PDA

View Full Version : 737NG VNAV Descents


Cakov
23rd Apr 2002, 18:50
Can anyone tell me whether a VNAV PATH descent will provide overspeed protection on the 737 NG?

I was always under the impression that if the aircraft were high on the descent profile, it would pitch further down and regain the path by accelerating, but that it would not exceed any limit speeds (it would revert to LVL CHANGE and the FMCCDU message would read OVERSPEED DISCONNECT). On three separate occasions recently, colleagues have witnessed the aircraft dive for the profile in VNAV PATH, and speed has increased until the clackers went off, at which point they manually reverted to LVL CHANGE and wound the speed back.

I certainly never saw this happen on the 737-300: has anything been changed?

kriskross
23rd Apr 2002, 20:27
Not an unusual problem on the NG. I would bet money that they were flying at the top end of the altitude capability of the aircraft i.e. above FL 370 ish, where the higher cruise speed of the NG i.e. .78 or more, brings you much closer to the MMO of the aircraft. If correct programming of the descent is not carried out, such as descent forecast winds etc, then as the aircraft retards and starts the VNAV Path descent, and goes high due to a strong tailwind, the aircraft will attempt to follow the path and pitch down to the MMO before Overspeed Disconnect will show.

The MMO protection to reduce thrust will not work because the thrust levers are already at idle with retard/arm showing on the FMAs.

With the first UK operator of the aircraft, we artificially put in a descent speed of .78/280kts to cater for just this situation, which caught us out as well. With a lower cost index that my current company are using for max efficiency, and due to higher fuel prices, the descent speed is much lower, but the Mach is still .78, which is still on the descent page.

I teach crews to be aware of this situation, especially with a strong tailwind component, even if the descent has been fully programmed, and be ready to go LVL CHG with speedbrake hand at the ready. Another way would be to programme for a VNAV SPD descent and keep an eye on the descent profile, adjusting as necessary till below about FL340 or so when the gap between descent speed and MMO should be sufficient to allow for a Path descent.

Hope this helps

Stan Woolley
24th Apr 2002, 08:33
Cavok

I find descending a little early using the 'Des Now' feature works well from high level and avoids clackers/speedbrakes etc.

seat 0A
24th Apr 2002, 09:13
Exactly, Flanker, that`s what I do.
Sitting there until the clackers go off?

The 747-400 has the same tendency, I seem to recall.

I find that the speed control of the NG in general is inferior to the PG. Ever tried to land a heavy -900 in 55 kts gusts? Nothing short of magic if you keep the speed below the barberpole. The speedtape is so sensitive, it seems.

Seagul1
24th Apr 2002, 12:15
try vnav speed descents initially and then at a lower alt go back to path if ure still around it, works most of the time only thing specially on the 700's she keeps pitching up and down even for little speed changes (not to good for the breakfast) ;)

FutureJetPilot
9th Oct 2003, 21:32
Here is a Boeing ISAR concerning this issue :

BOEING 737-600 IN-SERVICE ACTIVITY REPORT 98-01
FEB 04/98

98-01-3462-00 (737-700) FLIGHT MANAGEMENT SYSTEM - PATH DESCENT SPEED

Initiation of the overspeed clacker has been reported by Boeing Flight Test pilots during vertical navigation (VNAV) path descents on the Model 737-700 airplane. The economy (ECON) target speeds incorporated in the Update 10 Series FMC are closer to VMO/MMO (maximum operating velocity/maximum operating mach number) than for previous FMC versions. As a result, the Model 737-700 will have more tendency to initiate the overspeed clacker than on Model 737-300 to -500 airplanes during VNAV path descents. When a VNAV path descent is being flown, the FMC is controlling to a vertical speed to maintain the desired path. This issue is not applicable when flying a VNAV speed descent where the FMC is controlling to a specific target speed.
Boeing is formulating options to address this issue in a future update to the Update 10.0 FMC software. (Status: Open)


This report is a little old, the 737-800 CBT (having the update 10.2A for the FMC) states that in a path desc :

“if you let speed increase to 10 knots or more than the target, the FMC shows the scratchpad message DRAG REQUIRED”

and also when u are below the airport restriction altitude “if you do not use the speedbrake when the DRAG REQUIRED message shows, VNAV disconnects when the speed is more than 5 knots above the airport speed restriction. The FMC shows the scratchpad message OVERSPEED DISCONNECT” and u’ll see the amber [CWS P] annunciation on the FMA.

I think this is a sufficient overspeed protection.

To avoid these messages and the speedbrake, the best thing I would do is to put in all available and most accurate data (winds, temp, weight…), if nevertheless the FMC still calculates a late ToD in a notable number of occasions, I would cheat on the wind and put in a more tailwind(or less headwind) than the forecast, I think this would be better(with practice) than use of speed desc, because of the workload and the non-economical speed desc.

If anyone has something on this pls post a message

A.H. (possibly a 737NG pilot in the near future)

Farty Flaps
10th Oct 2003, 00:31
Another factor that produces an over speed is when a short cut is entered on lnav during a vnav descent.
The logic commands an energy saving speed reduction,with a drag required message. the speed barkes are insufficient to provide a slow down and a wallow back to the new path so the aircraft noses over to regain the path, having just nosed up to slow down.(go figure) It happens quickly and is obviously exacerbated by winds and altitude.

I tend to slect lvl change whilst the pnf enters the shortcut and then tidy it up back into vnav.or a combination of previous tips ad the above.

Ps most dramatic effect on descent point is achieved by adjusting the programmed tail/head wind at a lower alt.i.e 10000' due to the proximity of the 250 k limit and the need to get the speed off earlier or later as the case may be, before pasing 10000'. This is the only point the aircraft seems to know it cant get down and slow down so it kicks the arse out of it and descends much earlier.

Jungle rules no science.

your bird major

crvm
10th Oct 2003, 15:25
You will never know the *actual* tail wind, so one way is to assume that's it's always more, so input with this in mind, this will give you some room. Since it is slaved to PTH you have to keep a heads up on speed. It will revert to LVL CH 1 knot prior to the barber poll.....

BrunoF
11th Oct 2003, 00:15
Hi,

Why didn't they make the FMC reduce the speed by itself when it reaches 5kts before VMO?

Thank you,
Bruno Francescoli.

cat 3a
11th Oct 2003, 02:56
ASSUME LESS H-WIND OR MORE TAIL WIND
ALSO BY STARTING THE DESCENT A FEW MILES EARLY WITH THE DESC NOW HELPS