It may be worth noting that most IAP minima are predicated on CDFA. If the plate specifies CDFA, I guess that there are no minima published for D’n’D.
I thought there were a number of factors that go into determining the visibility minima…eg couples autopilot, CDFA, approach lighting etc…. so that as you forego each element you pay a penalty in terms of increased visibility minimum….typically using CDFA on a NPA reduces the visibility requirement by around 2-300m or so I thought..,in any case CDFA is not specified for St Mary’s as far as I can see…
RobertL18C wrote:
The CDFA does require better situational awareness and instrument skills as you are checking off that you are within +/- 100 feet off the target altitudes every 1 DME, while maintaining a stable Vref.
Maybe. I do find CDFA much easier to fly. I was “brought up” with DnD when I took my IR in 1987. CDFA wasn’t even being discussed back then. When I renewed the IR in 2014 after not having flown IFR for 18 years (and not flown at all for 17 of those years) CDFA was there and it was like a revelation. Why wasn’t an NPA done like that from the start?
You don’t even need a DME or GPS — checking altitude with reference to timing works reasonably well.
Timothy wrote:
I wasn’t using GPS ILS, it was not necessary as I was on a published approach. I was doing CDFA based on the GPS range to touchdown.
Thanks for the reports back and learning value for the st of us. Out of interest, what was the plan with originally requested approach to Runway 14? All the best.
I have found in training that some people do not realise that flying RoD based on GS is a Euclidean mathematical certainty (as opposed to a good starting point.)
In other words, if you have a GS readout (whether from DME or GPS), and you fly a really accurate RoD appropriate to GP angle and GS, you will fly the slope as if it were a glideslope.
For a 3° slope, 5fpm per kt is quite accurate enough to give a descent as precise as a glideslope, and really you only have to remember the three obvious numbers: 300 for 60, 450 for 90 and 600 for 120, then work to the nearest 50fpm per 10 knots to keep you within two reds and two whites all the way down.
This is why it is important to be stable, and also why life is a bit easier in a retractable (as for all aircraft it seems to be true that dropping the gear without touching trim gives you a 3° descent without a change in speed.)
Once people have internalised that, NPA and CDFA become trivial.
If I have a student who is prone to “needle chase” on the ILS, I cover up the OBS/HSI and get them to fly heading and RoD only so they can see how amazingly accurately they arrive at the MAP.
Balliol,
That would have been OBS and RoD vs GS and GPS distance in the normal way.
You have seen the papers I have written for the CAA on the subject, I believe?
No I haven’t I don’t think, I have been snowed under with email traffic recently so may have missed it though. How do you calculate minima for an unpublished approach like that then?
Drop me an email and I’ll send the papers. The CAA have been cogitating about them for years, and indeed they came up again today at a meeting in the Belgrano.
I don’t really want to dribble the information, like minima calculation, out, point by point. The whole thing has to be seen as a whole in the context of contemporary attitudes to regulation and risk.
Graham wrote:
Now now, it was an NDB approach…
It also wasn’t anywhere near zero-zero. From the accident investigation report:
SPECI EGHE 201631Z 19016KT 4500 BR SCT002 BKN003 17/17 Q1017
Observation made as a result of the accident.
