When Being Meticulous Isn‘t Enough

According to what investigators reported, the Cessna T206H was being operated by the commercial pilot on a positioning flight to nearby Teterboro Airport (KTEB). The pilot was the only occupant and was killed. He apparently was en route to KTEB to pick up the airplane's owner and take him out to the east end of New York's Long Island, where the owner had a home.

A friend of the pilot had a Cessna 182, was thinking of upgrading to a Cessna 206, and was invited by the pilot to meet him at KCDW to see the T206H.

The T206H is a high-wing, fixed-gear, six-seat aircraft dubbed the Stationair. The accident airplane was manufactured in 2009. It had a turbocharged Lycoming TIO-540-AJ1A engine rated at 310 horsepower at 2,500 rpm. Maximum takeoff weight for the model is 3,600 pounds, with a typical useful load of 1,251 pounds. It has a maximum cruise speed of 178 knots, a stall speed of 54 knots, and requires a takeoff ground roll of only 915 feet. Its service ceiling is 27,000 feet, and the maximum range is 630 nautical miles.

When the friend arrived at KCDW at about 9:30 a.m., the pilot had finished the preflight. The friend said the pilot was "fine, his usual self, and doing a good job that morning."

The pilot's friend was inside the FBO when he heard the airplane's engine start. It held on the ramp for a few minutes before taxiing out.

According to an audio recording, the pilot's first contact was with ground control to make a request to taxi. He advised having the ATIS and that he was going to Teterboro. He was instructed to taxi for departure from Runway 22 at the intersection with taxiway November. Runway 04/22 is 4,552 feet long by 80 feet wide.

Investigators were told by a student pilot that while the airplane was holding on November, the engine went from almost idle to full power about five times. The student was quoted as saying that the engine didn't "sound right," but the Safety Board report did not offer more of a description of what "right" was.

The pilot's friend said that he saw the airplane in its takeoff roll. He said the engine didn't sound as loud as he thought it should, and the airplane didn't seem to be moving as fast as he thought it should.

Visual meteorological conditions prevailed that morning, and an airport surveillance camera was rolling. Although the color video it captured was not of the highest quality, what it shows is nonetheless chilling. From a vantage point at roof level of a hangar, we can see a panorama including about half the length of Runway 22. We see the departure end in the distance on the right third of the image, a parallel taxiway in about the center, and hangars on the left. A timer superimposed at the lower right begins ticking away at 10:00:00 a.m.

There is no visible activity until about 10:02:43, when an airplane appears on the right, already airborne at almost treetop height. It climbs at an energetic rate on runway heading, soon becoming just a white dot in the distance and disappearing from view.

At 10:04:08, an airplane that had landed suddenly appears, already firmly planted on the runway. It slows, turns off the runway at a far intersection, and holds awaiting clearance to taxi.

At 10:05:17, another airplane appears on the right side of the image, looking as if it also has just landed. However, this airplane doesn't slow down. It continues along the runway on the ground and, at 10:05:21, begins to lift off within what looks like about 100 feet of the intersection where the previous airplane was holding. This is the Cessna T206H. The airplane climbs slowly, hesitating as it reaches what appears to be the limit of ground effect, as if to hover there. Then, it climbs a bit more. It seems to level off at an altitude even with the treetops. When it's beyond the end of the runway, at about 10:05:38, it makes a turn to the left and sinks a bit.

At 10:05:50, the Cessna, now tiny in the distance, remains level with the treetops and very small in the video. Then, it appears to enter a slow and slight descent. At 10:06:02, either because the trees are higher or it has sunk lower, the airplane disappears from view. At 10:06:20, a large, black mushroom cloud begins rapidly rising from behind the trees on the far left of the picture. The airplane has crashed.

The times shown on the surveillance video differed from the accident time determined by the NTSB. The Safety Board said impact occurred at 10:02 a.m.

The Safety Board did not include a timed transcript of communications between the pilot and air traffic control in its report. But an audio recording reveals that in issuing the takeoff clearance, the controller stated that the wind was calm and the pilot was cleared for takeoff with a left downwind departure toward Teterboro. The pilot acknowledged the clearance in a voice that sounded calm, strong and very business-like. Shortly thereafter, he radioed, "63 Tango Victor has a problem, declaring emergency, returning to field immediately." Again, the pilot sounded calm, in command of the situation and business-like. The controller responded immediately, "63 Tango Victor, anything you need is approved, any runway, cleared to land." The pilot responded, "Cleared to land, unable to maintain power, 63 Tango Victor."

The accident site was in a wooded area to the left and about one-third of a nautical mile from the departure end of Runway 22. The airplane hit several trees as it descended to the ground. Impact was in a nose down attitude, and the wreckage was found inverted. The propeller blades did not show the usual scratching and bending evidence that is indicative of rotation during the impact sequence.

The engine separated from its mounts during the impact sequence but did sustain some fire and heat damage. Investigators determined that the airplane had plenty of fuel on board, and fuel had been getting to the engine. There was no evidence of blockages in the intake or exhaust systems, and nothing to indicate magneto, fuel pump or oil pump failure. Fire and heat damage to the turbocharger system couldn't hide evidence that its turbine wheel was not rotating during the impact sequence. Further, investigators noticed that the spark plugs had black sooty deposits on their electrodes. This pointed toward carbon fouling as a result of the engine being run with an excessively rich mixture. That could happen as a result of turbocharger failure.

