View Full Version : is there any descent rate limitation for B737NG?
goodwill
6th Oct 2010, 14:40
Just wonder any limitation for max descent rate with speedbrakes extended on B737NG ?
of course assum that within Vmo/Mmo conditions.
Thanks!
Assuming spoiler mod done, no. Just need to watch the cabin rate.
FCS Explorer
6th Oct 2010, 18:31
i think you can dial in something like -7300 in the V/S-window
a bit scary
and u might get to some pitch limit
STBYRUD
6th Oct 2010, 22:04
For some reason I have never seen anything higher than 4650fpm, even when a higher selection is made on the MCP - possibly there is some kind of hard limit that the FCCs impose when using the autoflight system, but it might just be coincidence - no idea.
plain-plane
7th Oct 2010, 09:01
You can do very very very high rates of descent...
however the autopilot struggles in this config:
LVL change, gear down, F5, speed brake out.. and a speed og 230ish...
But then again, do one really need to do that... you could try it out in the sim:ok:
it should give a descent angle close to 12 deg.:E
and a speed og 230ish...- let's hope I don't have to fly/sit in the ship after you:ugh:
STBYRUD, the autoflight system can use higher rates, had today a nice and (short time) steady 5500fpm descent rate in VNAV, without overspeeding or any other issue. Haven't used very high rates in V/S though, usually only in level change or VNAV.
cosmo kramer
8th Oct 2010, 00:55
If being kept high by ATC or having step down descends over mountainous terrain, a very effective way to get down is to reduce to clean. When cleared for lower level, go V/S and dial in something like 5-6000 feet ROD (leave the speed at clean). Depending on weight pitch will go below 5-7 degs and speed will increase very fast. When approaching the VMO reduce ROD as required to manage the speed. When reaching the next restrain the speed is already set for deceleration too. Super for losing altitude really really fast.
Much more effective than LVL CHG and increasing the speed, since you don't get the pitch down as fast. And also less work since you don't have to dial the speed up and down.
Great fun too. :)
misd-agin
8th Oct 2010, 01:25
Quote:
and a speed og 230ish...
- let's hope I don't have to fly/sit in the ship after you:ugh:
BOAC - that's 20 kts under the flap 5 limit and in agreement with Boeing's recommendations. What is your complaint?
and in agreement with Boeing's recommendations. - can you quote your reference where Boeing recommend using Flap 5 at 230kts?
plain-plane
8th Oct 2010, 08:24
well i agree with BOAC (did you only read the first 3 lines of my post?).:hmm:
but this question was about limitations,,, not about nice, good idea, or the like.
the fact the A/C can do it, does not mean you should be doing it, hence my advice to try it in the sim...;)
keep the shiny side up...
misd- found it yet?
plain-plane - no, I read it all. 'Try it in the sim' and you will probably be beaten around the ears with a wet fish. The OP's 'question' was not about flap!
MrHorgy
9th Oct 2010, 23:54
Whilst I agree descent planning where possible should be managed so flight at or near limitation speed is avoided, I fail to see what the problem is with making selection as long as it is below placard speed.
The wording in most operations manuals is normally along the lines of "not recommended" or "should be avoided". This is not the same as "Not permitted". If it was a airframe risk, or damage may occur, it would be forbidden. There are times and places where selection of flap 1/5 at 235 kts cannot be avoided. If Boeing, or airlines see it as such a risk, they should contact the provider of local ATS units and ask them to provide arrivals that are designed with different tolerances in mind.
Horgy
There are times and places where selection of flap 1/5 at 235 kts cannot be avoided - can you tell me a few? I'm intrigued.
Of course you can use any part of the aeroplane up to its 'limit' (and, indeed, beyond) but there are very good reasons for not doing so unless you are in dire straits. It is not up to Boeing to change procedures - they are not flying the aeroplane.
Personally I refrain from using speedbrakes against flaps. Speedbrakes for me come out at high speed only. Once Flaps are out I just stay high on profile and drop the gear a few miles earlier. That aside I don't know what the maximum obtainable rate of descent is with just speedbrakes out. I would say that if you bring it back to Clean speed and then just drop the nose you can reach a maximum ROD at -10° BA. Go beyond that and technically you are in an airplane upset condition. Your last option then is to sideslip it manually. But expect a few dozen sick passengers and start updating your CV :)