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View Full Version : A slack technique. Using flaps as speed-brakes in the 737.


Hudson
19th May 2004, 05:04
The 737 FCTM states: " The flaps are not normally used for increasing the rate of descent".

Elsewhere a paragraph says: "Normal descents are made in the clean configuration to pattern or instrument approach altitudes". I read that as selecting flaps 1 and 5 in level flight at the recommended speed.

From personal observation I would say that on most occasions the flaps are used as speed-brakes because especially on a low drag approach procedure flaps 1 and 5 are invariably selected while the aircraft is still descending towards the outer marker, rather than in level flight at idle thrust. This usually results in the flap selection being made at a speed less than the maximum for the flap position but faster than the recommended flap extension.

Is this just lazy flying procedure or is it perfectly acceptable to ignore the Boeing recommendation that the flaps should be extended in level flight? (except landing flap, of course)

Atlanta-Driver
19th May 2004, 06:30
Never heard or read that Boeing would recommend extending flaps while in level flight alone. Flew the 727, 737, 747 and DC9.

What I think Mr B meas by " The flaps are not normally used for increasing the rate of descent". is that flaps should normally used as speed brakes in the initial approach segment (Read STARS) for the following reasons:

Expected speed in many STARS are much higher that the usual speeds to configure aircraft with the first segment of flaps. One has the use of Speedbrake/Spoiler or in extreme cases landing gear.
Using flaps as speedbrakes is not a good practice as it increase wear & tear of all flap components not least the jackscrews and drivemotors. The possible reconfiguration of aircraft due to ATC speed requirements is time consuming (1-5 with B747 takes it's sweet time and vice versa). Is selecting flaps back and forth a good idea?

But if you are heading down the glide, speeds aremoslty quite low and well within flap speed range.

These profiles are guidelines and should not prevent PF using his/hers own judgement on what is best for any given situation. Also many airlines have procedures such as lowdrag approaches with late configuration of aircraft (Saving fuel) that differ from those of Boeing.

Hope this made some sense

AD

Hudson
19th May 2004, 12:25
Further to my initial post, it is instructive to see the following in the B737-200 FCTM under the sub-paragraph Normal Maneuvring and Flap Extension:

"The flaps extend mainly aft and slightly down to flap 15, increasing drag very little. Deceleration, descent and flap extension are relatively incompatable. Normally, extend the flaps on the flap/speed schedule in level flight using the minimum thrust required to maintain altitude while decelerating, and trim out the resultant pitch changes as they occur".

All of the above would suggest that Boeing recommend extending flaps from up to flap 15 in level decelerating flight - not in a descent.

alexban
19th May 2004, 21:47
You are right- flap positions 1-15 will not increase very much the drag.Selecting flaps during descent,especialy on classics,it's not so indicated due to posibility of an increase in speed (gusts,tail wind,etc) and a low flap limit -230 kt.
with thrust at idle,speed 210 kts (min clean) you'll descent normally with around 1100'/min. So ,if you descent with 1500' and you start extending flap,there is a big chance of overspeeding the flap(due to external factors like wind ).You'll have to use speedbrakes,and this is not such a nice manouvre.And if you have a tail wind of more than 20 kts,then you have some work on your hands.
The 737 has good aerodinamics and poor speedbrakes,it's sometimes hard to 'brake' and reduce speed.
That is why it's indicated to start extending flaps in level flight.
Brgds Alex

Dehavillanddriver
20th May 2004, 07:59
I am venturing a guess here, but I reckon that the FCTM is written that way because the americans fly level into the glideslope and come down configured.

They don't as a rule (as far as I can tell) use the low drag - speed bleed type approach profile.

Though I do stand to be corrected.

Hudson
20th May 2004, 10:32
From my experience it is no trouble to momentarily hold level flight to drop the flaps through to Flaps 5 then tip over into the descent. The low drag recommendation by Boeing (737-200 FCTM) is gear down and flaps 15 at GS intercept (having already set flap 5 in level flight) then descend at flap 15 manoeuvre speed until ready for landing flap.

One can rationalise as much as one likes, but I think it boils down to a lazy flying practice to push out the initial flaps while descending, rather than in level flight.

BOAC
20th May 2004, 10:49
In UK at least you will often be descending when asked to reduce speed and ATC will not be overjoyed at your levelling off, I suspect.

unruly
20th May 2004, 16:59
I would rather make more use of the speedbrake than deploying flaps and slats to decelerate. For one thing, the slight vibrations it induces is more tolerable than a rapid change in pitch (ballooning effect) that flaps may induce in a descending aircraft. This is less pronounced in level flight. And Atlanta-Driver is right. Less stress on the screwjacks and flap motors.

Other aircraft types though have higher speed ratings for their flaps/slats, like the DC9. These planes could make use of their flaps as drag devices as these could be deployed at 280kts (Flaps5), 240kts(Flaps15) down to 210kts(flaps25) and 180kts(Flpas40/50).

LEM
20th May 2004, 19:49
Often to decelerate in a descent, instead of levelling off momentarily, a selection of VS-500 will satisfy both requirements (decelerating and continuing your descent).

