Landing Performance Monitor Could Prevent Runway Overruns

Incident at Toronto illustrates need

It may be useful for aircraft to have a landing system that is analogous to the system used on takeoff – one that advises when things are going badly and the landing should be abandoned.

The case of the Aug. 2 Air France overrun at Toronto comes to mind, as well as the Southwest Airlines overrun at Burbank, Calif., on March 5, 2000. After touchdown in that incident, the captain later told National Transportation Safety Board (NTSB) investigators that the end of the runway appeared to be closer than it should have been, and the crew thought they might hit the blast fence wall (see ASW, Aug. 81, and Aug. 26, 2002).

Recall the virtues of a Take-Off Performance Monitoring System (TOPM). This was an addition to the flight management guidance system (FMGS) suggested to preclude the sort of takeoff overrun accident that happened to the MK Airlines 747 freighter at Halifax on Oct. 14, 2004 (see ASW, Nov. 1, 2004). It is possible that a fatigued crew may have reused the takeoff performance data card for their prior takeoff out of Hartford, Conn., or mis-set the derated power they calculated for the Halifax runway – and realized the miscalculation far too late to retrieve the situation.

It was postulated that mandatory data entry of the many takeoff performance factors required would avoid mistakes and that the TOPM would monitor in real-time the satisfactory progress of the takeoff, avoiding any tail-strikes or panicky over rotations in or beyond the overrun. A takeoff performance monitor would enhance safety by annunciating any acceleration discrepancy. But now the recent accident to an A340-300 at Toronto has highlighted a similarly inexact situation in the approach and landing evolution. Is a Landing Performance Monitor (LPM) technologically feasible?

Highly automated jets can now robotically handle pretty well everything but the takeoff and the landing. Admittedly, there is autoland – but that is restricted to the visibility limitations under the relatively stable conditions of mist, fog and heavy drizzle. Autothrottle and instrument landing system (ILS) autoland are unable to cope adequately with unstable atmospheric conditions such as large gust factors, cross-winds, and directionally variable strong winds found beneath a thunderstorm. Autoland also requires an ILS equipped runway and a qualified crew. It also demands that multiple onboard systems be serviceable. So in point of fact, pilots are usually required to visually fly the last few hundred feet to touchdown and the landing roll-out. Unlike parametrically bound systems, they are prone to errors of fatigue, indecision, uncertainty and illusions.

Add in a tailwind and – as with Air France at Toronto, even though it was reasonably close to the correct threshold speed – the ensuing float can waste a lot of runway. What happens in those few moments between crossing the runway threshold and selecting reverse can make the difference between a successful arrival and a career in tatters.

Airline pilots very rarely carry out “bolters” (touch and go’s). Sometimes a non-handling pilot can see something dire happening that the pilot flying (PF) cannot – a problem sometimes referred to as task fixation. In times of stress, it can become task saturation (the point at which situational awareness is superseded by adrenaline). A pilot is really only in control of his approach’s “setup.” Once he’s crossed the threshold, he’s wholly unaware of runway behind him and is looking fairly close in for his landing flare cues. He may be concentrating upon a smooth touchdown to the extent that he’s blissfully ignorant of the rate at which he’s running out of post-touchdown deceleration bitumen. In fact, once past the threshold, he just does not know accurately what roll-out distance remains – so his destiny is in the initial setup, and thereafter rests wholly upon his decision to press on or go-round. Many pilots just don’t make such a conscious decision because they are unaware of any impending peril. By the time fright arrives, flight has gone. Most runways only add the red runway lights for the last 1,000 feet to the far threshold. That point is rather late for trying to do anything other than stand on the brakes.

The factors within the pilot’s control that determine the touchdown point and the landing energy dissipation within the remaining runway available might be as follows:

Threshold airspeed (& density altitude)
Each extra knot will cause landing distance to increase by about 2 percent (gust factors added to Vref speed will similarly increase roll-out). In fact, the entry argument here would be groundspeed ? to accommodate the wind component and the varying effects of density altitude (landing at Mexico City on a hot day for instance).

Threshold crossing height
VASIS or PAPI lights, based nominally upon a 50-foot pilot’s eye height for a touchdown 500 to 2,000 feet in on a 9,000-foot runway

Aircraft mass
Which of course generates inertia ? which is the problem (within pilot control, to an extent)

Power levers to idle point
Pilot assessment based upon his target threshold speed (whether auto-throttled or not)

Spoiler actuation
Controlled by “weight on wheels” ground/air sensing circuitry

Flap setting
Full (or something less than ? for handling considerations)

Autobrake setting / anti-skid
Or manual braking (either style braking is compromised by a low friction surface that’s awash with water, hail, slush, etc.)

Flare height
Too high a flare will extend the touchdown point (especially with a tailwind)

Reverse selection point / Amount
And deselection point (and whether a reverser or two is locked out and unserviceable)

Brake fade (during late roll-out)
With steel brakes (not really a factor with carbon brakes)

Disregarded here is tire tread condition, runway contamination, aerodynamic braking (nosewheel placement), wind component, runway slope, rubber deposits at the far end, runway, flooding or hail/snow, up elevator (forcing main-gear into the runway for better braking) and, of course, pilot error. There may also be the psychology of knowing what the runway overruns are (ravine, ocean, cliff, gravel arrestor bed, barrier or highway). But obviously, although beyond a pilot’s control, these are still valid entry arguments for achievable stopping distances as well as quantifying and minimizing risk.

Uncertainty and cross-cockpit disagreement can also play a role. What might happen if a captain decides his first officer (F/O) has landed too deep? In the Qantas 747 scenario at Bangkok, the F/O’s instinct to go round was countermanded by his commander, the handling became suddenly complicated by habit patterns and configuration – and they overran (see ASW, April 1, 2002). Time is always acutely of the essence, communication is sparse and one never gets a second chance to get it right. Most company Standard Operating Procedures have the selection of reverse as the last-chance point for conversion to a touch and go. But what happens if the F/O momentarily advances the thrust levers to convert to touch and go and the skipper instantly takes over and retards them to reverse? Well, for starters, the spoilers will retract and not redeploy once a throttle goes forward of idle. That’s sowed the seeds of an overrun because braking on a wet runway will now be severely degraded.

Part of the solution is an obstacle free overrun safety area. But perhaps technology has something to offer. Nobody wants to see an airplane sitting in a ravine if there’s an optional extra that will assist pilots in making early go-round or touch-and-go decisions.

As R�al Levasseur, Transportation Safety Board (TSB) of Canada investigator for the Air France overrun at Toronto commented, “There’s no way, having touched down 4,000 feet in, that they could’ve stopped on that 5,000 feet of wet runway remaining.” So what precisely is the answer to avoiding such challenges? A Landing Performance Monitor (LPM) would consider all the relevant factors, meaningfully embrace some data-scatter or variability, resolve whether the energy dissipation challenge is feasible, apply a safety margin and then only tell the pilots (audibly) NO (i.e., it’s not needed to say YES). If the LPM were to err on the safe side, that would be no bad thing.

Most pilots sooner or later get to see the runway end approaching at a speed they never want to see again, but a landing performance monitor could change that.