Pilots: Switching from Navigation (LNAV/VNAV) to a heading?

[invertedattitude]

Wondering if there is a procedure you guys follow for an unplanned switch from regular navigation to heading mode in en-route airspace? (Vector)

Reason I ask, is that it seems to take quite sometime once a heading is issued to the time the airplane starts to turn, even factoring in lag in the radar return it seems to take a somewhat longer time to commence the turn rather than simply dialing the heading and pushing HDG. It's also not uniform either in the length of time from one flight to the next.

Anyway just being curious, thanks for the replies in advance.

[Stinky]

If you're on autopilot and punching in the new heading on an FMS it takes a few seconds before you start turning. Spinning the heading bug is almost instantaneous. Vectors for an approach will usually be in HDG mode so the turns will be quicker. Enroute vectors I do on the FMS because I'll end up switching back to that mode anyway once cleared en route.

[Jaques Strappe]

That is a really interesting question. All the types I have flown have a simple heading bug and heading select which will take it out of LNAV right away. The only thing I can think of other than delayed crew response would be aircraft speed. I would imagine however, that if you are working in a terminal, all the speeds will be similar.

[whipline]

It also depends on your altitude with some airplanes. Either the aircraft, FCTM or SOP's will dictate a smaller bank angle when at high altitudes.

[invertedattitude]

It also depends on your altitude with some airplanes. Either the aircraft, FCTM or SOP's will dictate a smaller bank angle when at high altitudes.

I thought this might be a reason.

Thanks to the replies so far, and I'm speaking of enroute high level airspace (FL290 and up) IE fast movers.

[invertedattitude]

If you're on autopilot and punching in the new heading on an FMS it takes a few seconds before you start turning. Spinning the heading bug is almost instantaneous. Vectors for an approach will usually be in HDG mode so the turns will be quicker. Enroute vectors I do on the FMS because I'll end up switching back to that mode anyway once cleared en route.

That's interesting, I didn't know a heading could be punched into an FMS.

So if you switch to HDG use the bug, do you need to re-input Present position direct your next fix? Will it do that on it's own? Or if you simply switch back to LNAV will it attempt to re-intercept it's original course? Might answer my second part to the question: Why does it take seemingly even longer to turn back from a vector

[rightseatwonder]

I'd say that the bank angle limit either manually set or automatically set at high altitudes and heavy weights would be the most likely culprit. The plane I fly is only manual adjustment but not sure if there is an FMC auto function on other types. But still its just set new heading then punch the button.

cheers

[Rotten Apple #1]

Hi Inverted. On the NG, you can't manually input the heading into the FMS. It would have to have been part of a vector on an arrival (DTW). Can't speak for other types.

On most vectors, we would re-enter "direct to" the fix and proceed direct to the fix upon the end of the vector. Depending how far off course laterally, the LNAV function may reengage the original track to the fix, but that's not normally how we would do it.

Cheers

(Pass a howdy to my buddies in Riverview)

[Liquid Charlie]

We use the UNS units and yes there is a lag when a heading is inputted via "command heading" - the unit also asks for a confirmation so there is another little delay - on tight vectors where the controller is almost keeping you in a constant turn the unit will not keep up and we either hand spank it or go back to the heading bug. I always thought it was a function of digital to analog with our steam driven airplanes -- interesting to see it also happens with completely digital systems.

[complexintentions]

To be honest I'm surprised that any SOP's would allow one pilot to be heads-down punching on an FMS unnecessarily in a a terminal environment...a lot of keystrokes for something that can be accomplished with the twist of a knob...no? Enroute is a little different of course.

On the B777 the bank angle is automatic dependent on speed/weight/etc, it does have manual settings for other angles, but even those will be limited by the FBW if you select one too high for the conditions. I've only ever come out of "Auto" once, when a controller specified a high rate of turn on crude vectors. I figured if he asked for it he must have a good reason to do so...

[invertedattitude]

Hi Inverted. On the NG, you can't manually input the heading into the FMS. It would have to have been part of a vector on an arrival (DTW). Can't speak for other types.

On most vectors, we would re-enter "direct to" the fix and proceed direct to the fix upon the end of the vector. Depending how far off course laterally, the LNAV function may reengage the original track to the fix, but that's not normally how we would do it.

Cheers

(Pass a howdy to my buddies in Riverview)

Thanks for the info, you guys are always happy to accomodate our needs which everyone appreciates.

I'll pass it on, don't be afraid to stop by sometime!