ATC 'Maintain present heading' instruction
 

Can someone explain me why sometimes ATC orders you to 'maintain heading' after being cleared to a certain waypoint - sometimes 1 hour flighttime or even more further down the route?

E.g. ATC clears you direct BLBLA and asks you to report your heading once established inbound BLBLA. Once established inbound BLBLA, you report your heading and ATC orders you to 'maintain that heading'

Has it something to do with different lateral separation requirements for aircraft flying 'own navigation' and for aircraft flying on an assigned heading?

I understand that ATC requires you to fly headings in during descent and approach in terminal areas, but a lot of times it also happens enroute at high levels.

Thanks for yor reply!
Stephen

ATC will give you the best direct routing available most of the time. If you are then told to maintain the heading it will be to separate you against other traffic at or going through your level. The lateral separation limits don't change whether you are on a heading or not but when Radar Separation is applied separation has to be ensured, hence the use of headings. If you are given a direct and immediately afterward told to continue on the heading it will be for the same reason but the controller has decided to let you pick the optimal heading to your way point rather than estimate it him or herself. It’s a good method if the waypoint is a long way off or there is a strong wind playing havoc with the holding headings. Often the traffic you are on a heading against is not on (or not yet on) the same frequency, so it will not be obvious to the flight deck what is going on.

now assume loss of radio contact
 

how long would YOU maintain the heading, until you realised you were no longer on RT.
assuming a reasonable difference in track made good and track required.....just to keep us thinking along the same lines....Bumz

Maintain Heading?
 

The reason we send you direct then ask you to "maintain heading" is exactly for the reasons you describe.

Where an aircraft is overflying a sector i.e. flying direct to a waypoint an hour down the line, we lock you on that heading if we have aircraft above you to descend down to an underlying airfield or aircraft departing from airfields beneath that have to climb up through you. Generally we will lock the overflight on a heading and vector the climbers or descenders around your heading as the overflying aircraft.

There are no set parameters about lateral distance apart for climbing or descending aircraft through others but the CAA merely insists that we 'ensure' separation. For example Airbus aircraft 'anticipate' turning at reporting points on autopilot and sometimes turn before you expect them to. As a very general rule, I expect some colleague flak here!, I would say roughly 15 miles lateral on head-on routes, maybe slightly less on similar direction routes would require us to use headings for climb/descend throughs.

As regards R/T fail or loss of contact the procedures are quite clear. In the Area Control environment if you are unable to get 2 way R/T with an appropriate sector, you maintain the heading for a period of 3 mins from the time it was issued then squawk R/T fail and fly the shortest possible route to rejoin your flight plan. The rules are slightly different in the TMA or SID STAR environment.

Hope this helps.

DC

The infamous "fly by". A pain in the neck and potentially dangerous too.

:mad:

And so does the Boeings. I know you can create "Overfly" waypoints in the bus. Still havent seen it in a Boeing, maybe someone can enlighten me.

duece

Compared to the 'bus, the Boeings do it on a neglectable scale, hardly noticeable on Radar.
Even when flying towards an obtuse angle, the 'bus will turn (depending on angle and speed) several miles before the waypoint to "catch" the next track.
When it gets really annoying is when the angle between tracks is acute, then the turn can "happen" up to 15NM before the waypoint. Not good and safe enough!

wow, I thought boeings were bad with a mile and a half or so. But 15 miles??? Can it really be that much?


duece

Very minor point and not related to the thread, but I'm sure I was taught you "maintian" a level and "continue" on a heading?? I stand to be corrected if I'm wrong...

With regard to the comms failure - when being radar vectored according to the rules we wait 3 minutes and then return to the flight planned route. (UK local requirement).

However, we have been sent to a point on our flight planned route whatever miles ahead. This suits us and suits ATC. One could argue that we are not being radar vectored we are simply agreeing with ATC that we will maintain the heading that is currently taking us to that reporting point. Thus we would in effect maintain the heading for 3 minutes after R/T fail and then if necessary adjust heading to maintainn the track to that reporting point and thereafter follow the filed route.

As for the problem with fly-by waypoints. There is nothing new there. It is not only modern aircraft that can take advantage of that - old steam driven aircraft can also (and have done also) in enroute airspace.

Remember that when enroute we navigate to RNP5 - we must remain within 5nm of the route centerline. Thus the aircraft can at any time be up to 5nm to one side of the ATS route and still be considdered to be navigating to the required accuracy. When at a turn point (intersection or beacon) it is perfectly allowed for the aircraft to "cut the corner" or "anticipate the turn" or whatever one likes to call it provided that the flight remains within the requirement of RNP5. Imagine leaving a 90deg turn overhead a VOR until the VOR indicates station passage as say M.80 and FL350 - how far would that put the aircraft off track initially!!!!

I believe that is one of the reasons why radar controllers lock aircraft on headings to ensure radar separation.

Regards,

DFC

RNP5 is an entirely theoretical value. A deviation of 5NM to the left or right of track in reality does not happen. In fact the tracking of a BRNAV equipped acft is exact to the mile.
As for the fly by, it makes a huge difference if an acft such as the Airbus does it extensively as described above or only on a minor scale as also described above. It is a safety issue.

