737 Speebrakes

[Lt Manuel Hung]

Dozing the back of an orange 737 yesterday on the way to work, my Monday morning snooze was rudely awakened by the use of the speedbrakes. Not unusual as such, except this was quite late in the approach and continued intermittently almost until landing which is unusual. Leaving aside the "isn't it about time you had the speed nailed laddie?" questions, having made plenty of duff approaches myself, I'm just curious to know if there are restrictions on speedbrake usage on the 73 below flap limiting speed?

[dvt]

On the NG we get a warning light below 800 ft AGL if they're deployed. Can't us them above 300 kts or past the lfight detent. No restriction on usage with flaps deployed.

Sounds like the pilot might be new on the NG. It takes awhile for the new guys to figure out that the speedbrakes are pretty worthless. They're just an airplane vibrator. They don't really appreciably increase you rate of descent for the noise you get.

Best way to descend in an NG is to slow down first with a momentary level off. Optimum drag for descent on a long ILS final from altitude would be Flaps 5 or 10, assuming you wouldn't be droning through ice.

[Wizofoz]

dvt,

Are you saying that the NG speedbrakes are even LESS useful than the brakes on the "Classic" series aircraft?

AW NUTS!

I`m about to convert onto the NG and was hoping the handle might DO something now!!

That being said, the speed brakes are resnobly effective at higher speeds, the trick is to get your profile sorted out before the 10 000` speed transition.

BTW BIK, how extensive/expensive is the mod, and does it also remove the 270kt restriction after de-icing?

[dvt]

wizwoz

I've never flown the classics, but I'll tell you that this was the first thing I noticed about this highly efficient wing. It just wants to keep flying. You'll find it surprisingly difficult to descend and decelerate at the same time in an NG cleanly configured. You can do only do one of these tasks well. For example, let's say your 250 clean in your descent and ATC tells you to slow to 210 kts. You'll find the NG loses very little speed even while it's slowed its rate descent to 400fpm in LVL CHG. And the "boards" don't help you very much either.

Here are the speedbrake numbers from my operating manuel...

Target Speed Rate of Descent
Clean/with Speedbrakes
.78/280 2100fpm/3000fpm
250 1700fpm/2300fpm
Vref40+70 1100fpm/1400fpm

As you can see, don't look for much help from your speedbrakes. That being said, let me say that the NG is a wonderful plane! It has many tools that allow you to think way ahead of the airplane, to the point of rarely using the speedbrakes at all. I arm them prior to descent, and forget about them mostly. You'll love it! Enjoy your training.

[Blip]

Here's something I noticed that I find very useful. It seems to agree with dvt's figures.

Approaching the top of a 300 kt descent, we were instructed by atc to descend at 250 kts. This then meant we were suddenly about 4000 ft high on profile.

I noticed that when speedbrakes were fully deployed at 250 kts, the descent angle increased by almost exactly 1 degree (from say 3.1 deg to 4.1) (Comparing 250 kts with speedbrakes down vs 250 kt with speedbrakes at flight detent).

That's roughly a 30 percent increase in descent angle which seems to agree with the rate of descent increasing from 1700 fpm to 2300 fpm.

Thinking of the one in sixty rule, 1 degree equates to:

1nm in 60 nm, or
6000 ft in 60 nm, or
1000 ft in 10 nm.

So if I am 3000 ft high on profile, I know it will take approximately 30 nm to get back on profile.

If I'm 1800 ft high, it will take another 18 nm.

I've checked this rule of thumb on other flights and it seems to be a reliable one.


Of course we're talking about air nm, not ground nm. An allowance would have to be made for the effect of wind.

dvt's figures suggest that the 30 percent difference applies at 210 kts as well (27 percent actually) but I haven't had the opportunity to try this one out on line.

I think if you keep this 30 percent figure in your mind, you can then use your speedbrakes more thoughtfully.

You can choose to:

increase the descent path by 30 percent, or

increase the deceleration in level flight by 30 percent, or

increase the descent path by 15 percent and increase the deceleration by 15 percent, or

use any other combination that suites you.

:)

[AirSpd]

I can only speak for those of us who still drive the original -200:

Boeing 732 Operations Manual Vol 1 limitations ( L.10.10 ) states: "Do not deploy the speedbrakes in flight at radio altitudes less than 1000 feet."
As for the speed limitation, there's non, except do not deploy past the inflight detent in flight.
Lastly, I've found the speedbrakes to be very effective in slowing down this bird when the need arises.

It's all about airspeed....

[PifPaf]

Just to complete AIRSPD's message, Boeing recommends, for the -200:
1 - speedbrake + flaps can be used, but should be avoided;
2 - speedbrake cannot be used with flaps 15 or more.

PP

[Cross Check]

Another Boeing contradiction ... how unusual :ooh:

The 733/4/5 AOM has the same entry (L.10.5) re use of Speedbrakes below 1000'RA, but then recommends speedbrakes not be used for speed reduction below 1000' and 800'(AGL) in the FTM.

Either way I suspect this is more from a CYA position in regards to Boeing's stabilised approach criteria (1000' in IMC) - if you had to use speedbrakes on 4 mile final you couldn't possibly be stabilised and require an immediate go-around :* If in their next breath Boeing state you could be "non-stabilised" (by interpretation, not literally) prior to 500' (in VMC) and kiiiinda stabilised prior to 300' (circling approach) ... can't half tell they're somewhat concerned about crews getting excessive sink rates happening on the ILS. Yeah sure the book says: "use of speedbrakes can result in rapid roll rates" (I find it a little sluggish actually), "use of speedbrakes with flaps extended is not recommended", "speedbrakes should be retracted at Flaps 15 or greater", and then "if circumstances dictate use of speedbrakes with flaps extended, avoid high sink rates" (... well derrr) which standard does one choose?

Don't get me wrong, I'm all for a nice, smooth, stabilised approach (preferably well before 1000') and no last-minute manoeuvres from me thank-you kindly, but sometimes Mother Nature and ATC have other ideas (sorry guys I know ya under pressure and doing ya job). If it's <Vref+20 and <1000fpm under control it's still stabilised in my book.

Besides, on a slatless/flapless Vref+55 approach (or even a Flaps 15 approach) I've found them rather useful. :eek:

Oh, and thanks Blip - nicely put mate! :ok:

[PifPaf]

Hummm,

Boeing contradictions... Have you found many of them? (as we did here?!?!)
Maybe it's a good topic...