We're probably getting a bit far off the topic here
alf in respect of single navaid non-precision approaches. However, as you have raised the issue of not to "drive and drive", I'd suggest that there's not always a lot of alternative. Classic case from this part of the world is a VOR or NDB at the wrong end of the aerodrome for the only possible straight-in approach.
The base turn procedure is longer than normal because of the need to provide a reasonable length for the final approach segment. Sure, you make a reasonable calculation of time and ROD to work out a profile descent but, in absence of a FAF, you'll never really know how the profile is actually working out. There are, of course, three possible scenarios here.
- Everything works out exactly right, with nothing to worry about. I don't know about you but, for me, this is really more like a "hit or miss" type of scenario. I've seldom seen this work in practice because there are too many things that can go wrong.
- Descent profile is wrong and you end up too high to reach the MDA before the MAPt (let alone before the VDP!). The implication here is too obvious.
- Descent profile is wrong and you end up at MDA in level flight to the MAPt, or VDP. This is, of course, the "dive and drive" scenario.
And, of course, you'll never know your VDP without some sort of distance reference.
Like most pilots around here, I use GPS as a backup for distance. I select the relevant navaid from the database and base all my calculations on that, making due allowance for the approximate distance from the navaid to the threshold. It's pretty rough and ready but normally works out reasonably well here because we don't get a lot of wind effect.
However, you need a lot of confidence in the distance information from the GPS (bearing in mind that it's operating in enroute mode at the time). You also need some caution and a fair bit of good gut instinct. Without all of that, I'd suggest that there are places in the world where a single navaid procedure can only be flown as "dive and drive".
As for Baro-VNAV, this is closer to the topic because these procedures have a Decision Altitude. This is because they have a glideslope. For these reasons, a check altitude is always needed for these procedures.
It is even more important to catch glidepath errors in a Baro-VNAV procedure because the glidepath is generated barometrically. This introduces a complication that doesn't exist in a precision procedure, but is catered for in the design of the procedure and the specification of a temperature range within which the procedure is usable.
There is, of course, no OM, MM or IM associated with this type of approach, but there
is a distance reference, so I guess that brings us back on topic. At least for a while!
