Protected area on IAP

[bookworm]

PANS-OPS vol 1 requires that an aircraft establish inbound (half-scale or +-5 deg) before descending on the inbound leg of an IAP. However, it seems to make no similar requirement of the descent outbound on leaving the IAF for the course reversal.

What's the extent of the primary protected area for the outbound leg, particularly a teardrop? (I don't have PANS-OPS vol 2, which I presume contains the answer.)

Thanks.

[keithl]

Bookworm, I see you haven't had a reply to this so far, so I'll just offer this as a "hang on there" and to keep the thread alive. Check the "Assumptions" in the "Initial Approach" section of Vol. I.
I think it may be there.

This is Ozexpat's territory, really, but if he doesn't mind - or is still celebrating New Year - I'll be able to check my Vols I and II on Monday. I'll get back to you then.

[bookworm]

Thanks Keith. Can't see it in vol 1 -- it says that it guarantees 300 m (984 ft) of vertical separation in the primary area, but doesn't specify the width of the primary area.

[OzExpat]

G'day keithl and bookworm. Sorry to have been away so long and missing the start of this but I was well and truly (and very happily) sidetracked on NYE. :E Anyway, I'm a long way away from my own books right now (am on annual Xmas/NY break in Oz), but think I can provide some answers to this.

The protection area for a "teardrop" procedure, known in Pans Ops as a Base Turn, is quite large and will vary somewhat according to the maximum speed for the particular procedure and the wind velocity that is used in the design. Outbound timing will also make a difference to the overall dimensions, though this can be reduced with a DME distance limit on the outbound leg.

Even so, the area is considerably larger for the Initial Approach segment than the final segment. However, it isn't Pans Ops that requires aircraft to remain within the tracking tolerances that you have specified. These standards, which seem to be fairly universally applied, have grown up over the years and, to be brutally honest, they are not reflected in the current Pans Ops design standards.

The basis of the design standards are the root-sum-square of a number of adverse circumstances. These circumstances include the bearing error in the transmitter and its' monitor, and the tracking accuracy assumed for the pilot. Note that these are not applied cumulatively but as a route-sum-square which assumes that not all of the errors will necessarily be adverse. I have an idea that a previous incarnation of the design criteria applied all the "errors" cumulatively and was considered to be the worst case scenario. This could be how the stated pilot tracking standards came about, because it was felt that pilots should be able to track well within the designed, cumulative error.

It might be a good idea to ask the world's Aviation Regulators why they have not changed the pilot tracking standards, to stay in line with the criteria. I suspect that the answer will be that there has been no evidence to date to suggest that a tighter tracking standard is needed. This is probably a practical assessment that recognises that, overall, the design criteria is still a little more conservative than it needs to be.

Of course, having flown many NDB-based procedures in anger myself, I still can't believe how one can stay within +-5 degrees of track when the needle is wandering all over the place! Like most people, I prefer the greater stability and accuracy of a VHF navaid but, as the RSS error budget for such navaids is smaller than for a NDB, I remain unconvinced that the well-known half-scale deflection is completely safe in all circumstances. But, then, I'm naturally conservative!

A full discussion of the width of the protection area for a Base Turn procedure is a bit impractical without reference to Vol 2 of Pans Ops. However, I can describe the area associated with a straight-line approach, where the Initial, Intermediate and Final segments result in an overall course change of less than 120 degrees at the FAF.

In this instance, the Initial Segment has a general half-width of +- 5 NM (that is, on each side of the nominal track). This area will be wider if the initial segment commences more than about 27 NM from the navaid, but I won't complicate matters with that sort of consideration. So, for the case I've started to describe, the first 2.5 NM each side of the nominal track is considered "primary area", where you get the promised 300 metres of minimum obstacle clearance. The area of 2.5 NM outside the primary area is called "secondary area", where MOC linearly reduces from 300 metres to 0 at the outer edges.

