Protected Areas for Procedures

[NotaLOT]

Dear All,

I am trying to get my head around protected areas around procedures.

PANS-OPS (Doc 8168) states that all procedures depict tracks that pilots should attempt to maintain.

As I understand it, protected areas should be obstacle free in case the aircraft is not able to stay exactly on track - i.e. it offers a buffer of space that the aircraft will to a high probability always fit inside when flying the procedure.

Is my understanding correct? In that case, is there a probability assigned to the protected area (e.g. 99% of the time the aircraft will be inside the protected area)?

If I am now doing a noise assessment say, or any other analysis that requires knowledge of the aircraft position in space, would I use the nominal procedure track to locate the aircraft, or should I also take into account the fact that the aircraft may not necessarily be always on track?

I have had some very useful feedback on my previous questions on this forum, so I hope you will help me again.

Regards,

NotaLOT

[alphacentauri]

G'day NotaLOT,

As I understand it, protected areas should be obstacle free in case the aircraft is not able to stay exactly on track - i.e. it offers a buffer of space that the aircraft will to a high probability always fit inside when flying the procedure.


Yes this is correct.

The protections areas are based on an idea of aircraft containment, and take into account system accuracies. To this end the areas sepcified in PANS-OPS are designed around 3 standard deviations, or 99.7% containment. Have a look at PANS-OPS Vol2 Part1 Section2 Ch2 Para 2.8 for ground based navaids.

Also have a look at Part 3 Section 1 Ch2 for RNAV info.

Hope this answers your question

Alpha

[NotaLOT]

Thanks Alpha, and what about the second part of my question?

Is the probability of the aircraft going slightly off track (but within the protected area) high enough to justify assuming that it could be located anywhere within the protected area, or is it reasonable to assume (for the purposes of a noise assessment for example) that it will fly on track?

[alphacentauri]

NotaLOT,

Are you familiar with the concept of secondary areas, as they apply to MOC reduction? ie the protection areas are broken up into primary and secondary areas. The primary area is usually half of the 3SD width at any given point and this area gets full (100%) MOC. The secondary areas then have the MOC reducing linearly to zero at the out edge of the splay.

For protection areas in final approach it is my understanding that, the primary areas roughly correspond to half scale deflection (VOR/RNAV) or +/-5 (NDB). Since the aircraft can legally fly anywhere within these tolerances we have to assume that the aircraft will be anywhere in the primary area. Once the aircraft has exceeded tracking tolerances, then it is assumed the aircraft will conduct a missed approach so that the probability of the aircraft being outside this primary area decreases the further out you get. (Hence the reducing MOC in the secondary area). This is also valid in other phases of flight (enroute, initial approach,etc)

To specifically refer to noise corridors, (this is just my opinion) yes you could reasonably assume the aircraft will fly on track, especially with todays nav and autopilot systems. However, you probably would need to consider aircraft that still use steam driven nav systems or no autopilots and they could be anywhere in that primary area.

Hope this helps

Alpha

[NotaLOT]

Thanks Alpha, that makes it much clearer. I appreciate the advice.