FLIGHT International, 22 February 1962
Automatic Landing
ECONOMICS
AND
TIMING
OT so long ago it was confidently stated that a human being
N
could not safely control a machine that travelled in excess of
70 m.p.h.; a little later, that an aircraft could not fly faster than
sound; and then that the civil jet aircraft was technically and
economically impossible. At the time that each of these statements
was made it was also impossible to demonstrate conclusively that
these things were reasonably attainable. And so it is with automatic
landing.
Let us look at the present situation. In the first place, all-weather
operation in the absolute sense is not now and probably never will
be achieveable. Snow on the runway, very high crosswinds, icy
runway in conjunction with strong crosswinds, or several inches of
water lying in the take-off or landing path will defeat the conventional airliner—perhaps as yet unrecognized but different limitations
will also affect operation of civil VTO aircraft.
We should perhaps re-define our objective as "foul weather"
operation, and, more specifically, to operation in very low
visibilities with acceptable runway conditions.
In the second place it should be recognized that the technique
of automatic landing is fairly well understood in so far that BLEU
(Blind Landing Experimental Unit) have now amassed some
8,000-10,000 automatically controlled landings without accident.
Thirdly we must appreciate that the progression to automatic
landing in very low visibilities must be an orderly one, step by step,
which may take many years to complete.
Too many misconceptions about automatic landing are becoming
current at the moment, first because the many variables are only
now being closely studied and, second, because many of those who
are beginning to appreciate the competitive possibilities are misunderstanding or underestimating the technicalities. On this last
subject there is an important difference between the British and
American approaches to the problem. Britain now has, by the
exertions of BLEU, an automatic landing system in use, and in
progressively wider operation. Because this system is designed to
military requirements, it is not entirely suitable for airline use and
much of the present development work is directed towards adapting the proven basic system to the more demanding civil environment, and with providing as nearly 100 per cent reliability as
possible. In America, on the other hand, an established military
system does not exist and a civil airline order has not been placed
for which the FAA can spell out a precise safety requirement.
A great deal of money is being spent in original research, but the
general airline trend is to develop the present coupled approach
system, rather than to modify an experimental military system to civil
requirements. The natural inclination is to try to produce something
a great deal simpler than what one might call the "10-"' system and
to limit the objective to an allegedly economical mid-point between
full-blown automatic landing and the present level of coupled
approach.
The choice between the "10"'" level and the extended coupled
approach—or even of any improved system at all—depends largely
on economic considerations. Is the money invested going to be
balanced by tangible benefits, either in terms of immediately
apparent operating economies, or by competitive advantages over
rival airlines, both factors being ultimately apparent as increased
revenue ?
Fig 1, "the weather problem" indicates one basis upon which the
economic estimates are made. The operator must know how often
his airliners are at present prevented from completing scheduled
flights and how much could be gained by improving landing
capability by a specific amount. One of the curves shows the
percentage of any one year for which ceiling and visibility fell to
given levels at major airports in the continental USA. The other
curve shows the percentages for major airports all over the world,
including the USA. British records indicate that London Airport
falls somewhere between the two curves in average years, but that
there are certain years in which the percentages are significantly
higher—when BEA, for example, loses hundreds of thousands of
pounds through cancelled and delayed flights over a period of a
few days.
The left-hand end of the curves must be an extrapolation of the
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remainder, because little quantitative observation is carried out at
these very low minima. The horizontal bracket indicates the range
of landing minima at present applied to airliners of the various
types. The 300ft ceiling and £ mile visibility are the minima now
generally applied to the big jets, and they are not met for approximately 1} per cent of an average year.
From the purely economic point of view, the 200ft and 100ft
levels are well worth bringing within the capabilities of the present
jets; and the "big four" airline autopilot manufacturers in the USA
are busy extending the capabilities of their existing autopilots into
this region.
Other economic arguments are being put forward to show that
the cost of fitting new or improved automatics would not be recovered if the minima aimed at were below what would in Britain
be regarded as a relatively high level, on the assumption that equipment of an appropriate capability and safety level, and therefore
complexity, is added to an existing design of operational aircraft.
Boeing, for example, have plotted these curves of revenue saved
versus installation cost and have come to the conclusion that the
curves cross at around the 100ft ceiling minimum. This is also
approximately the minimum "go-around" height, below which an
airliner would probably contact the ground even after overshoot
action had been initiated.
But these assumptions concerning equipment and aircraft apply
specifically to the American situation and not to an airliner designed
from the outset to accept the necessary automatics of the required
level of safety—and everything else that this implies. There is little
doubt that the most uneconomic way of obtaining automatic
landing capability is to fit the equipment to an aircraft not specifically designed to accommodate it. The equipment will inevitably
be complex, because one cannot assume that the FAA would set
STATISTICS FRC
SERVICE UNIFORM SUMMARY OF
SURFACE OBSERVATIONS, PART D
FAA
MINIMA
S WORLD
CONCLUDING
-P-U.S.A)'
FOB
CONTINENTAL
OCCURRENCE
PER CENT
OF YEAR
1
£L
KSL
?OQ
300
.4C
C E I L I N G (FT.)
VISIBILITYCMILES)
Fig I. Incidence of various weather minima
Fig 2. Cost of providing automatics for landing