737NG Common VNAV - direct to followed by speed jumping to FLAPS speed

[DirectVELAG]

I've noticed that in some specific circumstances, the FMC behaves in a strange way regarding speed.

For example, you are descending at 250kts following the STAR. You get given a DIRECT TO another waypoint on the approach, but when executed, the target speed will jump straight to FLAPS (UP speed) and the DECEL point will be removed. You are then left at whatever speed you were doing (250kts) with no shallower path angle in which to decelerate. This tends to be when the distance to this new waypoint is greater than say 50nm, and if executed before reaching the original DECEL point.

I'm thinking this has something to do with either a) approach logic, or b) the logic with cruise descent if initiated more than xx miles before TOD (80nm rings a bell)?

Can anyone shed any light?

Thanks

[FlyingStone]

This is usually due to executing a direct to a waypoint, that has a geometric angle path coded towards it.

For example, if you have FF36 and CF36 waypoints for an approach to runway, sometimes there will be a coded 3 degree path between the two waypoints. If you insert and execute a direct to FF36, the FMC will generate a 3 degree path from your current position to that waypoint, and (in the eyes of the FMC) there will no longer be a need for a DECEL waypoint, or for you to fly at whatever speed you have programmed, because you can just "simply" follow the 3 degree path all the way to the runway, even though that is easier said then done.

Depending on your company policy with manual waypoint creation, you can fix it by manually inserting the "offending" waypoint, which means it won't have the 3 degree path coded towards it anymore, and you will be able to restore normal VNAV profile.

[DirectVELAG]

thanks, will keep an eye on the 3 degree thing

[LOWI]

Yeah I know there are 3 ways around this:

** Let's say you're cleared to the CF32 waypoint **

1) Before EXEC the direct, select ABEAM PTS and then execute. Probably the easiest way. You can tidy up all the abeam points as long as you only leave one point before the CF32 waypoint.

2) Create a waypoint before the CF32. Many possibilities here. You can find a waypoint nearby (if you're on HDG SEL). You can take your present POS from FMC and insert it with your current trk + a random distance. You can even create an inbound radial to your CF32 point. Many others but essentially the goal is to create a waypoint before the CF32 to trick the FMC into getting out of this so-called approach logic.

3) My personal favourite shown to me a few years ago by a clever F/O.
Enter AAA waypoint, don't EXEC. This will create a DISCONT after AAA. DELETE CF32. Then you'll have AAA, DISCONT, RW32. Then re-enter CF32 after AAA. Bring CF32 to the top and now EXEC.
Apparently this recalculates the 3 deg path only from CF32 waypoint and removes the FLAPS UP SPD from the DESC page.
I've only seen it done once by the F/O and it worked.
Only risk to this is, let's say CF32 has a speed or altitude constraint, when you re-enter CF32, you need to also re-enter the speed/altitude constraint too.

[BraceBrace]

Originally Posted by LOWI

3) My personal favourite shown to me a few years ago by a clever F/O.
Enter AAA waypoint, don't EXEC. This will create a DISCONT after AAA. DELETE CF32. Then you'll have AAA, DISCONT, RW32. Then re-enter CF32 after AAA. Bring CF32 to the top and now EXEC.
Apparently this recalculates the 3 deg path only from CF32 waypoint and removes the FLAPS UP SPD from the DESC page.
I've only seen it done once by the F/O and it worked.
Only risk to this is, let's say CF32 has a speed or altitude constraint, when you re-enter CF32, you need to also re-enter the speed/altitude constraint too.

Sounds very complicated.

It's been a while but if you want the FMC to recalculate the path, just reset a cruise altitude/level and once the path is calculated re-engage the path descent?

[LOWI]

Originally Posted by BraceBrace

Sounds very complicated.

It's been a while but if you want the FMC to recalculate the path, just reset a cruise altitude/level and once the path is calculated re-engage the path descent?

That's why I've only seen it done once but it does work!

Maybe one day Boeing will introduce FPA on the MCP just like the Bus. But we do have the FPA symbol on the PFD which can help fly a certain glide angle like standard 3 degrees.

[DirectVELAG]

Thanks for the replies. I've tried all of them in the past, sometimes one works, sometimes the other - usually have most success with putting a new cruise alt then pressing alt intv. Maybe it also depends on the distance to go until the CF?

[LOWI]

Few other little things to mention (all shown by very good guys!)

