GPS substitution for navaids - Europe generally - is it allowed? (and low vis ops)

Ibra wrote:

The protected area of LNAV is based on lateral 0.3nm

Actually it is twice that value, 0.6 NM.

NCYankee wrote:

Ibra wrote: The protected area of LNAV is based on lateral 0.3nm

Actually it is twice that value, 0.6 NM.

@NCYankee, I though it is .45nm (1.5TSE, 1.5×0.3=4.5) as it could be .3 nm without you noticing and then half the scale adds .15nm.
Hence .45nm each side for primary area.
Am I wrong?

NCYankee wrote:

Actually it is twice that value, 0.6 NM.

Yes full-scale lateral deflection of CDI is at 0.3nm, this is only half of 0.6nm surveyed primary area, the primary area has flat MOC, then there is the secondary area where MOC goes linearly to 0ft, I am supposed to fly 1/2 CDI deflection or go missed, on non-WAAS that means staying within +/-0.15nm of center-line (about 1/4 of primary area), what can go wrong? long story short there’s a lot of leeway even in non-WAAS GPS, when executing under CDFA, one will go missed 2nm way from MAPT, that would surely keeps everything in whatever ‘NDB protected area’?

Of course, it’s a serious safety issue for someone flying DnD, cruising parallel to runway at MDH and full scale CDI deflection, then being so unlucky that before reaching his MAPT he goes and hit some downwind pylons, hey we can’t save everybody….

Even if NDB straight-in minima is problematic for our unlucky guy above, I am sure the NDB circling minima if any are published are still usable for non-WAAS LNAV guidance, including at full scale CDI deflection

arj1 wrote:

I though it is .45nm (1.5TSE, 1.5×0.3=4.5) as it could be .3 nm without you noticing and then half the scale adds .15nm.
Hence .45nm each side for primary area.
Am I wrong?

This is from TERPS:

According to PANS-OPS, the obstacle clearance area of an LNAV approach at the FAF extends to 1.45 NM to each side of the final approach track. Half of this (the primary area) has full obstacle clearance, then the clearance reduces linearly to zero at the edge. (This decreases to 0.95 NM closer to the runway, but when you compare with an NDB approach the NDB is usually the FAF so the area around the FAF is the critical one.)

In contrast, the obstacle clearance area at the NDB extends to 1 NM to each side!

(I don’t claim this is reasonable – I’m just taking the figures from PANS-OPS.)

Airborne_Again wrote:

In contrast, the obstacle clearance area at the NDB extends to 1 NM to each side!

That will depend if NDB is at FAF with guidance on the tail? or at the MAPT with guidance on the head?

But I really don’t see any design issues at FAF for both? there are some near MAPT & MDH, I doubt it’s meaningful to compare the geometry of LNAV protection surface to the NDB protection surface?

• First, I am assuming the majority of NCO operators fly LNAV with GNSS otherwise these discussions happens on different planets and GPS is very precise

• Second, the real question, does flying on GPS keeps one always inside NDB protection surface all the time from FAF to MAPT? I am sure one can’t fly outside NDB obstacle area on GPS unless they really decided to do so, especially with angular CDI scaling if NDB IAP is loaded from database, you could argue there is an issue with ‘large angular guidance error’ for non-WAAS GPS near FAF & MAPT but you should be inside whatever +/-1nm…

• Third, if some LNAV FMS acts like black-box, yes definitely ban it’s NDB substitution but LNAV on GPS boxes TSO129/146, one should always stay within NDB protection design

This also works when destination airport is loaded in FPL without even having an NDB IAP in GPS database !

Doing OBS mode on GPS/FMS with ENR DCT toward some NDB FIX (without loading NDB IAP as approach or AD in FPL as destination), flying abeam at +5nm or +2nm is not very wise…

Ibra wrote:

That will depend if NDB is at FAF with guidance on the tail? or at the MAPT with guidance on the head?

As, I wrote, at the FAF.

CDI scaling on a non-SBAS device is relatively simple:

The width of the obstacle clearance area is not directly related to CDI scaling, although of course both depend on the RNP specification.

Airborne_Again wrote:

The width of the obstacle clearance area is not directly related to CDI scaling

Indeed, but from risk-based perspective one should only care if his CDI goes outside the protected area during the substitution?

I am sure Garmin can update their GPS-W such that CDI scaling mimic the primary protected area of an NDB given that GPS is inherently accurate, I don’t think it’s very challenging to degrade sensitivity & accuracy to match whatever the NDB does?

Ibra wrote:

Indeed, but from risk-based perspective one should only care if his CDI goes outside the protected area during the substitution?

Obviously one doesn’t as the CDI scaling and protected area width are different!