I've been avoiding stating the difference between precision and non-precision approaches as I discuss minima because it's too long for a parenthetical remark, so now I'll give it its own post. They are two classes of instrument approach procedures, based on the equipment available in the vicinity of the airport, and in the aircraft. The same airport, indeed the same runway, may have both a precision and a non-precision approach. I believe all Canadian precision approaches include a corresponding non-precision approach, but I haven't searched all my approach plates to make sure.
A precision approach provides both horizontal and vertical guidance to the runway. In other words, it's tells you if you're deviating to the left or right and it tells you if you're too high or too low, before you ever see the runway or any of the ten items associated with the runway visual environment. By far the most common type of precision approach, and really the only one available to civilians is the ILS. (It stands for Instrument Landing System, but we only ever say "eye-ell-ess.") Imagine a beam that extends from the desired touchdown point on the runway back up the approach, and imagine the pilot using instruments in the cockpit to keep the aircraft centred in the beam. That's not quite how the ILS works, but it's an adequate model. If you haven't found the runway by the time you reach decision height, usually 200' agl, you fly the missed.
A non-precision (NP) approach gives only horizontal guidance. That is, it tells you if you are off to the left or the right, but not how far above or below the glidepath you are. Of course it has to give some vertical guidance, which it does in the form of telling what altitude to start at, and what altitude to be at by each of a number of points, sometimes specified by DME (distance measuring equipment -- yes, aviation has some of the stupidest abbreviations) or passing overhead a radio beacon called an NDB (non-directional beacon). The lowest altitude you may descend to before sighting the airport is called the minimum descent altitude, or MDA. You stay at the MDA until you either see the runway or reach the missed approach point (MAP). If you see the runway, and it's possible to land safely from where you are, then you do so, otherwise you fly the missed.
One tricky thing about a NP approach is that the MAP may not be a specified point like "the beacon that broadcasts 'HI' in Morse code." It might be an amount of time past the beacon that broadcasts 'HI' in Morse cose, determined by how fast you're flying through the air, the wind strength and direction, and a little table in the corner of the approach plate. I think that sounds worse than it is, so I'll show an example.
I have here an expired approach plate for the ILS or NDB RWY 09 approach to the John G. Diefenbaker International Airport in Saskatoon, Saskatchewan. (Sorry, I don't have a scanner, and I don't know where to find it online). We'll imagine the pilot doesn't have an ILS receiver in her airplane, perfectly legal in Canada, so she'll be doing the NDB approach. The diagram shows the XE NDB (i.e. the beacon that broadcasts Morse -..- . in all directions) aligned with runway 09 and located on the ground 3.6 miles before the runway threshhold. According to the plate, once the pilot is cleared for the approach, and is within 100 nm of the airport, she may descend
She knows her true air speed for the approach is 120 knots, and we'll say she knows she is facing a fifteen knot headwind, so her groundspeed is 105 knots. That's faster than 90 knots, but slower than 110. From the table there's a 26 second spread over that 20 knot range, and five knots is a quarter of that, so six or seven seconds longer than the 110 knot case: giving a timing of 2:05. The pilot works this all out before starting the approach. On reaching the beacon she will start her stopwatch, reduce the power, and descend promptly to 2020 feet. For the best chance of landing, she must get there before her two minutes and five seconds have elapsed, because MDA or not, when the time is up, she's leaving. Most NP approaches are designed such that if you see the runway at MDA just as the time elapses you still have to go missed because they bring you out right above the threshold, too high to land. The trick is to chop and drop so that you are established at the MDA in time to see the runway.
This is where you want to look at the advisory visibility and calculate when you actually have to see the runway. The advisory visibility for the approach above is one mile. At 105 knots, she covers a mile in about 34 seconds, so she needs to be at MDA within 1:31 of the beacon to make this approach. When she crosses the beacon a 700 foot per minute descent rate should be enough to make that happen.
If the cloud bases are at 1900', no matter how fast she gets down and no matter how good the visibility underneath is, she's not going to see anything at 2020', and is really going to be wishing she had an ILS, because the DH for the ILS is 1853'
Technically, only ILS approaches have a DH. NP approaches may be based on a variety of navigational aids, but they all have an MDA and may have a time. GPS approaches for the time being are considered NP, but that may change.
1. I understand that in the United States she would need a separate descent clearance, but in Canada "cleared for an approach" allows her to descend.
2. Yes, 1853'. Not 1854' and definitely not 1852'. I may do another post on how they come up with these numbers.