Of course extreme cases are extreme cases.

Hudson
21st May 2004, 12:19
If you are cleared to descend to an altitude then I would have thought that the clearance automatically permits you to temporarily level off at any altitude from top of descent to cleared lower altitude. Presumably by the time the first notch of flap is needed the aircraft would have been at 250 knots reducing, and nearing 3000 ft agl. The most efficient deceleration is level flight idle thrust. Selecting VS to 500 fpm merely delays speed bleed back.

Sniff
21st May 2004, 13:41
Hudson,

Don't forget ATC require a minimum rate of climb/descent of 500fpm when changing levels.

Unless below TA into EGSS,KK,LL when other rules apply...

4dogs
21st May 2004, 15:29
Hudson.

A level, idle thrust deceleration may be the most efficient in smooth air but may be a poor choice when descending into turbulence. A decelerating descending speed bleed flight path (as mentioned by LEM) can often be far more efficient at satisfying passenger and cabin crew comfort considerations as well as permitting a bit more flexibility at achieving your desired energy gate.

I just reread my FTCM which covers all models and I would have to read a hell of a lot into it to come to your conclusion. Unless you have a communication from the author that states that his/her intention was that anything other than a level deceleration was not permitted (interesting for stabilised approaches!), then I think you are inventing a bit of folklore.

I think level decels are the product of laziness and a lack of finesse, but remain as part of every pilot's toolkit. I am just off to search out some words that I can claim to support my case.....

Stay Alive,

Atlanta-Driver
21st May 2004, 15:32
More like "Maintain 2000fpm minimum and slow to 250 kts" add a crossing restriction to that, heya and we just another day in Europe.

AD

willfly380
22nd May 2004, 05:44
drag flap or no, but have you ever tried to capture gs when clean?well what i mean is if your flaps are not 5/10 when you capture gs ,boy you will have to drop your gear in a hurry. i have flown the 737 23456789. not once have had a problem with flap speed been exceeded. if you select flap 10 and then have 3000 vs then you will have a problem. but if you are in lvl chg mode or if flying manually a rod of upto 1500 would never get you in trouble.on the 200s we used a rod of 1000 to start reducing our speed and to get into slot for flap extension.you got to be abit ahead in your planning.
as far as being level for flap extension,it is not possible all the time, yes when you can then why not.
cheers and happy landing
ps. the last 73 fctm which i have doesnt call for level flight for flap extension.

Hudson
22nd May 2004, 09:43
4Dogs

Read my second post carefully. I didn't invent this Boeing advice - it is there in black and bloody white and I quoted from the B737-200 FCTM Date August 20th 1976 - Page 04-60-02, first paragraph, line 5 which states:

"Normally, extend the flaps on the flap/speed schedule IN LEVEL FLIGHT" (my capitals).

Succeeding Boeing FCTM's have been apparently been the subject of significant legal re-editing in order to minimise the risk of litigation eagles seizing upon the slightest excuse to make a killing. There was far more amplification of Boeing intent in the early FCTM than you see now. The responsibility of how Boeing advice is interpreted is now left to individual operators - with the resultant wide ranging and often questionable points of view.

I certainly do not recall stating (as you have interpreted) that Boeing do not permit flap extension in other than level flight. Where on earth did you get that from?

Also I fail to see the correlation between extending flaps at the correct speed in level flight and in passenger and cabin staff comfort if the flaps are extended in descent.

Cap 56
22nd May 2004, 14:43
Flaps are extended to meet ATC speed requirements and to comply with the speed limits for the approach to be flown.

Using flaps as speed brakes causes extensive wear and can cause flap problems in the long run.

Speed brakes and gear should be used to keep the speed under control if the profile requires a descent that will cause the body thrust to accelerate the acft above the design maneuver speed for the current flap setting.

Requesting extra miles is an option but is not always available if you are already to deep inside the procedure.

A good knowledge of available decent rates for all various configurations is essential and part of the basic flying skills.

dvt
22nd May 2004, 18:25
Your configuration always is dependent on your situation.

Nothing decelerates a 737-800 as good as idle power and level flight. Now that I'm on the 767, I miss this 737-800 quality quite a bit. To get the same rate of clean deceleration in a 767, I'd have to have a shallow climb.

Flaps 10/180 kias is useful for intermediate approach manuevering.

Farty Flaps
22nd May 2004, 19:03
I think there is a subtle difference between using the flaps as a last resort to maintain profile just within speeds and a gradual continuous descent with smooth deceleration and configuration changes, well below the limitations to hit a loc/glide intercept exactly on slope, five flaps and idle power.
ie 230 kts clean will give you plenty of penetration until you know you wont drift high. Then select clean speed and dirty up avoiding the amber. I would always use the speed brake/vs if atc/proc required slower and there was any thermal activity, due to the a/p commanding 500feet rod to facilitate slow down, when on average you need 1100/1300 in this sort of phase to maintain 3 degrees. I prefer this to level flight especially if you get more vectors than you need , continuous can be adjusted to suit, levelling you may just be dragging things around the sky. Level flight is a bit like fixed card adf work , parallelling the bearing, methodical:continous done properly is more like an rmi , fluid.
Dodgy analogy I know

Right thats as clear as mud then.

