40 plus additives.
Their rationale is that if an asymmetric flap occurs during extension, it is best to have the higher speed for controllability reasons rather than the lower speed for the planned for final setting.

Apologies for the rather convoluted explanation, but is this an approved Boeing procedure (i.e. don't set the final MCP speed until the flaps have stopped operating)? Or is it a company initiated procedure, rightly or wrongly, designed to pre-empt a potential flap non-normal? How valid is that company procedure and is it widely used by other Boeing 737 operators even though not mentioned in the FCTM?

Stan Woolley
22nd Aug 2018, 10:12
Which mode is being used?
If I remember correctly when VNAV is being used the bug automatically reduces to the speed when the new setting is made on the flap handle, so I don’t see a reason for changing it when using FLCH or some other mode.
Probably somebody with a bored brain that too big for its own good flexing their ego, on the other hand it’s possible that there might be a good reason for doing so that I’m not aware of.

BluSdUp
22nd Aug 2018, 10:32
Unless You want flapload relief on a regular basis.
The 737 is a glider on a CDA approach as I assume everyone are doing now..
The deceleration to speed selected is so gentle and slow that there is plenty of time to reverse the selection up to a higher speed if there is an issue.
I would strongly advise against outsmarting Boeing.
An operator I know for a long time started off with massively modifying the new 737 -800 operation to fit the old 737- 200 SOP.
It almost ended in tears many times until it sunk in that Boeing actually knew what they were doing.
It did not help that the local CAA had no clue, and resisted a few changes.
Re-inventing the wheel or the picture I like : Re-inventing the Dynomite, Nobel`s brother tried that . And promptly blew himself up.

Happy Landings
Cpt B

DogSpew
27th Aug 2018, 09:02
This method of setting the command speed is ONLY applicable to Flap Extension using the Alternate System.

The 737 FCTM states: "When extending the flaps using the alternate system, the recommended method for setting command speed differs from the method used during normal flap extension. Since the flaps extend more slowly when using the alternate system, it is recommended that the crew delay setting the new command speed until the flaps reach the selected position. This method may prevent the crew from inadvertently getting into a low airspeed condition if attention to airspeed is diverted while accomplishing other duties."

There is nothing here that talks about flap asymmetry, or other such nonsense.

Judd
27th Aug 2018, 14:52
There is nothing here that talks about flap asymmetry, or other such nonsense

Actually it is an SOP with a certain Chinese operator.

D-OCHO
2nd Sep 2018, 21:19
If I remember correctly when VNAV is being used the bug automatically reduces to the speed when the new setting is made on the flap handle
If you would have looked correctly.
VNAV reduces the speed in reference to the actual flap setting. For example when the flap is half way between 2 settings the speed is also half way.

Car RAMROD
3rd Sep 2018, 00:36
FCTM states depending on weight, select flap 1 and immediately wind MCP speed back to 190 knots.


My bolding above.

No the FCTM does not state "immediately". Well, not that I can find in the one I've got.


Mine doesn't state whether to do it "immediately" or to wait until the flap has made it to the selected position before dialling in the command speed. To me it seems open to do it whichever way you'd prefer. I would prefer to see the flap indicator needles start moving before I bug the speed back; I guess that's almost "immediately" though.

Skyjob
3rd Sep 2018, 10:23
If I remember correctly when VNAV is being used the bug automatically reduces to the speed when the new setting is made on the flap handle
VNAV reduces the speed in reference to the actual flap setting. For example when the flap is half way between 2 settings the speed is also half way.

Nearly correct.
VNAV commands the new speed WHEN flap gauge indicates flap selection.
When flaps 1 is selected, flaps 1 will be selected on speed tape WHEN flaps 1 is reached by flaps indicator.
Cause for some confusion to this, as described misinterpretation by D-OCHO's post, is when transitioning from 1-5, where the flap lever passes the (usually non selected) flaps 2 position, so when flap 2 is passed on the flaps gauge the speed bug will temporarily bug flap 2 speed, before placing bug at flaps 5 when that selection is reached.
Next time in simulator, try selecting landing flaps from up and monitor speed bug with VNAV connected, it will step down in steps of -20/10 its continuously as flap position indicator passes flap positions.

Their rationale is that if an asymmetric flap occurs during extension, it is best to have the higher speed for controllability reasons rather than the lower speed for the planned for final setting.
You are exactly correct, it is rational, and so this is what VNAV logic does as well, so why not embed it into SOP for manual flight or non-VNAV flying.

VNAV protects the aircraft, its functions are rational and logical.
It works on the most known principle of automation: input->output, therefore flaps gauge input->flap speed bugged.
The above also results in the biggest problem using VNAV: {not so good smelling pieces} in-> {worse smelling pieces} out, consternating pilots in the process what's happening and then starting to not trust VNAV.

Judd
3rd Sep 2018, 14:04
so why not embed it into SOP for manual flight or non-VNAV flying.

One would have thought that if the manufacturer (all things considered), recommended in their manuals leaving MCP speed selection until the flaps reached their new selected setting during non VNAV operations, it would be made SOP by the manufacturer. But there is no such recommendation. There must be a reason for this.

galdian
3rd Sep 2018, 17:07
OK to throw into the mix:
- previous operator standard was to wind back speed after requested flap selected, something like "speed check, flaps..." and then confirm "flaps..., green light".
- present operator is the same regards winding speed back after flap initiation ie "speed check, flaps..." but NO further confirmation after that (I assume it's assumed the flaps will reach their selected position - but certainly not verified by any standard call).
Fortunately present operators procedures are "flexible" so I continue to make the call for my own awareness.

Separate: from memory the Classic logic was that if you "selected gear down/flap15" and immediately wound the speed back you would get the gear warning horn until the gear was locked; this was revamped in the NG.
Maybe memory not so good, happy to be corrected.

Cheers!

Capt Pit Bull
3rd Sep 2018, 17:15
pointless spool up