Instrument Flying Procedures Question

[Hugh Been'ad]

A good one for IREs, Trappers, StanOs and spotters. I would be grateful if anyone could provide the definitive (and correct) answer on the following, a challenge perhaps!

When on an Instrument Procedure in the overhead of a facility, what is the outbound tolerance? i.e. you are "beacons outbound" and descending to set yourself up for the inbound turn. Is it +/- 30 degrees or +/- 5 degrees. I know it is definitely +/- 5 degrees from the FAF but haven't been able to get past rumour and hearsay for an answer from leaving the beacon.

I have looked through PANS-OPS 8168 for the past 45 mins and my eyes are bleeding :uhoh:

http://dcaa.slv.dk:8000/icaodocs/Doc%208168%20-%20Aircraft%20Operations/

[TheInquisitor]

Within +/- 30 deg from the outbound when overhead the facility.

You are correct about +/- 5 deg on the inbound (to commence / continue descent) - not sure if it's from the FAF or the 'profile' descent point....since on a pure NDB, I think overhead the facility IS your FAF....AHHH, now my ears are bleeding!

[bArt2]

When you are proceeding to the Initial Approach Fix (i.e. in this case overhead) you must be within 30º of the outbound course to be allowed to directly intercept it, passing overhead you turn and then descend (assuming there is a descend published in the outbound leg).

This requirement to be within 30º is there to assure that you stay in the protected airspace after passing the IAF (assuming you stick to the speed restrictions) and therefore there is in my opinion no need to have another restriction for the descend. It would be sensible however to first turn to an intercept heading before starting descend (my opinion)

If you where not within 30º you have to go via the holding which will then put you within 30º at the completion of the holding.

This explanation is about the civil procedures, military (DoD-Terps) are slightly different as there is no need to be within 30º to intercept the initial approach segment. However an IAF of a Tacan procedure will normally never be overhead the TACAN due to the bigger cone of confusion related to the TACAN system, therefore I assume you where talking about a civil procedure (VOR or NDB)

After (or at the end of) turning inbound you can not descend to the next altitude (mostly FAF altitude) before you are within 5º of the radial/course you are intercepting. (Also valid in mil procedure)

I've never heard about being within 30º before starting descend, I suppose that comes from a misinterpretation of the first point I explained.

I hope this helps, greetings Bart

[Hugh Been'ad]

To add more "meat to the bones" the Filton ILS/DME procedure (see link below) has you arriving on a track of 275 degress (within the 30 degrees required for ARRIVING) and then departing on 293 degrees once overhead the Beacon.

The example I am trying to explain is if your Handling Pilot was suffering from "a lack of ability" or wind* (or the hand of god), and failed to fly directly over the Tacan, could he legitimately decend with the procedure if he is more than 5 degrees off the outbound track of 293 degrees.

http://www.aidu.mod.uk/images/tap.jpg

p.s. Thanks for the responses so far!
p.p.s *The reference to wind is about PGF and Coriolis, not the results of a good curry!

[Role1a]

In that example I would say you need to be within 5deg of 293 from the NDB before a descent can be initiated.

R1a

[Flymee 2 Da Moon]

IIRC from my days as an IRE, standard IF limits apply on the initial segment of the instrument approach which included +/- 5 degrees on any radial tracking(Airways or 'beacon outbound´). This would also be used as the tolerance on the relative bearing, outbound on a NDB approach (Question for the pedants; is it 'a NDB' or 'an NDB'?).

[Emerson Cahooners]

Just over a year since I ceased to be an IRE (I'm Staff filth now:() but I believe the answer is +/- 5 Deg. That said in the cone of silence, while the CDB is dancing all over the place, you can commence your descent.

[Brain Potter]

You can start descent as soon as you cross the beacon/fix and don't have to be fully-established on the outbound track. However, you must be within 5 degrees to start descent once pointing inbound (the intermediate segment) because the separation in the primary area is now reduced from 300m to 150m.

To back my thoughts this was lifted from my Air Law notes:

Track Reversal Manoeuvre:

The track reversal procedure (the turn from inbound to outbound) may be in the form of a procedure or base turn. The 3 generally recognized manoeuvres related to the reversal procedure are the 45/180 degree procedure turn, the 80/260 degree procedure turn and the Base turn.

Flight Procedures for Racetrack and Reversal Procedures:

Entry. Unless the procedure specifies particular entry restrictions, reversal procedures shall be entered from a track within +/- 30 degree of the outbound track of the reversal procedure. However for base turns where this sector would not include the reciprocal of the inbound track, the entry sector is expanded to include it.

Descent. The aircraft is to cross the fix or facility and fly outbound on the specified track , descending as necessary to the specified altitude. If a further descent is specified after the inbound turn, this descent is not to be started until established on the inbound track. Established is considered to be within half-scale ILS localizer deflection or within +/- 5 degrees of the required bearing for the NDB or VOR.

Of course the tracking prior to the Intermediate fix is still subject to the standard Instrument Rating limits.

[bArt2]

The example I am trying to explain is if your Handling Pilot was suffering from "a lack of ability" or wind* (or the hand of god), and failed to fly directly over the Tacan

He would have to grossly f*ck-up. For example at 8 DME an angle of 5º equals about 0,7 Nm. To end up more than this 1Nm before the station he would have a tracking error of about 45º.

If he can not track to the station more accurate than 45º I can't see how he will manage to know when he is within 5º :E

P.S. You can not replace the example NDB procedure with a TACAN since A tacan station will normally not be used as an IAF, in case of a TACAN approach the IAF is allways a radial/dme fix (as I mentioned before)

Greetings, Bart