Date: 4 Mar 94 13:34:51 GMT (Fri)
From: Dr Peter B Ladkin <pbl@compsci.stirling.ac.uk>
Subject: Re: Autopilot landings in `zero visibility'

In RISKS-15.62, Simson Garfinkel says: 

> I was on one of the few aircraft to land in the Boston blizzard today. There
> was zero visibility. [...]
> And I wondered which would have been RISKier: landing on autopilot, or landing
> on human pilot.

It's well to wonder, but in this case there might not have been the option.
There are three categories of Instrument Landing System (ILS) approaches, Cat
I, II and III, and Cat III is further subdivided into A, B, and C. The
categories are differentiated according to the minimum weather conditions
required for landing. An ILS is, abstractly, a couple of radio homing beams.
One, the `localiser', beams down the centerline of the runway, so you can tell
if you're left or right of it, and another beams up at an angle, usually
between 3-5 degrees, from the touchdown point - the `glide slope'. You or your
favorite autopilot are supposed to follow the beams from 5-15 miles out. In
order to land legally for most Cat I Instrument Landing System approaches,
besides the usual visibility conditions, some part of the runway, its lighting
or its environment must be visible when you're roughly 200 feet above the
ground (and therefore a few more hundred feet from touchdown).  Cat II
`minimums' are lower, Cat III lower still.  Furthermore, for air carriers,
operation is only permitted with certain values of `Runway Visual Range'.
Special crew and aircraft certification is required for Cat II and III, and
certain modes of operation are mandatory.  It is possible that the landing
described was made under Cat IIIA, in which case use of some automated systems
is mandatory, and hand-flying is not an option.

A further question is: what form of safety analysis has been done to ensure
that the requirement to use automated landing systems rather than people is
appropriate for Cat III landings? Perhaps those RISKS readers who have
extensive dealings with the regulatory authorities and the airplane
manufacturers could tell us?

Peter Ladkin