May 1, 2005
By Barry Schiff
Barry Schiff is a retired airline captain for TWA.
Approaching the west coast of Ireland, our flight engineer gives us the latest Paris weather: "Ceiling's still zero, Skipper; visibility's holding 300 feet."
We learn about the need to shoot a Category IIIb ILS approach at Paris after a night Atlantic crossing. The rising sun bursts into the cockpit, an intrusive wake-up call, and we begin to prepare for the challenge. A Cat IIIb approach in a Lockheed 1011 is a no-brainer. I could teach any pilot to do it (as long as nothing goes wrong).
After descending into the Paris area and being vectored toward the localizer, we select the autoland mode and engage the second autopilot. If one of the two should fail, a missed approach is mandatory even though the remaining autopilot could complete the approach and landing safely.
The first officer calls out, "Localizer alive," and the big Lockheed rolls gently onto final. A minute later, he calls, "Glideslope alive." We extend the flaps to 22 degrees and lower 10 wheels from their wells.
The glideslope needle centers, and we extend the flaps fully. The nose of the TriStar dips slightly, and the autopilots establish an initial sink rate of 680 fpm to approximately parallel the glideslope. Seconds later, they make the corrections needed to intercept and track the glideslope. The wings slice into the undercast, and the autothrottles maintain 138 knots.
Both ILS receivers are constantly fed into a vote-and-veto box. If a suspect signal is received, the computers reject it and allow the approach to continue using the other, more reliable signals. The outer marker flashes blue dashes, and I call, "Outer marker." The flight engineer is monitoring instruments on both sides of the panel and calls, "Flags checked."
The radio altimeters show us descending through 1,500 feet agl. This signals the autopilots to roll in some nose-up trim to prepare for the possibility of a missed approach. The autopilots also take control of the rudder and now can automatically prevent yaw created by the failure of an underwing engine during the remainder of the approach (without pedal-pushing help from a pilot, thank you).
The tower clears us to land and reports that the ceiling is still zero and the runway visual range is holding steady at 300 feet.
At 500 feet agl, the copilot calls out, "One hundred and thirty-eight knots, sink seven."
My left hand holds the yoke loosely; my index finger rests lightly against the go-around switch in case a problem develops.
My right hand is on the throttles, and I feel them making slight and occasional thrust changes. I also feel my pulse picking up a bit.
The autopilots have been crabbing the airplane to remain on the localizer, but at 150 feet they dip a wing to compensate for the crosswind and add top rudder to align the Lockheed with the runway. Although the autopilots can land with a 44-knot direct crosswind, CAT IIIb landings are limited to 10 knots.
The middle marker spews a short stream of dots and dashes, the radio altimeters say that we are descending through 100 feet, and the ILS needles remain centered; there is no sense of being so close to the ground, no approach lights reflecting through the dimensionless world of gray. I shift slightly in my seat trying not to let my crew notice. It's crunch time.
For most airplanes on a CAT IIIb approach, pilots use a 50-foot decision height, and a missed approach is required if the touchdown-zone lights are not seen when the landing gear is 50 feet above the runway.
The Lockheed benefits from an exemption. We do not have to see anything outside the aircraft before touchdown, and we don't. This eliminates needing a decision height. We instead use an alert height of 50 feet, the height above which a missed approach must be made if a required ground or airborne component fails. The approach and landing may be continued if such a failure occurs below 50 feet.
Amber lights on the radio altimeters signal that the main landing-gear wheels are descending through 50 feet, and the autopilots shift to the flare mode. The throttles move aft, and the attitude indicator confirms that flaring has begun. At 10 feet, the autothrottle system retards the thrust levers fully and trips itself off. The computers program the flare for a 2-feet-per-second sink rate at touchdown. We still cannot see anything through the milky mist. The radio altimeter shows 10 feet, 5 feet. We can almost feel the main landing gear stretching for the ground.
The mains touch with a mild thump, the ground spoilers deploy, and still we see nothing. The autopilots lower the nose at a programmed rate, and then just as the nosewheel is about to touch, we make out a stream of centerline lights disappearing beneath the nose. I pray that no vehicle has strayed onto the runway. We would have no warning of the impending collision.
The autopilots shift to the rollout mode and steer the nosewheel to track the localizer.
I engage the thrust reversers. They seem more effective at making noise than slowing us. At 80 knots I come out of reverse, apply normal braking, disconnect the autopilots, and steer toward the ghostly image of a high-speed taxiway.
Now the tough part, trying to find the gate without bumping into something on the way.
Visit the author's Web site ( www.barryschiff.com).
Safety and Education,
The NTSB has organized a safety seminar May 10 to focus on aerodynamic stalls and loss of control, a leading cause of general aviation fatalities.
According to the most recent Joseph T. Nall Report, in 2010 there were 43 accidents involving weather, and 28 of them were fatal. In fact, weather accidents are the most consistently fatal types of accidents.
The management team running Chelton Flight Systems and S-Tec Corp. in Mineral Wells, Texas, for parent Cobham Avionics saw an opportunity and bought in.
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