VNAV Path Descent after the TOD on the 737-400

[Lawrence of Quebec]

I like to know what the VNAV path descent is based on and how the TOD point is calculated.

Is it based on a 3 degree descent angle?
If so, is it not uncommon for the engines to be applying significant power (50 + percent N1) on a VNAV path descent to hold that 3 degree angle and appropriate selected descent speed?

Not a 37 driver, but I've always thought at the TOD the throttles go all the way to idle.

Cheers.

[Sink Rate]

I quote from the FMC user Guide written by Bill Bulfer:

"VNAV PATH uses idle thrust and pitch control to maintain a vertical path similar to a glideslope in 3 dimensions, except there is no ground based beam. The target speed (entered in the FMC) is used for planning purposes only. There is no attempt to to maintain the TGT SPD unless the airplane gets too slow. VNAV PATH is slaved to the path. It is the pilots responsibility to control the speed."

From my own experience if the aircraft accelerates much beyond the target speed whilst trying to maintain the path it will annunciate "DRAG REQUIRED" on the FMC. If it gets much slower than the TGT SPD it adds power to maintain the path (I think!).

Hope this helps.

VBW,

SR

[Lawrence of Quebec]

Thanks SR.

I just want to confirm:

So with the autothrottle engaged, the throttles will move all the way to idle at the TOD.

As for the descent angle. The VNAV path descent angle is similar to a glideslope, but it's by no means based on a shallow 3 degree descent angle. 3 degrees is pretty shallow for an idle descent.

[minuteman]

To quote the Boeing Manual:

"The path mode controls descent to fly a vertical path which complies with altitude and speed restrictions in the flight plan. The speed mode controls descent at a fixed speed and complies with altitude and speed restrictions in the flight plan."

Both modes use idle thrust, but path mode will induce a level off if there is an "at" or "at or below" altitude restriction. When it gets to its next idle thrust descent point, off you go. Speed descents make no allowances for this, and will continue down to the lowest restriction altitude but maintain crossing restraints (with no level off.)

Path is normally used, but I prefer level change. Much easier!

As for TOD, it computes back from its end of descent point (usually from a selected approach) to your cruise altitude. DRAG REQUIRED appears when there's a bit more tailwind than it expected and a higher speed is required to maintain the profile.

I hope that helps!

[Jambo Buana]

VNAV PATH works well if all the info you put into the computer is perfect. If you put in forecast winds for your descent, ISA deviation, landing QNH etc. then the thrust levers close at TOD and don't open again until you level off at your end of descent waypoint.
If however you cockup the winds, or as I heard happened to a friend of mine, the FO put in the wind 180 degrees out at 100kts, you can either end up really high or really low.
If you end up high the a/c will nose over and hit VMO/MMO and go initially DRAG REQUIRED, ie use the speedbrake, and then if your still high, OVERSPEED DISCONNECT, and drops you into level change. Its then up to you to get down in so much as the a/c can't do any more for you with the automatics.
If you get low on the projected path, it will allow airspeed to drop to target minus 15kts then will add power to maintain approx 1000fpm until flying back into the path when it will reclose the thrust levers and try again to go down with them closed.
Thats about it.

[DontSink]

VNAV path is not based on a 3º descent, but if you have no altitude restrictions during descend and light winds, it will be something very close to a 3º descent. For example you`re cruising at 350 and the next point to be is the LOC capture point to be at 3000 feet and at 180 kts. You enter that into the FMC and VNAV path will put there at that altitude and speed. But since it`s path we`re talking about, it does garantee that you will be there at 3000 feet, but doesn`t garantee the speed! It will be slaved to altitude. If you wanna comply with the speed, you should use VNAV speed.

If you feed the FMC with expected winds and speed you wanna fly, I usually use 320 Kts for descend and 250 bellow FL100 and 180 to be at the IAF, it always work pretty well!

Cheers!

[whoop-whoop]

Jambo Buana

Funny how it's always the F/O that that makes the mistakes.

In my experience it tends to be the F/O's that know how to optimize the use of the FMC, and anyway that's why there are two of us to continually crosscheck, not only each other but also what the A/C is doing.

Don't forget good old airmanship and your 3 x table with a decelleration allowance +/- a bit for headwind,tailwind,high/low QNH,TAI etc. This will soon reveal any programing errors. ;)

[sabenapilot]

VNAV is without any doubt the most unhandy mode available to the pilots.
Besides, at most airports in Europe you are vectored along an S or U shaped track onto the ILS as soon as you descent below FL100, so your FMS has no idea of what your plans will be!

[QAVION]

"As for the descent angle. The VNAV path descent angle is similar to a glideslope, but it's by no means based on a shallow 3 degree descent angle. 3 degrees is pretty shallow for an idle descent."

Descent is influenced by Cost Index. The higher the Cost Index, the faster you will descend (Elevators used to maintain the path angle and throttles at idle... at least to the first restriction). If there are unforseen headwinds, the aircraft should come out of idle thrust to maintain the path.

Rgds.
Q.

[stevevanda]

There are 3 pitch controlled decent modes: level change (MCP), path and speed descent (FMC). All methods use pitch control to slave the A/C to either airspeed, flight path or target speed, respectively. The path and speed descent modes allow for manual or ECON target speed settings, for which ECON speed is a function of the cost index (standard setting of 40 for VNAV climb optimizing). As I understand it, with

Path descent mode,

1 – target speed is set (ECON or manual)
2 – manual pitch is set (via spoileron and thrust levers) to control the path speed
3 – pitch auto-controlled (via AFDS) to slave the A/C to the flight path (FPA) at the T/D
4 – target speed maintained at E/D

Speed descent and level change mode,

1 – target speed is set (ECON or manual)
2 – manual pitch is set (via spoileron and thrust levers) to control the flight path
3 – pitch auto-controlled (via AFDS) to slave the A/C to the target speed at the T/D
4 – target speed maintained at E/D

Hence, thrust and spoileron are optional for pitch control in descent mode!

Regards,
Stephen Wood :)

PS you should also use pitch control (thrust and spoilerons) to keep your A/C on its G/S path during ILS approaches!

[ 25 November 2001: Message edited by: Stephen Wood ]