Cessna published information on turbocharger failures in "Pilot Safety and Warning Supplements," issued in 1998. It warned that, "A failure of the turbocharger system will cause either an overboost condition or some degree of power loss." If power loss results, Cessna warned, "...it may be further complicated by an overly rich mixture. This rich mixture condition may be so severe as to cause a total power failure." Cessna noted that partial power might be restored by leaning the mixture.

A turbocharger takes the energy from heat normally expelled through the engine's exhaust system and compresses the exhaust gases to be fed to the intake manifold. That results in greater manifold pressure than the current atmospheric pressure. When atmospheric pressure diminishes with altitude, engine performance also diminishes. A "normalized" turbocharger system is intended to provide sea level engine performance (or close to it) at high altitude. A "ground-boosted" system is intended to boost the horsepower output of an engine. The exhaust gas rotates a turbine wheel, which is on the same shaft as a compressor, at high speed. The compressor pulls in filtered air, which is then compressed and sent to the engine.

Investigators could not find any preimpact problems with the moving parts, which were recovered, nor with the wastegate, which is intended to open and allow exhaust gas to bypass the turbocharger, limiting speed of rotation and boost.

Here's where the investigators really became detectives. Working closely with engine manufacturer Lycoming, and also relying on expertise of Hartzell Engine Technologies, which manufactured the turbocharging system, sophisticated techniques such as infrared spectroscopy, X-rays and CT scanning produced some astonishing results.

Maintenance records indicated that the owner was meticulous about seeing to it that any issue was addressed. The engine was receiving regular oil changes and oil analysis. As far back as January 21, 2013, however, maintenance personnel noticed oil on the inlet scroll of the turbocharger and on the belly of the airplane. They replaced the turbocharger.

In maintenance on May 22, 2013, they found pooled oil in three cylinders. At the time that was dealt with, the maintenance records did not indicate that the check valve on the turbocharger oil supply line was cleaned or replaced.

Proper turbocharger operation depends on a clean and sufficient engine oil supply. The Safety Board quoted Hartzell as saying that "...the turbocharger operates at speeds over 100,000 rpm and temperatures exceeding 1,650 degrees and oil is required at the correct flow rate and pressure to lubricate the bearings, stabilize the rotating shaft and bearings and act as a coolant."

X-ray examination of the oil supply check valve, located upstream of the turbocharger to regulate the supply of oil it received, showed that an internal spring was angled by about 5 degrees. CT scanning and X-ray study found that there was a small gap between internal components. Foreign material was found lodged there. To prevent oil from building up in the turbocharger when the engine is shut down, the check valve is not supposed to allow any oil to get through when the oil pressure drops below 8 pounds per square inch (psi). When the valve from the accident airplane was tested, oil was getting through as low as 1 psi, and when the pressure was raised to 5 psi, it became nearly a straight jet of oil.

The NTSB determined that the probable cause of this accident was a loss of engine power due to a malfunction of the turbocharging system likely due to contaminated oil. Also causal were the pilot's decision to continue takeoff, although the airplane was not performing normally, and his failure to maintain adequate airspeed following the loss of engine power, which resulted in the airplane exceeding its critical angle of attack and an aerodynamic stall. Contributing to the accident was the engine manufacturer's inadequate guidance regarding inspection and maintenance of turbocharged engines.

As far back as 1994, as a result of its investigations, the Safety Board was concerned that pilots did not have all the information needed for adequately handling emergencies related to turbocharger failure. It called on the FAA to require manufacturers to include that information in handbooks and airplane manuals. It wanted the FAA to include such a requirement in airplane certification regulations. The FAA instead issued a policy letter on February 16, 1995, adding turbocharger failure procedures to the list of things that the agency should consider when deciding whether to approve the emergency procedures section of an Aircraft Flight Manual. The NTSB said at the time what the FAA did was acceptable.

On May 13, 2008, after investigating an accident involving turbocharger failure, the NTSB again called for FAA action to require that emergency procedures published for pilots and aircraft operators specifically address turbochargers. It seemed the emergency procedures for power loss presented in the Pilot Operating Handbook for the turbocharged airplane involved in that accident called for advancing the mixture control to full rich. Remember, it's been established that going full rich in some circumstances can cause a total loss of engine power. The Safety Board found that in the previous 15 years, there had been 23 accidents or incidents involving turbochargers, resulting in 23 fatalities and three injuries. It also expressed concern that "...no regulatory requirement has been established to mandate the incorporation of the recommended safety information into the appropriate POHs." Even though the NTSB's probable cause of the Caldwell accident does not cite lack of action by the FAA, I'm wondering out loud whether it should, given the fact that issues raised years ago by the Safety Board once again are on the table.

The Caldwell accident underscores a few things to keep in mind regardless of whether you're flying behind turbo or normally aspirated engines. First, clean oil is always better than contaminated oil. Second, when maintaining any airplane system, replacing a part may not be sufficient; there may be ancillary preventive steps worth taking. Third, memorize emergency procedures for those occasions when you won't have time to find and run a complete checklist. Fourth, try to learn in advance whether there might be unintended consequences when you take prescribed emergency actions. Fifth, if something doesn't sound right, look right, perform right or produces a funny feeling in the pit of your stomach, stay on the ground and thoroughly investigate it.

Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other news concerning the National Transportation Safety Board. To subscribe, visit www.ntsbreporter.us or write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.