denopa wrote:
But your comment on flying 2 dots down to aim for the treshold instead of a touch down zone could be mistaken as such, and I’d rather call it out.
Not at all. The Visual Approach is totally dumb. It appears that it aims for a TCH of 50’ regardless. My only point was that, experimentally, if you fly the glidepath it takes you approximately to the centre of the runway, if you fly two dots low it takes you to the threshold. That is not a recommendation, just an observation, made because I am lucky enough to have a GTN with up to date software and database on my sim, so can play with it and report.
Cobalt is absolutely right that, as @Airborne_Again and I have said a number of times, this is purely a theoretical discussion. No-one is going to use this to LPV minima, preferably not at all in IMC. The way it was shown in the video that started this discussion seems somewhat reasonable, the pilot seems to have had surface contact from about 700’ and could see the runway from about 600’, which is not threatening.
Peter wrote:
FAR 91.175 which in superb clarity says exactly thatSec. 91.175
Takeoff and landing under IFR.
(a) Instrument approaches to civil airports.
Unless otherwise authorized by the Administrator, when an instrument letdown to a civil airport is necessary, each person operating an aircraft, except a military aircraft of the United States, shall use a standard instrument approach procedure prescribed for the airport in part 97 of this chapter.
I don’t agree that that is the least bit clear. When is “an instrument letdown to a civil airport necessary”? If I do it by DR is it OK? If I could do it by DR (making the instrument letdown not necessary) may I use a DIY approach, because the instrument letdown was not necessary?
If you want clarity, you say that an aircraft may not descend below the IFR enroute minima except on a published approach procedure.
But they say that in Germany and people still crash on DIY approaches, so it clearly doesn’t work.
Timothy wrote:
I don’t agree that that is the least bit clear. When is “an instrument letdown to a civil airport necessary”? If I do it by DR is it OK? If I could do it by DR (making the instrument letdown not necessary) may I use a DIY approach, because the instrument letdown was not necessary?
No, you can’t descend below the Minimum IFR Altitude, regardless of type of navigation or DR unless you use a part 97 approach. There are no DIY approaches in the US for civil aircraft.
The construction of “when an instrument letdown to a civil airport is necessary” in part 91.175 means that if the let down is in less than VMC conditions for the airspace involved, and a standard instrument procedure must be used for the descent. For an aircraft that can operate in VMC from an IFR minimum altitude to a landing at the airport, it is not necessary to conduct an instrument letdown and a standard instrument approach procedure does not need to be used. But if this can not be done under VMC, a standard instrument procedure must be used.
One is not authorized to cruise at an altitude below the MIA (Minimum IFR Altitude – 91.177) or descend below it except when landing which is governed by 91.175. 91.175 defines what is required for landing under IFR, or in other words descending below the lowest IFR altitude or the Minimum IFR Altitude (MIA). So if one were able to depart and remain in class G airspace, one could takeoff, climb to an MIA, and they could cruise to their destination, but not let down below the MIA in the destination area unless the conditions were VMC at the MIA. Since there is virtually no class G airspace left in the US where this could be done, it is largely an academic question. I have filed IFR to an airport that has no approaches, but am required to specify an alternate that does have a part 97 approach. I can navigate to my destination via conventional or RNAV. ATC will not allow a descent any lower than their minimum vectoring altitude and if I don’t break out, I can continue to an alternate or hold waiting for the weather to lift, so the flight can be finished in VMC.
Sec. 91.177 Minimum altitudes for IFR operations.
(a) Operation of aircraft at minimum altitudes. Except when necessary for takeoff or landing, or unless otherwise authorized by the FAA, no person may operate an aircraft under IFR below—
(1) The applicable minimum altitudes prescribed in parts 95 and 97 of this chapter. However, if both a MEA and a MOCA are prescribed for a particular route or route segment, a person may operate an aircraft below the MEA down to, but not below, the MOCA, provided the applicable navigation signals are available. For aircraft using VOR for navigation, this applies only when the aircraft is within 22 nautical miles of that VOR (based on the reasonable estimate by the pilot operating the aircraft of that distance); or
(2) If no applicable minimum altitude is prescribed in parts 95 and 97 of this chapter, then—
(i) In the case of operations over an area designated as a mountainous area in part 95 of this chapter, an altitude of 2,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown; or
(ii) In any other case, an altitude of 1,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown.
NCYankee wrote:
The construction of “when an instrument letdown to a civil airport is necessary” in part 91.175 means that if the let down is in less than VMC conditions for the airspace involved, and a standard instrument procedure must be used for the descent.
Of course it does. I am just saying (as someone who sometimes has regularly to decide on arcane misconstruals of law) that it is not the fine bit of drafting it was held up to be.
There was a case in the UK recently where someone took an artwork from a gallery because he liked it and wanted it at home. The charge of theft could not be upheld because theft entails “with intent to permanently deprive”, whereas this man intended to return the painting when he was done.
Drafting law is a tricky business 