The principle here is that the primary area is where the aircraft can be expected to be, given all errors, on a 2-sigma basis. The secondary area can be considered as a protective buffer, but is an area where aircraft should never be.

Anyway, in some respects, the area that I've described will be larger than the area for a base turn, most notably at the IAF. In most other respects, it is a much smaller area than is provided in a base turn. However, by way of illustration, if the segment length is 5 NM and the aircraft remains within the primary area (2.5 NM maximum off-track allowance), the average angle of divergence from the IAF to the IF is a bit over 26.5 degrees. If the segment length is more like 10 NM, the average angle of divergence from IAF to IF is a bit over 14 degrees, which is easily within the tracking tolerances required of the pilot.

That's not the whole story, however, because the protection area begins to narrow at the IF. It reaches it's narrowest point at the FAF and then narrows even further, until reaching the MAPt. Thus, the pilot must not accept the full 2.5 NM off track in the Initial Segment because the aircraft will end up outside the procedure's protection area some time after passing the IF.

In the final analysis, therefore, the pilot tracking accuracy standards that are used in many parts of the world are probably a pretty realistic average. There is no doubt that they keep aircraft safe, but I'm not convinced that the aircraft will always be within the primary area for a straight-line approach.

It's probably a bit less of a problem with a base turn because the criteria includes allowances for intercepting the initial approach track from any track within +- 30 degrees of that track. And this includes a fairly generous allowance for drift due to wind, while turning to intercept the initial approach track.

If this hasn't answered your question very clearly, just holler and I'll try again.

[bookworm]

Thanks OzExpat that's just what I was after.

That's not the whole story, however, because the protection area begins to narrow at the IF. It reaches it's narrowest point at the FAF and then narrows even further, until reaching the MAPt. Thus, the pilot must not accept the full 2.5 NM off track in the Initial Segment because the aircraft will end up outside the procedure's protection area some time after passing the IF.

I suppose the issue at the heart of this is the proximity to track required outbound before commencing descent to the base turn level.

One way of looking at it is that even if 2.5 nm off track at the end of the outbound leg at the altitude for the base turn, the pilot can still make the base turn and establish within the tighter tolerances required beyound the IF before further descent inbound.

I suppose one could envisage a situation where an obstacle beyond the IF is far enough away to be outside that primary area for the intermediate segment but within 2.5 nm and high enough to be an issue even at base turn level.

[reynoldsno1]

Just to add another point - the initial approach speeds for reversal procedures are also very important, and they apply from overhead on the outbound leg . I have heard many pilots complain of overshooting the inbound track whilst turning, and most were surprised to learn that they were actually going too fast for the procedure...

[OzExpat]

bookworm... the Initial Segment of a Base Turn procedure normally ends upon completion of the turn. It goes without saying that there may, or may not, be an IF in such a procedure. However, I believe it would be a very bad idea to be 2.5 NM off track at the end of the outbound leg because :

If the aircraft is on the outside of the procedure, there may not be a lot of obstacle clearance. As you correctly state, an obstacle in the secondary area can be higher than in the primary area, so the pilot may have 100 feet, OR LESS, of clearance above it. This situation could easily remain true all the way through the turn.

If the aircraft is on the inside of the procedure, it's unlikely that there will be any chance of intercepting the inbound track directly, even at an extreme bank angle. Thus a severe overshoot is highly likely to occur. This might well place the aircraft out in the secondary area and, at the same time, increase the time needed for the intercept - thereby reducing the time available for descent on the inbound leg.
Neither of these situations are desireable and it needs to be remembered that the splay angle between outbound and inbound tracks is based on a particular maximum speed, as reynoldsno1 appropriately points out. If the aircraft is not close to the outbound track at the end of the outbound leg, an overshoot of the inbound track is almost certain. There are only a few circumstances in which a procedure will keep an aircraft within the primary protection area and still allow an efficient intercept of the inbound track.