Writing

When advisory guidance flies you into terrain

The data and charts behind this post: LNAV terrain viewer.

An airplane crash about a week ago has been sticking with me. It was most likely pilot error, but an Epic turboprop flew into a rock near Steamboat Springs, Colorado… based on a guidance option from the on-board computer.

Accident record for N98FK: Aviation Safety Network

Vertical profile of the KSBS approach from the FAF (MABKY) to the runway: the +V and flight-track lines descend through the 9,100 ft MDA and meet a terrain peak about 3 nm from the runway, with minimum clearance of -47.2 ft

It was a non-precision approach, which means there’s an MDA (minimum descent altitude) which a pilot is not permitted to descend below unless the airport is in sight. That’s why this is pilot error: the approach should have stopped descent at 9,100 feet. But the airplane will let the autopilot continue a descent following an advisory vertical guidance (+V) as long as there’s a non-circling approach available for the airport (which is the case at SBS). And at most airports, if you fly the +V you probably won’t hit anything.

But through either the pilot’s mistake or lack of awareness regarding approach limitations, he let the airplane continue flying on this unofficial line and it flew him (and three other people) into a peak; and since it was beyond the final approach fix (FAF), some terrain warnings are inhibited.

I grabbed elevation data and the FAA approaches for the continental U.S., and there are six airports with approaches where FAF->runway will have the same result. Three of those support +V guidance, so those three will let an autopilot knowingly identify where you are, where you want to go, and fly you into the rock.

I mapped them all, plus another fifteen of them which get <100 feet from an impact: garrettbernard.com/lnav-terrain.

There are other conditions where the autopilot will fly you into the ground. An ILS approach will keep flying the airplane into a runway if the pilot doesn’t take over, or a -VS descent will end at the ground if you don’t level off somewhere. But this one is different, because the airplane knows what you’re trying to do and also knows it won’t work, yet gives you the option to continue.

This one bothers me because the pilot made a mistake, and people are dead because the computer offered an option that would 100% end in tragedy.

+V guidance should not be available at airports where the guidance will crash you into a mountain.