Of course if you knew ANYTHING about procedure design, you'd realise what a stupid statement that was :rolleyes:

Go read the definitions for 'fly by' and 'fly over' :)

Only potentially dangerous if you don't know wtfwhn ;)

Which means that you remain within 5NM for 95% of your route ..... sometimes (for 5%) you could actually be MORE than 5NM off track (legally ;) )

Pprune Radar,

When was the last time you worked in front of a Radar screen and noticed that the Airbus turns considerably earlier than any other type of acft?
That it is the issue here and not fly by as such.
Of course I know that a waypoint can't be overflown on the spot in every circumstance, but it still makes a difference when it is 10NM before (Airbus) or 2NM before (Boeing).
And BTW I'm not claiming to carry the ultimate wisdom re ATC (must be a NATS thing...).

Spuds (non-NATS and non-UK ATCO, yes, ATC is being done elsewhere too...)

I'm unsure how locking an a/c on a heading assures separation. I've heard some arguments which were factually incorrect, and am yet to be convinced. If an a/c is tracking somewhere, and you want him to keep doing that, why say anything at all?

Ferris, try and picture a square with points marked a & b across the bottom, and c & d across the top...

c . . . . d


a . . . . b



Now aircraft No 1 is flying from a to d, and aircraft No 2 is flying from b to a. No 1 is at FL 380, No 2 at 400 descending 360...

Still with me?

If I tell No1 and No2 to "continue present heading" then I can prove my separation because both aircraft were under my direct control and, assuming the headings were providing the required distance (5miles), would not hit each other...

If I leave both aircraft on own nav then the possibility exists that either aircraft may have an intermediate point on the route that would erode the separation and I would therefore not have PROVED it.

I hope that makes sense, next week intermediate Nuclear physics for the retired hostie!!!:cool:

That is exactly what I am talking about. Your example doesn't "prove" separation. It's a furphy.

Continue present heading will not guarantee to keep the aircraft apart laterally. If the wind is different at any of the levels between and including FL370- FL390, then the possibilty exists that the aircraft will drift closer together until they experience the exact same wind at the exact same level (FL380). Lateral sep could be lost within those levels before vert exists.

If, however, the aircraft are told to "track direct to d" and " track direct to a" respectively, that guarantees they will remain on set paths (which is what you are seeing on the radar) despite moving air masses, and therefore known distances apart. Aircraft told to "track direct XXXXX" don't have mysterious intermediate points in their route, unless not following the clearance.

I imagine that this is an old-fashioned technique from pre-RNAV days, when headings were probably more accurate than 'own nav'.

I think the point is that ATC have control over the aircraft's direction, rather than the pilot's, ergo "proven" separation.

That said, where I work aircraft on the SID's are deemed to be on "known tracks" and lateral separation can be used as if they were on headings.

PPRune Radar,

I prefer to think it more in the terms of in 100,000 flying hours, the flight with be within 5nm of the centerline 95% of the time - we'll be long retired before we get to the 95,000 mark! :)

However, while we can in theory cut corners (and the UK CAA IR flight test required it for commercial expediency there is one requirement that prevents one from pushing it..............remaining within the airway (controlled airspace) at all times.

Isn't turn anticipation for a fly-by waypoint changed if the angle of bank limitation is set to a low level? i.e. limiting the bank angle changes the turn radius requiring the turn to be started earlier to achieve the correct intercept of the outbound track!

Regards,

DFC

separation
 

Ferris,

Methinks you are being a little pedantic. If I have two a/c in proximity to each other and they are on their own nav, they can turn unexpectedly and come into confliction. If they are maintaining level separation, then this is not a factor. 'Track direct' is not a recognised technique, nor does it ensure separation, however nice the idea may be.

If I want to do a climb through, I will instruct them to fly or continue a present heading as a radar heading thus locking them on to a 'virutally' non deviating track.

We are not stupid, we know winds change, what we are doing is locking a/c onto a resultant track. Any Atco that did this to ensure the minimum 3 miles separation would either be very skilfull, or very silly. The A/c in question may pass each other with ten miles separation in the end, but if they are crossing tracks at oblique angles, a 15 degree heading change by one or the other may make a comfortable situation become sticky. Hence the locking of headings. It also means that a busy ATCO can get on with the other controlling tasks without constantly monitoring the climb through.

If, whilst locking A/c onto headings we can give one a direct routeing to a waypoint, then it's a win win situation for us and the pilot, who gets more direct routeing and an earlier climb/descent to final level.

Working in the London TMA, we have to use headings to achieve what we need to do. Whether these are controller chosen headings or 'locked on' pilot headings is a moot point if it is providing the required separation.

More twaddle.

I don't think I'm being pedantic- quite the opposite. I think I am questioning other pedants who are certain locking a/c on headings makes things safer. I am questioning that (something that doesn't go down very well in this industry). Stuff like How? Mysterious intermediate waypoints? Wx avoidance? Or some other random departure from clearance?

Sure, I can think of lots of examples where using headings is useful (do it every day {btw "Working in the London TMA, we have to use headings to achieve what we need to do"- well guess what: In the eleven-and-a-half-thousand movements per-man per-year UAE, we use headings to achieve what we do, too.:rolleyes: }). But this thread was about the practice of using headings to somehow assure route-following (and therefore separation). Something I question. With good reason if you believe