Thank you all for the great replies.
I have always assumed that the SDFs must be checked during the descent… well you will fail your IR test if you don’t so hey it must be that way 
But are they meaningful with respect to obstacle clearance?
I loaded a GPS track of one flight of that IAP into Oziexplorer and loaded an old 1:25k Ordnance Survey map. And I see that the track flown contains a pretty generous obstacle clearance (map uses metres BTW). The “Track” databox is shown immediately to the right of, and below, the corresponding track point.
The current radio masts are about 850ft high (the old map shows them at 216m; ignore that) and here we are at about 1600ft
Further down, at the MDA, the absolute clearance narrows to about 430ft and actually if you fly that IAP in VMC it is that bit which looks a lot more scary. The track gets a bit bendy here because I disengaged the autopilot, went to full flap, and was hand flying
Furthermore, there is no way you will get anywhere near those masts laterally because you have GPS guidance. The distance is about 500m. On the CAA 1:500k map it looks like this
This IAP is currently unpublished because Shoreham lost its ATC, although ATC has more or less always been there on a part-time basis. Hilariously, of all the GPS databases out there which removed the IAPs immediately they were axed from the AIP, the KLN94 one never removed the IAPs, and long may it last
Airborne_Again wrote:
The yellow tables in Peter’s example are not part of the procedure design – they are advisory!
That is correct. So checking them is only required when they match step-down fixes, but I find them good assistance in flying the approach as a CDA vs a D&D and they are above the step down fixes anyway…so useful if you do not have vertical guidance.
In any case, I would like to see one of those examples where the advisory vertical guidance registered in the database does not ensure terrain clearance…do yuo happen to know one?
Antonio wrote:
In any case, I would like to see one of those examples where the advisory vertical guidance registered in the database does not ensure terrain clearance…do yuo happen to know one?
There is one airport in the US which is commonly used as example where there is a ridge just before the airport, unfortunately I don’t remember the name. You have terrain clearance down too the MDA, of course, but not below that if you follow the nominal glidepath.
Another example is Stuttgart airport before the runway extension. Runway 07 had an NDB approach with an advisory ILS! The nominal ILS glideslope did clear a hill in the approach, but not if you were half deflection below, so NDB minima applied.
Peter wrote:
Furthermore, there is no way you will get anywhere near those masts laterally because you have GPS guidance. The distance is about 500m
Is this an LNAV approach?
Yes; GPS/LNAV.
Peter wrote:
The distance is about 500m
THen the MOC design criteria should be +250ft.
It is an interesting study that you did…I guess you could do it for every approach procedure, but is it not what onboard navigation guidance does anyway?
There is also an additional vertical criteria that descent path should be no more than 3.7 deg or something like that (1000fpm @ 150 kts gs) or else you need especial procedures, so this may also drive some of the step-down altitudes. Perhaps this had an influence in the Shoreham design?
Not that you would be doing that, but if you were to design an IAP, then 250ft MOC should be legal between FAF and MAP if the descent path criteria is met.
Airborne_Again wrote:
You have terrain clearance down too the MDA, of course, but not below that if you follow the nominal glidepath.
I see…so you must be visual before MDA, which you should anyway.Airborne_Again wrote:
Runway 07 had an NDB approach with an advisory ILS! The nominal ILS glideslope did clear a hill in the approach, but not if you were half deflection below, so NDB minima applied.
Wow! That is really interesting! I assume it is no longer active?
I would be more worried about advisory guidance outside of the FAF, where clearance may or may not be good, but I do not know if that guidance is active on GPS approaches or it only activates inside the FAF…do you know?
Antonio wrote:
I see…so you must be visual before MDA, which you should anyway
Nope. You shouldn’t descend below MDA until you have the required visual references. You need to be visual by MAP.
Well, yes, on a nonprecision approach (like this one) you don’t get assured obstacle clearance below the MDA.
But that’s a different topic… There is one airport in the US (Stanley something?) where there is a rock there, and there are probably others.
Peter wrote:
Furthermore, there is no way you will get anywhere near those masts laterally because you have GPS guidance. The distance is about 500m.
Peter showed me the approach chart. It is a RNAV(GNSS) approach with LNAV minima. For that it is assumed that the total navigation accuracy is such that the aircraft will be within 0.3 NM of the final approach centreline at least 95% of the time. This figure includes piloting accuracy, not just guidance accuracy.
The total width of the approach area is 1.5*0.3+0.5=0.95 NM on each side of the centreline. The 1.5 factor is to get the 95% up to 99.7% – three standard deviations – and the 0.5 addition is a “buffer value” to cater for the remaining 0.3%. Half of the distance on each side of the centreline (the primary area), e.g. ≈0.48 NM on each side, has full minimum obstacle clearance of 294 ft. The outer half (the secondary area) has progressively less obstacle clearance reducing to zero at the full distance.
So the masts you mention are considered in the obstacle clearance calculations.
