Wings Magazine

Features Safety
Seeing Through Pea Soup

Forward Scatter RVR Sensor.

September 27, 2007  By Doug Morris


Many envision London’s weather as fog-prone, persisting for days on
end. However, I’ve flown to Heathrow numerous times and not once did we
have to resort to the heavy hitter, Category III autoland. Why the
meteorological mix-up? During the industrial revolution a thick, dirty,
yellowish green veil of pollution encompassed London and the
surrounding areas, frequently likened to the colour and consistency of
pea soup. The Clean Air Act in the 1950s ridded London of its green
glow, but the idiom, as thick as pea soup, stuck for thick fog.

Canada transmissometers – previously used to calculate distance seen
through fog and other obscuring phenomena – have given way to
forward-scatter RVR (Runway Visual Range) sensors providing pilots a
more precise reading in the pea soup. Every ILSequipped airport in
Canada has been modified with these sensors. But if you read the new TC
AIM (Transport Canada Aeronautical Information Manual) which replaced
the AIP (Aeronautical Information Publication) in 2005, it would have
you believe the transmissometer remains operational in Canada. Not so,
and in several e-mails and phone calls with Transport Canada, they
agreed this section of the TC AIM needs amending. Incidentally, recent
trips abroad to Europe and Asia confirm the old system of two
transmissometers mounted 250 feet apart are still at work providing RVR

These new
four-headed devices perched on a single 10-foot pole have two
lightemitting heads and two receivers to calculate how much of that
light is scattered (10 feet is a midpoint based on the most common fog
profile and flight deck heights). Brian Sherman, manager of precision
approach systems for Nav Canada and the pioneering engineer for the
device, likens the instrument to an “expensive coat rack.” Not only is
the new sensor less expensive, but it’s more reliable and accurate than
the old machinery, which required constant recalibration, was subject
to frost heaving and sometimes stopped working due to mechanical
problems. Many sensors perform the same task with just two heads, but
are thought to be less reliable in some weather conditions.

you drive in thick fog and see the headlights of an oncoming car, you
may see a halo effect caused by fog scattering the light in all
direction; the thicker the fog, the greater the light diffusion. The
same principle applies on the runway; the sensor records this scatter
effect and converts it into RVR data – in fact, the thicker the fog the
greater the accuracy. Every 15 seconds the sensors recalibrate
themselves due to ice, smoke, etc. The RVR sensor is always armed and
ready. Pilots flying in Canada should expect readouts from these
sensors whenever the prevailing visibility is one statute mile or less
and/or the RVR value for the designated runway(s) is 6,000 feet or less.


certain all pilots would guess where Nav Canada tested this device in
the early nineties. Yes, the foggiest airport in Canada, St. John’s,
Newfoundland a.k.a. Torbay, gave this sensor a workout before replacing
the transmissometers. The pole supporting the equipment, because of its
close proximity to the runway, needed to be frangible to give way to an
intruding wing. The sensors can be serviced by one person just by
pivoting the top of the pole by a hand crank. Additional sensors are
attached to determine ambient light levels and runway light intensity.
They even developed an algorithm just in case a spider decides to spin
its web on the sensor. The RVR sensor costs about $80,000 plus an extra
$20,000 for installation with Toronto Pearson having 13 of these
“expensive coat racks.”

Even passengers are starting to notice
these devices. Frequently co-located near the glide slope shack, they
have been assumed to have other purposes. In a recent overheard
conversation an onboard backseat ‘expert’ told his wife he thought it
was a radar detector used to make sure the pilot didn’t speed while on
the runway!

Hopefully, the next time you taxi by these
four-headed sensors, you’ll appreciate the lights are always on
ensuring accurate readings in pea soup.


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