When the Engine Goes 'Bang'!

Patrick R. Veillette, Ph.D. 08/03/2006 10:46:31 AM

Present day turbofan engines are so reliable that the average inflight shutdown rate is less than once in 10,000 flight hours. I like those odds. However, after reviewing several accidents involving misdiagnosis of engine malfunctions, some in the FAA and elsewhere are questioning whether current training adequately prepares flight crews for these relatively infrequent but potentially confusing and catastrophic events.

Like many issues in aviation, these questions arose from an accident. An American Eagle Jetstream 32 was inbound for an instrument nighttime landing at Raleigh-Durham, N.C., when the left engine IGN light illuminated as a result of a momentary negative torque condition from the prop speed levers being advanced to 100 percent with the power levers at flight idle. The pilot wrongly interpreted the light as indicating an engine failure and then failed to follow SOPs for such a failure, for a single-engine approach and go-around, and stall recovery. The commuter crashed in woods four miles from the runway, killing the pilot, copilot and 13 passengers, and seriously injuring five others.

Aside from citing the pilot's wrong interpretation and actions in the December 1994 accident, the NTSB faulted AMR Eagle's training for inadequately addressing the recognition of engine failure at low power, the aerodynamic effects of asymmetric thrust from a windmilling prop, and the high thrust on the powered engine.

These findings motivated the U.S. Aerospace Industries Association (AIA) and the European Association of Aerospace Industries (AECMA) to conduct an in-depth study of accidents and incidents involving engine malfunction, and more specifically, those of them involving incorrect crew response. The study covered such events between 1959 and 1996 and involving western-built commercial airplanes in excess of 60,000 pounds (27,216 kilograms.) All totaled there were 79 such events, 34 of them accidents.

While the majority of engine malfunctions are generally recognized and handled appropriately by flight crews, the AIA/AECMA study found that occasionally pilots have difficulty identifying the problem and sometimes react inappropriately. Ironically, among the reasons for this deficiency cited in the study was the reliability of today's turbine powerplants; so rarely do they go wrong that an entire generation of pilots has little or no experience in handling an honest-to-goodness engine malfunction.

The study also found evidence suggesting negative habit transfer was a factor in the incidents reviewed. This commonly occurs when previous training or experience in one type of airplane is inappropriate or even counterproductive when applied to another. According to James Reason, Ph.D., retired professor of psychology at Britain's University of Manchester and a world leading authority in human error, situations of negative habit transfer increase the chances of error by five-fold. Unfortunately the problem is most likely to occur during stressful, high-workload situations when pilots revert to old habits.

It should come as no surprise that 70 percent of the incorrect crew responses occurred during the performance-demanding takeoff and climb-out phases of flight. Aircraft certification regulations address the ability of the aircraft to continue flight safely after the failure of the most critical engine during the most critical point in the flight (FAR/JAR 25.107, 25.109, 25.145). However, continued safe flight is obviously contingent on the flight crew's recognition that a propulsion system has malfunctioned and then following the appropriate procedures.

As detailed in previous articles, malfunctions that occur moments before takeoff leave precious little time for the pilot to recognize and properly diagnose the problem. Of particular note were the substantial number of events -- 30 of the 79 -- that resulted in high-speed aborts, and 27 of those involved rejected takeoffs (RTO) past V1. All 27 overran the runway.

Engine compressor stalls/surges, flight deck alarms, sudden severe vibration and yaw can be so startling that they induce a pilot to reject a takeoff above V1. Yet in all 30 cases, the airplane would have flown satisfactorily had it been allowed to do so. In 1999, when the AIA/AECMA study was conducted, the number of RTOs above V1 was increasing in line with the increasing number of airplane departures, despite the obvious hazard of doing so and all of the authoritative published procedures advising otherwise. Why?

Loud bangs and teeth-rattling vibrations can lead a crew to believe their aircraft may not be airworthy. And because of technical limitations in simulator fidelity, these classic warning signs of an engine stall/surge appear to be unavailable in our training and thus could be major contributors to inappropriate crew response. In fact, in at least one case, a severe engine stall was thought to be a bomb, and the captain elected to abort the takeoff after V1.

By the way, the study's authors noted that compressor stalls/surges account for two-thirds of the engine malfunctions in today's turbofans. This is a change from earlier generation turbine designs in which uncontained failures were the principal malfunction. Since stall/surge is the primary engine malfunction, one would assume that it would be a regular item during initial, recurrent or simulator training. And yet, I cannot recall ever reviewing the matter or being exposed to a compressor stall/surge during a simulator session.

As with most pilots, my simulator training is dominated by engine failures, but most were the rather generic sudden loss of thrust or involved illumination of the engine fire warning light. The AIA/AECMA study expressed a concern that current training may be deficient in providing realistic symptoms of the malfunctions -- think "Bang! Bang!" and instrument-blurring vibrations -- most likely to induce an inappropriate pilot response. Further, the study said current training practices may also be deficient addressing the new definition of V1 and its significance in pilot decision making. (See "V1. . . . What Exactly Is It, and How Do We Apply It?" sidebar.)

