Tuesday, July 26, 2005

Minima and Missed

When you are approaching an airport through clouds, you aren't allowed to just point the airplane roughly in the direction you figure the airport is and go down until you find it, because of the great risk that you figured wrong and there's a hill there instead. You follow an instrument approach procedure, using cockpit instruments to aligning your airplane with radio signals that indicate a safe path to the runway. You have a bit of paper called an approach plate that tells you what frequency the radio signals are on, what morse code identifier verifies that you have tuned it correctly, where the obstacles are and useful things like that.

You descend, following the instructions on the approach plate, until you either find the airport or you don't. This differs from the rough method described in the first sentence of this post, because the approach plate defines very precisely when you're supposed to give up on finding the airport, and what you must do next, called a missed approach. For a precision approach, the plate lists a decision height. When the airplane reaches that altitude one pilot looks out the window and describes what he sees according to a template laid out in the company SOPs. It might be "Runway in sight, twelve o' clock" or "minima, no contact."

Minima is of course the plural of minimum, and I never really wondered what plural minima were involved before. There's the minimum altitude you're allowed to descend to and ...? There's only one instance of two thousand four hundred twenty-three feet, and you're at it. I might speculate that "minima" is easier to say than "minimum" and people might think it was singular, as data and agenda have come to be. Good speculation, except that I've seen company ops manuals where the standard call is "minimums." So it's factually and grammatically incorrect, and any English speaker can tell it's plural. What is this mysterious plural? The British even call final approach (the straight in towards the runway flying, right before you land) "finals". There's not more than one of them. It's one final approach.

I guess it's just pilot grammar, the way the word "missed" has become a noun. "The missed."

A missed approach isn't just for minima. If at any time during the approach you have messed up badly enough that you won't be configured properly for landing (because of airspeed, flaps, gear, captain's pants, etc.) or cannot be certain that you will remain within protected airspace (in this case protected from trees, radio towers and mountains) you must go missed even before you reach the decision height. Your company SOPs and your company culture (quite possibly diametrically opposed) will detail the conditions that require a missed.


Anonymous said...

And if you "miss" a busy airport you can get in a lot of trouble as well, I suppose.

Aviatrix said...

Visibility is just advisory for an approach. When the NFP makes the call "minima no contact" she's talking about altitude alone.

Avimentor said...

I'm curious about your last comment. US regs (14 CFR 91.175) allow the pilot to operate (descend) below the decision height or minimum descent altitude (or decision altitude if you prefer) under certain conditions. Those conditions all have to do with visibility.

In the US, if you can see the runway environment or the approach lights, you can descend down to 100' above the touchdown zone (lower under certain conditions). Visibility is what determines whether or not you can land, so visibility is much more than advisory - it's the controlling factor.

Maybe I misunderstood your comment or things are different in Canada?

Aviatrix said...

Yes, they are different. See my 27 July posting for details.