Ideally, should 737 flaps be extended in level flight or not.

[Centaurus]

The FCTM states somewhere that the flaps are not normally used for increasing descent rate and that normal descents are made in clean configuration to pattern or instrument approach altitude. An earlier FCTM stated that flaps are not designed to be used as speed brakes.

Despite these words of wisdom, it is common to see people selecting Flap 1 then Flap 5 not only while still descending but at significantly higher speeds than recommended. This is because the 737 is slow to lose speed while descending and so people simply drop the initial flaps to get the aircraft slowed down. From observation there seems to be a reluctance to momentarily level out during the descent in order to adhere to the FCTM recommendations stated above. Similarly with speed reduction requested by ATC. Immediate speed bleed is best done in level flight. But over what time period can ATC reasonably expect the new lower airspeed to be attained?

Am I alone in thinking that it is not playing the game when flaps are deliberately extended during descent in order to slow up? There is sometimes a thin dividing line between a perceived dogmatic approach to the FCTM advice and applying the principle of excellence in airmanship. In other words avoidance of near enough is good enough.

[junior_man]

Several ways to get a 737 down, all depend on the speed you will be at. Speed brake doesn't do much unless you are fast. It is quite effective at VMO but the buffet is awful.

Fast, Gear down (known as the Boeing speed brake) and pull the speed brake handle.

If you are going to slow down, you have to get the flaps out because the maneuver speeds are high (due to rudder issues). 1 & 5 don't do anything much to get the airplane down but flaps 10 does. Have to stow the speedbrakes before flaps 10 as the aircraft really buffets with flaps 10 out. Flaps 10 the airplane comes down fairly well.

Flaps between 0 and 5 takes a long time. Very loooong time if you find yourself fast or high. Gear down will get the little fat airplane slowing down while the flaps come out.

[shlittlenellie]

Provided that the flap speed schedule is observed, there is no problem with extending flaps in descending flight. Any other method of operation is inefficient and impractical and will lead to noise violations at any airport that requires a continuous descent approach.

The descent rate will decrease as the flaps are extended, however so will the ground speed and that takes you back to the basics of flying an approach. In brief: be in full control of the speed and energy of the aircraft and tackle it earlier rather than later.

There is nothing in the FCTM to state that flaps should not be extended during the descent. The point is simply that all pre-approach manoeuvring should be completed in a clean configuration to minimise fuel burn. On a 737 of any flavour the minimum clean speed is comfortable for any initial approach segment. Flaps should not be used as speed brakes, simply because they are not designed for that purpose and are ineffective, additionally there is a perfectly good speedbrake that at any speed remains more effective than flap and the gear is always going to create drag and therefore falls under the "approved" speedbrake label.

ATC when requesting a decrease in speed require a discernible decrease. Every ATCO is different and the speed decrease is not laid down. However, 1Kt per second seems to do the job and this can be achieved in the descent without resorting to extending the flaps!

[Intruder]

Does your FCTM say you should or have to level off to extend flaps? If not, what is wrong with extending them in a descent with minimal deceleration, just as you retract them in a climb?

The issue of using the flaps "as speedbrakes" is separate, though related. If ATC-mandated higher-than-desired speed causes late deceleration, flaps plus speedbrakes (or flaps plus gear in the case of airplanes like the 747 Classic where flpas + speedbrakes are prohibited) may be required at higher than "flap maneuvering speed" in order to decelerate, descend, and configure in time.

Granted, with perfect planning and execution you will not have to "use flaps as speedbrakes." However, we fly in a very dynamic environment...

[757manipulator]

If your held high late.......think ahead...speed back too clean, then when cleared to descend wind the speed up, when high speed..pop the brakes, all of a sudden you have a effective vertical profile control, when back on profile or below VS to slow down or to reduce your descent (works 99 times out of 100...on a boeing:E ) If all else fails deploy the boeing speed brake (gear).

[alloha]

well the only recomendation about flap/altitude thing is the 20000 ft and is only a recomendation.Persnonally i use F5 and IAS210-220 during descent .It gives u the best angle plus saves time from levelling of at 210 and then selecting F5 and start descending again .Most of the ATC in main european airports are trying to avoid any kind of level off during approach due to noise restrictions etc.This kind of approach makes their lives easier and the fellow neighbours have nothing to complain about.:ok:

[Centaurus]

Thanks for the replies from which one can deduce that the FCTM recommendation (at page 4.14 of my copy) which states clearly "Normal descents are made in the clean configuration to pattern altitude or instrument approach altitude." is unrealistic nonsense and Boeing should know better. Interestingly, on the same page Boeing say "Avoid using the landing gear for increasing drag. This minimises passenger discomfort and increases door life." Seems this is unrealistic nonsense too...

[junior_man]

Somebody tried it in the simulator and it worked fine. Unfortunately they neglected to try it in the real world.
Remeber the Boeing guys just fly over to Moses Lake and back.

[INLAK]

....(or flaps plus gear in the case of airplanes like the 747 Classic where flpas + speedbrakes are prohibited)....

