Apathy has forced take-off performance monitoring to be shelved.
David Learmount/LONDON
The US National Transportation Safety Board (NTSB) says that there were more than 4,000 take-off related accidents and serious incidents involving airliners in the USA between 1983 and 1990, resulting in 1,378 fatalities. The NTSB says that 8.7% of all accidents to large airliners occurred during take-off or aborted take-off. For regional airliners, the figure is higher, at 12.5%. Boeing says that 25% of all hull-loss accidents occur in the take-off phase.
Prototype equipment exists to warn pilots well before decision speed (V1) if their take-off is condemned to failure and, perhaps, disaster. Now, despite the fact that new research into take-off performance monitoring systems (TOPMS) indicates that this long-sought capability is feasible, further work has been shelved for lack of industry interest. Every take-off, therefore, is still at the mercy of a variety of risks, at least some of which could be avoided.
Nearly 15 years of research has gone into TOPMS, notably at NASA Langley, in the USA; at the UK's Bristol University aerospace faculty; and by the Netherlands' National Aerospace Laboratory (NLR). Their work has produced viable algorithms, software and prototype displays. Despite that, no aviation authority has a policy on TOPMS, no manufacturer plans to produce it and no airline wants to buy it.
Dr Ratan Khatwa, of the NLR's flying-qualities and flight-control-systems group, believes that TOPMS study at various levels has been around for so long that many organisations seem to have developed a "not-TOPMS-again" attitude to the subject, inhibiting interest in the latest research results.
TOPMS rely on inputs from sensors such as accelorometers to monitor and predict aircraft performance during the take-off and to compare that performance with the normal levels for the aircraft.
Decisions not to implement an operational TOPMS programme are based almost entirely on fear of false warnings. This is understandable because of the safety and operational problems which false warnings could bring. Industry and regulatory authorities have watched early TOPMS research teams struggle to define objectives, limitations, algorithms and means of display, and have observed those teams fight to prove the potential effectiveness, reliability and integrity of their research.
NASA has simulator-tested its head-down TOPMS display on a selection of 32 pilots from various backgrounds. It has also flown the display in its transport-systems research vehicle (TSRV), a Boeing 737-100. Flight-testing involved 85 sorties and a range of take-off decision-making scenarios. The system, still retained in the TSRV, was judged to improve pilot decision-making. Examination of the potential for head-up display (HUD) TOPMS warnings was recommended, together with improved performance-measuring techniques.
Khatwa notes that the NASA and NLR TOPMS algorithms are similar. Comparable, although not identical, dynamic cockpit-displays were the result. Meanwhile, the NLR has carried out extensive simulator tests, not only to identify the situations in which its TOPMS can be shown to improve pilot decision making, but also to define the combinations of circumstances (scenarios) where no improvement is apparent. Knowing what is not needed allows for the simplification of sensor systems, displays and warning systems. As Khatwa remarks, TOPMS "...must improve go/no-go decision-making, otherwise there is no point".
Just as important, says Khatwa, the NLR tests showed no scenarios in which pilot performance was degraded by it. It has always been feared that the additional display information might overload pilots in this critical flight phase.
NASA'S MANDATE
It was the crash just after take-off of an Air Florida Boeing 737-200 at an icy Washington National Airport on 13 January, 1982, which, more than any other single event, caused the US Federal Aviation Authority to mandate the NASA research. If a TOPMS had been installed in this aircraft, the crew would have had the information to abandon the take-off safely. Ice had blocked the engine-intake sensor probes, delivering erroneous engine-power readings on the flightdeck instruments. Acceleration was painfully slow and power was insufficient for the aircraft to climb, especially as there was also some ice on the wings. The aircraft hit the 14th Street bridge and crashed into the Potomac River, killing 74 of the 79 people on board.
The NLR points out that only 25% of rejected take-offs are engine-related, whereas the pre-take-off computations which determine decision speed (V1) are based on engine failure only, with all other parameters assumed to be precisely as forecast and no allowances made for performance degradation.
A TOPMS can either be displayed discretely, with its own head-down display (see diagrams), or on part of a HUD. According to Khatwa, it could also be part of an aircraft's digital flight-management system, its alert signals simplified and integrated with existing warning systems. The NLR adds: "TOPMS warning logic could be inhibited above a predefined speed, eg, 100kt [185km/h], which will prevent the system giving potentially dangerous false warnings at a very high speed." This arrangement is potentially promising, says Khatwa, because it would be cheaper and possibly easier to certificate. The NLR clearly states, however, that it requires further research to validate the concept - but it does not have plans to conduct such research.
The NLR's TOPMS can give early warning of the effects on take-off and stopping performance of wet runways, slush, windshear, unplanned-for runway gradient, poor engine performance/wrong throttle setting and events such as engine or tyre failure before V1.
