ATPL Systems - LNAV & Holding

[Shimmer]

Just doing a bit of study for my ATPL Systems exam coming up before I came across a bit of an odd one.

So with LNAV engaged, the FMC will select a sector entry to the holding pattern as a function of the actual aircraft track as the holding fix is passed (not heading).

Why would they base it off actual aircraft track, when if we were flying it we would choose a sector entry based off current heading? Also do the 5 degree buffers apply when choosing a sector entry, or will it just choose a sector entry purely on track?

Thanks for your help!

[Alex Whittingham]

A bit of a stunned silence here, probably because the JAA don't examine the FMC in such depth. I'll try to find you an answer tomorrow, in the meantime do you know which make of FMC you are being tested on?

[Rainboe]

FMC works on track and groundspeed. It is not greatly interested in heading and TAS. All its operation is based around track to a fix and groundspeed to get there. It would be adding an extra tier of operation to get it to calculate heading to a fix and adjust sector entry. Much easier for it to just look at track to fix and calculate sector entry from that. It is also more logical. When non-FMS flying, you don't have an accurate reading of your track, therefore calculating sector entry on heading is more logical.

[Shimmer]

I was starting to lose hope on this question,hanks for answering Rainboe :)

Alex, it's on the B767 FMC - though I thought it would be a generic feature in all FMC's?

[Fratemate]

Adding a little more meat to Rainboe's bone (maybe I should rephrase that :)), LNAV is very interested in your track and groundspeed but it does also factor in your TAS. It uses TAS to work out the angle of bank to use to fly the holding pattern it's drawn. Likewise, it uses that TAS to actually construct the dimensions of the hold. It only does this each time it passes over the hold's fix. So, if you come into the hold at warp factor 5 (not that you would, of course, but imagine for one moment that you're me and you've screwed it up AGAIN), then as you pass over the fix LNAV will draw a hold which would be really quite large indeed. Having got the beast under control and slowed down, LNAV will reduce the AoB in the turns because the hold it has drawn was based on a high TAS and it is simply following the drawing. Next time over the fix, it will re-calculate the dimensions of the hold based on your new, lower, TAS and draw a new pattern.

Bear in mind what Rainboe said re track. LNAV does not care about '3 x drift outbound' etc (which we know is only there to achieve what LNAV is better able to do). LNAV will follow the drawing on the map and that will be based on the info you enter into the 'Hold' page on the FMC and the TAS, as discussed above. It will simply fly a track and will join the hold accordingly. I can't say that I've ever been in a position to look at the 5 degree margins, but I know what you're trying to ask. It SEEMS to go for the simplest option, so would favour a sector 1 (parallel) over a sector 2 (offset) but I cannot be sure of this, so would add just about as much small print as is possible to my mutterings.

Why would they do this? Well, I think it's because it makes much more sense to fly a holding pattern whose dimensions lie within the airspace boundaries, rather than a theoretical 'pear shaped' one which you fly because you can't fly a track. In other words, LNAV CAN fly the actual pattern you see drawn on the approach charts. It can be adjusted so the legs are 6 miles long, 1.5 minutes etc and actually conforms to the physical dimensions of the pattern the authorities would like you to fly to keep you busting someone else's airspace etc. When you fly a pattern in your Seneca, you're doing your best with adjusting headings, outbound timings etc but the proof of the pudding is when you look at how it looks of a radar/simulator print out. There's LNAV with its beautifully flown race track pattern; all the correct dimensions and looking like someone has superimposed the approach chart on the radar. Then there's my manual attempts; looking like a demented pear drop whose 'fat' bit busts the local TMA boundary and whose long bit lasts too long because I cocked up the strength of the wind. So, why? Because it's better and you can :ok:

[Kirks gusset]

Just for info, not confusion! Most Boeings fly Track Up not Heading up, so there is an orientation issue here, consider the potential drift corrections aloft and each aircraft will treat these differently based on speed and altitude. The FMC draws the holding pattern entry and constantly corrects the legs, but only after the holding fix has been passed after joining the hold, not before. As most commercial holds are approached as part of a Star, by definition, the entry track as shown on the Star will be fairly constant.If you can get your hands on a Boeing Part B systems it is explained in detail.