The wording in the AOM is "..not recommended." Slightly different, but advisable not to use them anyway. At least F20 required to get a good descent rate at low speed, otherwise about 1000-1200fpm max.

[john_tullamarine]

Two considerations ..

(a) use of flaps at high speed will end up with a maintenance cost penalty and this can add up significantly for a routine usage pattern.

(b) the 20000 ft admonition is, if I recall, associated with not having demonstrated some certification requirements and may be associated with undesirable roll rates ... quite apart from the question of why would one want to drop out flap at that sort of level ?

[CaptainSandL]

My view is that you should try not to extend flaps until you are within 10kts of the block speed. ie if clean speed is 210, then don’t select flap 1 until you are below 220. This is simply being kind to your flaps! I don’t think it matters whether or not you are in the descent.

If you need more RoD then use the speedbrake. If you still need more then you should probably be telling ATC that you need more track miles. If you are doing everything that you reasonably can to get down and it is still looking tight, then tell ATC rather than fly in a non-standard config and get yourself boxed in.

Re the 20,000ft limitation, this is from the Boeing Airliner magazine:

"Several operators have asked Boeing why the Airplane Flight Manual has a limitation restricting the use of flaps above 20,000 feet. The reason for the limitation is simple; Boeing does not demonstrate or test (and therefore does not certify) airplanes for operations with flaps extended above 20,000 feet.

There is no Boeing procedure that requires the use of flaps above 20,000 feet. Since flaps are intended to be used during the takeoff and approach/land phases of flight, and since Boeing is not aware of any airports where operation would require the use of flaps above 20,000 feet, there is no need to certify the airplane in this configuration."

[Stan Woolley]

Quote

' "Normal descents are made in the clean configuration to pattern altitude or instrument approach altitude." is unrealistic nonsense and Boeing should know better.'

I disagree,in the vast majority of descents that statement is true or close enough to being true( under vectors F1 is generally selected somewhere around 12-15 track miles (4-5000ft or a bit lower slowing down if level)

I've really only seen the 'flaps as speedbrakes' as a problem since the NG came out with the 250kt limit and agree it is not best practise but there are a few occasions where leaving the speed around 220kts F5 +speedbrake will be the best option rather than getting really high and having to take the gear or even eventually throwing it away.Doing it as a matter of course is not acceptable.

[Intruder]

The wording in the AOM is "..not recommended."
Not in ours (747-200-300):

"Do not operate speedbrakes inflight with flaps extended."

[alexban]

Alloha: sorry mate,but you are one of many that developed a 'personal' SOP.
The 20000' is a limitation for flaps ,not a recomendation.Just read L.10.5 regarding this.
Regarding flap usage you should read the FCTM,where it says that 'selecting the next flap setting should be initiated when reaching the maneuver speed for the existing flap position'.
Actually,most european airports will give you a short level segment,in order to extend flap.This doesn't increase noise,cause when you'll come level you'll select lower speed,flaps,then lower speed again,flaps....so,no need to increase power.
The 3NM/250-210 IAS speed reduction is given for level flight.
Also,flaps 1-10 will provide NO increase in drag,only lift.It,s in the charts and the FCTM.
Also,flying with F5 at 220 kts will put you very close to the 230 kts limit for flap (on classic) And what if you encounter turbulance?
I don't see a problem in remaining high with speed ,cause I don't do it.When I start feeling unhappy with it I just inform ATC that I start reducing speed to...,that's all.I never got a refusal on this...No need for aerobatics at low level..

Brgds

[Stan Woolley]

Alexban
Quote......
'Also,flaps 1-10 will provide NO increase in drag,only lift.It,s in the charts and the FCTM'

Yeah right that's why we hold at minimum clean and even Flap 1 increases Fuel Flow by 10%. (Boeing FCTM)

Only fly NG's now so nowhere near flap limit at 220kts.Apart from anything else you can feel the drag everytime the flaps extend.

[Centaurus]

It's right off the subject I know but can anyone reading this who flies B737-200's with JT8D-15 or -17 tell me what is the N1 idle speed (closed throttle) at 35,000 ft at top of descent. I was in a simulator doing approach to stalls at high altitude (35,000 ft) and was surprised to note stable idle of 54% N1. I thought it was a lot lower from memory.

Thrust response at 35,000 ft to fast throttle opening was remarkably quick in this particular simulator (3 seconds from idle to 98%) and I doubted the fidelity. The FCTM warns of slow thrust response and certainly in CM56 engines in the 737 Classic sims I have operated, the thrust response from idle at high altitude is is very very slow as advertised. But there again the JT8D is a different design.

OK - back to config warnings!

[AerocatS2A]

Also,flaps 1-10 will provide NO increase in drag,only lift.It,s in the charts and the FCTM.

Doesn't lift necessarily create drag?