1. I'm on downwind for example and I'm planning a shortcut to the CF or FAF. I'm sat there trying to measure how many miles prior to the next waypoint so I can enter XXX/-5 for example. Then join this up to the CF or FAF. Instead, I can just select the CF or FA waypoint, insert it into FIX page, get the ABM and then insert it into the LEGS page. Join that up with the CF or FAF and voila.

2. If you're on HDG SEL for wx deviation or vectoring, you can go into the POS page 2, take your current co-ordinates, enter them into LEGS page, then take the next waypoint/+miles to give you a rough idea how long you will be on the heading for, especially for wx avoidance.

EDIT:
Extra point.
You're descending and ATC now tells you reduce speed as well (let's say 230kts). Some line trainer showed me. Go V/S -100fpm, SPD now dial in 230kts. Now press SPD button. This forces ARM mode. Thrust levers will be in idle anyway. Once you get to 230kts, go LVL CHG.
The line trainer told me that a lot of people use V/S to slow down and as the aircraft reaches the desired speed in V/S, it starts bringing the power back in. Guys will then press LVL CHG and then watch the thrust levers close once more. Maybe a bit OCD but I thought it was a nice touch of flying through the MCP!!

I'm always learning on this aircraft. I hold my hands up and be the first to admit I don't know everything but I have forgotten a lot! Sometimes I feel like I've forgotten more than I've learned.

[framer]

I'm on downwind for example and I'm planning a shortcut to the CF or FAF.

​​​​​I can just select the CF or FA waypoint, insert it into FIX page, get the ABM and then insert it into the LEGS page. Join that up with the CF or FAF and voila.​​

Im not saying it’s a bad idea, but just be careful you’re not falling into a common trap of having your head down explaining what you want done in the FMC to the PM who isn’t expecting a long series of FMC entries and hasn’t given this technique any thought previously, all while you’re at a busy airport at 6000ft. A lot of RA’s occur with SID’s only having 1000ft clearance from the inbound STAR’s.
I can easily imagine the ‘voila’ being more like a
PF - cool ok put the center fix in the fix page please
PM - ah ok what?CF 17R?
PF- yeah CF17R
( both of you looking at the FMC as the PM scrolls through to Fix page four)
PM- there it is
PF - select abeam please
PM - there it is
PF- ok now take that to the legs page please
PM - just select it?
PF- yeah select it and put it at the top
( no profile calculations are being run while this is happening and intermittently both pilots are looking at the FMC with nobody looking outside)
PM- that’s at the top
PF- Get rid of the disco please
PM- There it is, joins the CF
PF- Yip that’s it excecute
PM- excecute
PF- LNAV
PF- ahhh we’re 400ft high gear down

It’s very easy to draw both pilots attention inside the aircraft with the FMC in situations where the PF should really being looking out the window and making simple MCP adjustments.
Its debatable to me whether that’s a safer operation with lower mental workload than configuring conservatively while looking out the window.
The way you are actively looking to increase your knowledge is admirable, just pointing out that FMC manipulation is not always good airmanship.
​​​​​​​Cheers

[DirectVELAG]

Originally Posted by framer

Im not saying it’s a bad idea, but just be careful you’re not falling into a common trap of having your head down explaining what you want done in the FMC to the PM who isn’t expecting a long series of FMC entries and hasn’t given this technique any thought previously, all while you’re at a busy airport at 6000ft. A lot of RA’s occur with SID’s only having 1000ft clearance from the inbound STAR’s.
I can easily imagine the ‘voila’ being more like a
PF - cool ok put the center fix in the fix page please
PM - ah ok what?CF 17R?
PF- yeah CF17R
( both of you looking at the FMC as the PM scrolls through to Fix page four)
PM- there it is
PF - select abeam please
PM - there it is
PF- ok now take that to the legs page please
PM - just select it?
PF- yeah select it and put it at the top
( no profile calculations are being run while this is happening and intermittently both pilots are looking at the FMC with nobody looking outside)
PM- that’s at the top
PF- Get rid of the disco please
PM- There it is, joins the CF
PF- Yip that’s it excecute
PM- excecute
PF- LNAV
PF- ahhh we’re 400ft high gear down

It’s very easy to draw both pilots attention inside the aircraft with the FMC in situations where the PF should really being looking out the window and making simple MCP adjustments.
Its debatable to me whether that’s a safer operation with lower mental workload than configuring conservatively while looking out the window.
The way you are actively looking to increase your knowledge is admirable, just pointing out that FMC manipulation is not always good airmanship.
Cheers

thanks for that. thats exactly what i'm trying to avoid by getting an explanation of the FMC logic, to avoid button-hungry FOs messing about in the FMC trying random things.