The NLR's tests have been carried out using head-down displays, starting with simple comparative parameters with no predictive capability (Type 1 TOPMS) and progressing to Type 3 TOPMS, which has:
comparison of the achieved aircraft performance to a reference (nominal) performance with all engines operating (as in Type 1 TOPMS);
- performance prediction later in the take-off run;
- prediction of the ability of the aircraft to abort the take-off;
prediction of the ability of the aircraft to carry out required take-off performance (to clear "screen height") following engine failure at V1.
In the early stages, testing was done using fixed-base, part-task simulators. At all points, test participants preferred and achieved best results with the Type 3 TOPMS, says the NLR.
The dynamic TOPMS display (see diagram) appears on the navigation-display screens, being replaced automatically by navigation data at unstick. The NLR's adopted procedure is for the pilot-not-flying to have primary responsibility for monitoring the TOPMS data as part of his normal instrument scan, and to make the advisory, stop/go and/or rotate calls according to the information presented. If the captain is not flying, his calls are commands. If the pilot-not-flying is co-pilot, the calls are advisory.
Using the display...
when ground-roll starts, the nominal rotate speed (Vr) point (blue circle) and the estimated Vr point (yellow cross) coincide. If they move apart once the TOPMS sensors gather acceleration data, the direction and distance of movement shows the disparity between expected and actual performance, immediately alerting the pilots to the kind of decision which might be required;
- the aircraft symbol moves forward as the aircraft starts to roll. Its runway progress is indicated by the red distance markers;
- while the "aircraft" is between the brown (ground) bars, the take-off can be aborted without overrun, whatever other information is shown;
- with the "aircraft" between the blue bars, flight is possible; between the blue/brown overlap, either "go" or "stop" option can still be chosen;
- if a gap appears between the "can-stop" (brown) and "can-fly" (blue) bars, it indicates that, if engine failure occurred while the symbol was in the gap, the aircraft would overrun if take-off was aborted or fail to clear screen height if take-off was continued;
- if a red cross appears at the runway end, the blue bars disappear. It indicates that a safe take-off is impossible. If acceleration is well below normal, this advice could appear at quite low speed.
The non-flying pilot, following NLR procedures, would make the normal 80kt call; then, on entry to the "blue-bar" sector, he calls "can fly"; at exit from "brown-bar" sector, he calls "go", and, at Vr, he calls "rotate". If a "gap" appears while the symbol is in the "can-stop" sector, he calls "stop gap"; if the red cross appears while in the brown sector, then he calls "stop red cross".
In NLR tests, the situations in which the TOPMS cockpit always gave better results than the "no-TOPMS" cockpit were scenarios 4 and 5 (see diagram), where "stop gap", or "stop red cross" are appropriate calls. These show cases where TOPMS could show entry into a potentially disastrous situation, which could be prevented early in the take-off run.
Experiencing TOPMS
The NLR's research flight-simulator flightdeck is generic. Its instrument displays are similar to those of the Boeing 747-400 and it has four power levers. For TOPMS tests, however, the two left levers were tied, as were the two right ones, so the "aircraft" became a twin, representing the performance of a Fokker 100. The "runway" is visible, giving normal external visual cues.
Pilots participating in the NLR's test programme were sent a briefing "manual" before arrival, given a half-day's familiarisation with the flightdeck and practice at checklists, handling and procedures as for a normal line operation. Then the system-evaluation tests would start.
I was sent the briefing manual but, not being a participant in the test programme, I did not undergo familiarisation. Having carried out one non-incident "take-off" to experience the feel of the system, I reverted to the non-flying pilot's role, intending to find out how effective the TOPMS display is, both to understand and as a decision-making aid.
Take-off data were entered into the control display unit in the usual way for each take-off, which means that the take-off-speed bugs were set on the airspeed indicator (ASI) tape. V1 itself does not appear on the NLR's TOPMS display, only on the ASI.
Checks were completed, and take-off run commenced. After each take-off had been completed or rejected, the simulator was reset for the next.
For the first four runs, I was told what the scenario would be. After that, I was not. The simulator is programmed with 40 combinations of performance deficits, performance anomalies and "discrete events", or failures. Despite the lack of familiarisation, I found the display so easy to use that I made only one erroneous decision in about 20 take-offs and that error was not safety-critical.
Even sudden loss of the display was not upsetting, because it is part of the normal scan, so reversion to normal decision-making, using the speed bugs, warnings and engine performance indications, was no problem.
My experience with the display was like being re-educated about the considerations inherent in take-off decision-making.
Khatwa confirms that reports from pilots who had participated in the tests and returned to airlines have indicated that they feel better equipped to make take-off decisions.
Source: Flight International
