737 NG fmc des page

[josmison]

Hi

With speed restriction of 240/100 on VNAV PATH the aircraft will attempt to maintain 240knts below 10000 until the green circle symbolising the decelation point to flaps speed. However sometimes on the NG eventhough the restriction 240/100 have been preloaded, i noticed that from 10000 the aircraft will aim to decelate straight to flaps up speed. Anybody can assist with an explanation on that.

Thanks

[Skyjob]

Not got hand on the IT manuals for the FMC, but I believe from crew experience it has to do with no hard speed/altitude being defined.

Let me explain:
The FMC builds its descent profile backwards, from RW to a calculated T/D.
The final approach segment starts with a point which is coded as 165/1500 (for instance) where 155 is gear down/flap 15 speed for forecast landing weight.
If however such point has its restriction removed, essentially the aircraft does not have a final approach segment defined, even if the altitude is changed to 1500A to accommodate CDA this phenomena can happen; cold weather corrections for the same point like 1590 have no effect on this though.
When the point IS available, VNAV PTH will reach the point from a intermediate approach segment (flap 5) prior to final approach segment. This segment is defined as the waypoint after which the G/P angle is intercepted towards the final approach segment. Hence a 10nm final approach equivalent point in FMC such as (175/2000A) will be used by VNAV for such point. Equally such point could be defining a base turn, however.
If VNAV PTH is unable to define these points, is cannot gradually reduce the speed to UP from defined descent speeds flown, and it then sets the bug to achieve a ~10nm final at flap 5 speed. As the vertical path is not completed thus cannot correctly be created, the information for it is lost causing this anomaly.

Basically VNAV is operating in a degraded mode, trying to achieve what it is designed to do, but without all the information required, to make a perfect CDA.

[axl76fg]

Skyjob is correct!:ok:

[josmison]

Mucho gracias senior skyjob.

Sorry for the late reply. Very clearly explained and it makes sense.

Thanks again

[RVF750]

Yes. He is.

Practically, when you look in the LEGS page, you should see altitudes and speeds shown on the right side for each waypoint defined. Hard limits are larger and can have A or B at the end to signify not above or below limits.

You can add these or remove them. Say you find 220B/12000A alongside an intermediate descent point. If you wrote 220/ then pushed the LSK next to that entry, it alters only the speed. It's a funny old system, not as accurate as it should be, despite adding winds that can be pretty accurate. The key to how it is coping is the difference to VNAV SPEED and VNAV PATH modes. One is the machine a bit unhappy.......

Personally I like VS and MCP and using my brain. but tis a dying art now....

[OK465]

Say you find 220B/12000A alongside an intermediate descent point.

You won't.

The FMC doesn't support that format. You'll never see an A or B associated with a speed in the box. Altitude yes.

The NG box always treats a waypoint speed as at or below. The obvious example being a waypoint coded for 250 below 10,000. The NG FMS will cross this waypoint at 240 unless you go in and change the descent default.

[Denti]

Funny enough, it supports that format. It is just rarely used in database procedures. Might be a customer option though, god knows boeing has kinda a lot of those.

[Tom!]

Speed 220B works perfectly fine in my Boeing.

[Denti]

Works in my 737 perfectly as well.

[Skyjob]

OK465 you are actually incorrect, the format given is indeed supported for speed when entering a B, however not when entering A. Using a hard speed in FMC will result in attempt to achieve actual speed, thus a DRAG REQUIRED can be a direct result if leaving 210/ at a waypoint on 5nm from RWxx
Turkish Delight was correct in his example as the statement provided 220B/12000A is factually correctly used during a descent profile: crossing waypoint XXXXX at KIAS 220 or less and above 12000'.
The NG box always treats a waypoint speed as at or below. The obvious example being a waypoint coded for 250 below 10,000. The NG FMS will cross this waypoint at 240 unless you go in and change the descent default.
What you are quoting here is the VNAV PTH speed restriction which is always obeyed when used. Thus if high on profile the path will be "lost" in favour of maintaining a speed restriction below the altitude with deviation of -15kts/+10kts to the target speed. Hence the default of 240/100 as this commands "no more" than KIAS 250 below FL100. Thus VNAV PTH will maintain KIAS 250 until the path is regained from above. Simultaneously it displays the message UNABLE NEXT ALTITUDE when it calculates that (even with 250kts) the commanded target altitude cannot be achieved at the next hard altitude of a waypoint, so crews need to do act and do something like using speed brake, flaps or landing gear, which results in VNAV PTH recalculating the possibility of reaching the waypoint at the desired altitude, prompting further if unable to comply.

[OK465]

I stand corrected on the B.

Skyjob,

If the speed is descent stabilized at 240 with no drag or path problems below 10,000, the FMS will not accelerate the aircraft to 250 to cross a waypoint coded at a hard 250 if it can stay at 240. It will remain at 240 as it crosses that waypoint.

[Denti]

Do you have coded the general speed restriction as 240 below 10.000ft during descent? That might supercede the 250 fixed speed at a waypoint. 240/FL100 is the standard in our boeings, but then, it might be different in yours.

On the bus its of course - or + ;)

[Skyjob]

OK465 I think you misunderstood my point.
You are correct it will not accelerate to 250 KIAS, I did not state so.
What I stated was that if (for instance by ATC leaving you too high) VNAV is above VNAV PTH trajectory and above the speed limit FL is accelerating to regain the PTH, crossing the speed limit level it will level off, bleed off its speed to 240+10 maximum and then attempt to further regain its PTH while maintaining maximum 240+10..
The 240/100 serves as a speed restriction, with -15/+10kts allowance.
Any speed restriction coded after this point will be obeyed from that point onwards.
Including 220/ but also 220B/ thus allowing the airframe to reduce below 220 without forcing it to maintain it until after such point.

[Denti]

I might be wrong, but i guess the box takes the lowest speed limit and simply applies that. If you get a 220kts restriction by ATC and enter that as the below FL100 restriction it will follow that speed (with the limits quoted by skyjob), but you might get a scratchpad message UNABLE YYY KTS AT XXXXX to advise you that another speed limit cannot be met.

And i have to say, i often wondered about speed problems during arrival as well. I do understand the logic behind the 240 instead of 250, however other aircraft types (the bus for example) use 250 and create a 10kts difference which might become a problem if no speed control is applied by ATC.

[No Fly Zone]

As noted, SkyJob seems to have it right.
I'm NOT the expert (just one of geeks that wrote the software) but yes, he does get it.
Best wishes...