Of the remaining engine malfunctions noted in the AIA/AECMA study, 14 percent involved a loss of power for reasons unrelated to a compressor stall/surge, 6 percent involved a stuck throttle, 5 percent involved a fire warning, 3 percent involved no response to commanded power, and the remaining 8 percent involved a miscellaneous assortment.

A second significant finding from the study was the high number of events -- 27 out of 79 -- in which flight crews actually shut down a good engine. Obviously crews sometimes had difficulty identifying which engine was malfunctioning. Modern surge recovery systems tend to eliminate an EGT exceedance as a visible cue of a malfunctioning engine, thus making the identification of the affected engine more difficult. Quiet malfunctions that are slow to evolve and whose cues can be masked by automatic controls are thought to be a major contributor to inappropriate crew response.

Another worrisome finding was that in 14 instances the flight crews failed to maintain adequate control of the aircraft. Five of these events occurred on the ground during an asymmetric thrust condition while landing and the remainder involved a loss of control in flight.

Since the study was limited to large transport aircraft, it's important to compare its findings with the business jet statistics to see which apply to both.

First, of the 251 accidents and 808 reported incidents involving business jets in a recent 12-year period, powerplant malfunctions caused 15 accidents and 186 incidents. (See table at right.) Incidentally, those numbers correspond fairly well with the incident statistics compiled by the Transportation Safety Board of Canada. In a review of Canadian-registered aircraft involved in incidents between 1998 and 2003, a total of 135 engine failures were contained in the incident database. Of these, 61 experienced power failure, 61 had a component failure, and 13 involved some other malfunction within the powerplant system.

Some rather noteworthy engine failures occurred in the United States in the last year. On Nov. 28, 2005, a Beechjet 400A experienced a dual-engine flameout while en route from Indianapolis to Marco Island Airport, Fla. The crew declared an emergency and landed safely at Jacksonville International Airport. Kudos to them. The NTSB has not yet cited a cause for the flameouts.

This marked the second such event involving a Beechjet in recent history. On July 12, 2004, another 400A lost both engines while over the Gulf of Mexico, but the pilots were able to restart the right engine and safely divert to Sarasota, Fla. An initial NTSB report said that the aircraft had a lower-than-normal amount of anti-icing additive in its tanks.

Fortunately dual-engine failures are exceedingly rare. The only other dual-engine failure in the business jet data involved a Sabre 80 that crashed near Ironwood, Mich., in August 2000 after being struck by lightning. Both pilots were killed and two passengers seriously injured in that accident. (While this study looked solely at business jet engine failures, dual-engine failures have also occurred, albeit rarely, in turboprops. These involved a Short Brothers SD-3 in Scotland, a Swearingen in Arizona and an MU-2 near Lewiston, Idaho; all involved engine intake icing.)

All of the other turbofan engine failure events on business jets involved a failure of just one engine on the aircraft, and only three of the 201 engine failures resulted in fatal injuries. That's a decent track record over a 12-year period. The remaining 12 engine-failure accidents and 186 engine-failure incidents were resolved without further aircraft damage and without injury to anyone aboard.

If your last simulator training/checking session was anything like mine, you had a multitude of engine failures thrown at you with the most critical coming during those high-speed moments on takeoff. Out in the real world, you'll be glad to know that a total of only 23 engine failures were reported during the takeoff phase of flight. That's a pretty impressive number when you think about all of the business jet takeoffs in a dozen years. In eight of those events, RTOs resulted in two accidents and six incidents. In the other 15 events the pilots continued takeoff with the results being 12 incidents and three accidents.

Contrary to popular opinion, the vast majority of engine malfunctions occurred during the cruise phase of flight, which makes sense since that's its longest airborne exposure. When an engine fails at altitude the crew has plenty of time to perform checklists to correct the situation, a situation in stark contrast to the high workload situation during a typical simulator training session.

One particularly vulnerable set of circumstances that have not contributed to accident/incident statistics involves an engine failure during a departure from mountainous airports in IMC. While some operators strictly abide by the requirement to have an adequate performance margin to out-climb obstacles in the event of an engine failure, others will board multiple passengers in Aspen, load plenty of fuel and then blast into a low overcast even though there's no way the aircraft could meet the necessary obstacle clearance climb-out if it suddenly lost half its power. The operators who launch into such conditions without meeting those restrictive departure climb gradients are rolling the dice.

Surprisingly, there are no regulatory requirements to train pilots on propulsion system malfunction recognition. Even though the statistics don't suggest any cause of alarm, FAR Parts 61 and 121, JAR-OPS and JAR-FCL could and probably should be modified to address that shortcoming, along with the effect an engine failure has on airplane performance and controllability, and the subsequent control of the airplane.

There is no single solution to preventing incorrect responses by crews to an engine failure. Rather, a combination of enhanced awareness, training and simulation will be required.

The Air Transport Association and the FAA have developed a training aid entitled "Airplane Turbofan Engine Malfunction Recognition and Response," which includes a 22-minute video on a CD-ROM. (For further information, contact the FAA Engine and Propeller Directorate, ANE-110, 12 New England Executive Park, Burlington, MA 01803.) This training material is worth incorporating into your training programs and into simulator training exercises and could help you proceed intelligently and knowingly in the unlikely event you hear a "Bang! Bang!" in flight.

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