PS i know the flaps on th 737 dont contribute much drag normally as has been mentioned , but its more than you need on a wide vector if you loose an engine.

4dogs
23rd May 2004, 18:31
Hudson,

Well, it may have been there in the 76 edition but it isn't in the October 31, 2001 version.

So, does that mean that the original advice stands but is no longer written because of civil liability concerns? Or does it mean that the advice has been reviewed?

In 1976, American pilots in general (and those writing for Boeing in particular) would have maintained maximum speed to approach altitude, would have regularly intercepted ILS glideslope at the Outer Marker and would have used a Dive and drive technique for NPAs. While I would not argue about Boeing advice (or that of any other OEM) about how to operate their aeroplane in a general sense, I would have to have some hesitation about believing that the advice came down from Mt Sinai as a universal truth. We are all a product of our environment and there is always a sensible balance in achieving a sensible outcome.

When I read your posts, I concluded that we differ in regard to the latter conclusion.

When I read references in the FCTM about using the flaps or gear as speed brakes or as devices to increase the rate of descent, I read them as references to selecting those aerodynamic devices in and out as one does with the real airbrakes. I do not read them as precluding selection of flap as a function of manoeuvre speeds or the normal deceleration profile. I also read the word "normally" to be the most common of a range of options - a recommendation if you like but not a prohibition.

I read your posts to reflect so much of a belief in level flight flap selection that you were advocating interrupting descent to create a level segment to change configuration. IMHO, that belief would be an unthinking and pedantic application of a personal interpretation - hence my response. The fact that there is some 25 years between our comparative documents would not have helped.

I did not state that you had said "that Boeing do not permit flap extension in other than level flight", rather I intimated that your posts implied that that was the case but without any substantiating evidence. I sensed the assertion of facts or rules where none were intended. If I misunderstood your position, I apologise.

As to your final point, I was alluding to the fact that in 1976 and up until quite recently in some airspaces you could, if you so desired, maintain 300kts to glideslope intercept altitude and conduct a level decel to flap speed. My point was that if you were descending in turbulence (mechanical but often in this country thermal) such a speed profile would be very poor airmanship as opposed to a speed bleed descent. That I failed to make my point clear is my fault, particularly as I presumed that we operated in similar environments and with similar concerns about those who ride with us.

Stay Alive,

Farty Flaps
23rd May 2004, 21:52
Thats what i was going (trying) to say.
darn it:E

Prop's ????
24th May 2004, 03:10
Reference FCTM 737 NG (Oct 31, 2003), page 5.11 Approach.

Quote: “VNAV is the preferred descent mode when the FMS flight plan is programmed for the intended arrival.”

With this in mind, the planed descent profile is a constant rate descent with no level off before GS capture.

If we plan to fly a level segment to extend flaps then we would have to fly in V/S or LVL CHG to get to our level off altitude before the planed VNAV profile.

In some scenarios we end up doing this, but not for flap extension, but to comply with ATC requests.

Looks like Mr Boeing has removed a lot of information from the original FCTM.

Hudson
24th May 2004, 05:27
Thanks for all the replies - I have run out of touch paper to light the fuse. Interesting about "old" and new FCTM information. The reference (1976) to the desirability to extending initial flaps in level flight has obviously disappeared from later versions of the FCTM. For what reason, I do not know. Does this make the original advice invalid? I do not think so.

Until a couple of years back the 737 FCTM displayed a diagram of the typical circuit configuration and showed a recommended circuit width of between one to one and a half miles. For years I found this easy and comfortable to fly.

Suddenly a revised FCTM appears changing the recommended width to two miles. And the reason? "Changed parallel track to 2NM to standardise across all Boeing models". That is almost double the distance previously used. Up goes the fuel consumption.

Does that mean Boeing was wrong for all those years in between?

willfly380
24th May 2004, 06:42
hudson
my friend a lot has changed from 1970s and even 1980s.they at that time in one of their fctm/ptm have mentioned that windshear is no big deal, and even the term MICROBURST does not exist in that book.they have the definition as severe downdraft but no name.this is just to tell you that the book has changed and you will be better off getting a new edition .

4dogs
24th May 2004, 07:53
Props ????,

That approach statement is at page 4.11 in the FCTM version (300-900) that I was referring to. Can you confirm for me that the diagram for Missed approach for circling (p 4-36 in mine) still exists and that it still shows joining the missed approach track after the Missed Approach Point?

If so, it is an example of where the OEM's advice may be counter to domestic rules (Australian AIP) and thus requires sensible consideration rather than rote application.

Stay Alive,

Prop's ????
24th May 2004, 10:01
4dogs,

Yes it is still a part of the FCTM, it is now in 737 NG (31 Oct, 2003)5.57.

It is the same chart and text, just in new chapter.