[xetroV]

Thanks for the replies from which one can deduce that the FCTM recommendation (at page 4.14 of my copy) which states clearly "Normal descents are made in the clean configuration to pattern altitude or instrument approach altitude." is unrealistic nonsense and Boeing should know better.
There's nothing wrong with that statement, just with your interpretation of it. Nowhere does the FCTM say flap selection should be done in level flight (the recommendation reads "flight to XYZ altitude", not "flight at XYZ altitude"). This is just a recommendation to stay in clean configuration as long as practically possible, i.e. until reaching the pattern altitude or instrument approach altitude. That corresponds with another FCTM recommendation, one page earlier:

Plan the descent to arrive at traffic pattern altitude at flaps up manoeuvring speed approximately 12 miles from the runway when proceeding straight-in, or 8 miles out when making an abeam approach.
The "Descent Planning" and "Descent Rates" chapters provide some pretty useful guidelines and reference data to optimise the descent and deceleration planning.

Interestingly, on the same page Boeing say "Avoid using the landing gear for increasing drag. This minimises passenger discomfort and increases door life." Seems this is unrealistic nonsense too...
Again you seem to misinterpret the FCTM, possibly due to the somewhat ambiguous wording by Boeing. "Avoid" in this sentence is clearly meant as a recommendation, because just a couple of lines earlier the FCTM states:

When thrust requirements for anti-icing result in less than normal descent rates with speedbrakes extended, or if higher than normal descent rates are required by ATC clearance, the landing gear can be lowered to increase the rate of descent.
Boeings recommendations are certainly not unrealistic nonsence, but simple common sense, and applicable in the vast majority of approaches. I agree that using flaps as speedbrakes is not a great idea, and so does Boeing:

The flaps are normally not used for increasing the descent rate. Normal descents are made in the clean configuration to pattern or instrument approach altitude.
Note: "normally", not "never". Flying with flaps extended just below placard speed is not a sensible thing to do in general, but if it's necessary it's necessary. The same goes for early gear extension.

[alexban]

Stan:
TE flap positions 1-15 provide increased lift;positions 15-40 provide increased lift and drag. (FCOM 9.20.17)
Just get out of the plane and look at the way the flaps extend...
The 10% fuel burn for flaps 1 it's about holding...holding at a lower than clean speed....:8


Using landing gear for increased drag and using it to increase ROD can be diferent.
First case,you need to reduce speed,so at 250-260 kts you select gear down...very noisy ,and a lot of discomfort.
Second case,you descent at 210 kts,speedbrakes up,and you need anti-ice-so higher thrust....this will decrease you ROD,to increase it you can select gear down.....not do noisy or dscomfort due to already lower speed.
...:ok:
Brgds

[Stan Woolley]

Alex

The proportional increase in lift will be high compared to the increase in drag for F1-F15 but the drag does increase too, even if Boeing don't say so!

Actually since I first flew the 737 in 1989 I know how the flaps extend thanks.

[alexban]

Stan, you'll be surprised,my friend,how many 737 drivers doesn't know or doesn't care how the trailing flap works on the 737.
I know there should also be a change in drag,I guess you can't modify lift only,but this change is minimal,that's why B doesn't even mention it.
Sorry,but I have problems remembering the english terms to explain more from an aerodinamical p.o.v,but I guess you know already what I am talking about.
Brgds
Alex

[junior_man]

Flaps 1 , 2 and 5 do provide some additional drag, not much but some. Flaps 10 is a substantial amount of drag.
No restriction on the 737 200, 300 or 400 re speedbrake and flaps but very unpleasant buffet at 10+.

In a low traffic density airport you can fly the maneuver speed + 10 kts and with planning it works out fine. Go somewhere busy and it changes. Reasons to go to flaps 1-5 would be for just a little bit more drag, which it does provide. But generally it is because you need to be slower or will need to be at flaps 10 or 15 soon for maneuver speed or additional drag. BTW the -400 had the 250 kt flap 5 limit as well.
The speed brake works poorly in this airplane especially at slow speeds. The gear is less disturbing to the pax as they are used to the wind noise and find it normal whereas the buffet from the speedbrake is unusual to them. Gear speed is 280 kts and still not that bad in the back. Unlike the 727 there are no gear door etc and big wheelwells that open and close during the gear cycle. Noise is worst in the cockpit from the nose gear.

The world of ATC sometimes requires different descent profiles than the sim. Even flaps 1 provides extra drag, just not much. It takes a long time to go from flaps up to flaps 5 as they move slowly (they are moving aft a lot though).

Like most airplanes there are many different ways to fly it, some work better then others and some don't work in some situations. As you fly the airplane you will try different ways to get it down and slowed down and see the difference. You will soon separate the theory from the real world on the line.

[ZQA297/30]

(b) the 20000 ft admonition is, if I recall......... and may be associated with undesirable roll rates ...

Years ago, wanting to find out where the similar restriction in the L-1011 came from, I took a few loose minutes in a sim session, and tried it out at FL 250.
N'eer truer word were spake. The normally docile and ladylike queen of the skies became a real handfull in the roll axis with yours truly setting up what can only be described as a series of impressive P.I.O manoevres. If the real aircraft behaves like the sim at those altitudes, the restrictions are there for very good reasons.
I gained a newfound respect for "odd" limitations.

[Centaurus]

ExtroV. Thanks for taking the trouble to straightening me out about all my perceived "misinterpretations" Made me wonder how I haven't killed myself over all these years misinterpreting FCTM recommendations or advice.:confused: