A phenom in ice

But the system didn’t prevent the December 8, 2014, fatal crash of a Phenom 100 (N100EQ) on approach to Runway 14 at Gaithersburg, Maryland’s Montgomery County Airpark (GAI). The pilot and his two passengers were killed in the crash and post-crash fire, as were three people in a house. The crash site was just three quarters of a mile from the approach end of Runway 14.

The National Transportation Safety Board (NTSB) ruled that the probable cause of this accident was “the pilot’s conduct of an approach in structural icing conditions without turning on the airplane’s wing and horizontal stabilizer deice system, leading to ice accumulation on these surfaces, and without using the appropriate landing performance speeds for the weather conditions and airplane weight, as indicated in the airplane’s standard operating procedures, which together resulted in an aerodynamic stall at an altitude at which a recovery was not possible.”

The pilot took off from the Horace Williams Airport (IGX) in Chapel Hill, North Carolina, and the flight was uneventful until the airplane descended for the RNAV (GPS) Runway 14 approach to Montgomery County Airpark. Gaithersburg’s AWOS mentioned winds out of 40 degrees at six knots, visibility of 10 statute miles, few clouds at 2,100 feet agl, an overcast at 3,200 feet agl, and a temperature and dewpoint of minus 1 degree and minus 8 degrees Celsius, respectively. Investigator notes mention that several air carriers in the vicinity reported icing conditions in clouds with tops running from 4,300 to 5,500 feet msl. Moderate mixed icing between 4,000 and 5,000 feet was mentioned by a regional airliner.

By the time he was on approach the pilot had two strikes against him: He was flying at too low an airspeed and too high an AOA for the conditions at hand.The airplane was equipped with a cockpit voice and data recorder (CVDR), so we know that 15 minutes after takeoff—after a passenger in the right cockpit seat mentioned that the airplane was “in the clouds”—the pilot turned on the Phenom’s engine and wing and horizontal stabilizer (Wingstab) ice protection systems for about two minutes, and then turned them off. The engine inlets are heated with bleed air; the Wingstab system uses bleed air to inflate sections of the de-ice boots at intervals governed by a timer.

The pilot should have known that icing was a possibility in the descent, based on the AWOS report and his own observation. The Phenom flight manual says to turn on the ice protection systems whenever static or total air temperatures are 10 degrees Celsius or below, and visible moisture of any form is present. Moreover, the Phenom 100’s Descent checklist items in the Normal Icing Checklist section include reminders to turn on the engine and Wingstab deice systems. This action brings about changes to normal stall awareness parameters—changes designed to give the pilot more margin against a stall.

The Phenom stall protection system uses aircraft configuration—flap and landing gear position, as well as Mach number and Wingstab switch position—to provide stall warnings at lower angles of attack (AOAs) when flying in icing conditions. A red, low-airspeed awareness tape also shows higher stall thresholds when the Wingstab switch is in the On position. But while the accident airplane’s pitot-static heat was on, its Wingstab switch was Off, as was its engine inlet anti-ice.

At the time of the accident, in landing configuration and with Wingstab off, the stall warning AOA would have been the normal 21 degrees to trigger the aural stall warning, and 28.4 degrees for stick pusher activation. With Wingstab on, there would have been greater advance warning of a stall, with aural warning at 9.5 degrees AOA and stick pusher at 15.5 degrees AOA.

Turning on Wingstab also means the pilot must recalculate the landing reference speed (VREF) upwards, based on aircraft weight, to account for icing conditions. This, in turn, means recalculating landing distance and runway requirements.

Here, the pilot fell short once more. He used a 92-knot VREF, an airspeed that normally would have been safe with no icing, Wingstab off, and a landing weight less than the accident airplane’s actual weight. If he had used the Normal Icing Conditions Checklist and accurate weight, the pilot should have flown the approach at a VREF of 126 knots to account for icing. Of course, this would have meant using considerably more runway than normal.

By the time he was on approach the pilot had two strikes against him: He was flying at too low an airspeed and too high an AOA for the conditions at hand. By leaving Wingstab off and setting a VREF as though icing and landing weight weren’t issues, the Phenom’s stall protection systems had essentially been bypassed.

The NTSB investigator in charge said the airplane had been in icing conditions for 15 minutes and was on course and glidepath until 300 feet agl. Then the airplane rolled right 21 degrees. One second later, at 88 knots, the aural stall warning sounded, and continued sounding. Then came a series of roll oscillations, followed by a 100-degree roll before impact. These stall characteristics were consistent with ice accumulations, the investigator said.

The pilot’s skills had been questioned in the past. He’d had an accident in a TBM 700 at Montgomery County Airpark in August 2010, and been re-examined, with emphasis on balked landings, go-arounds, and missed approaches.

Based on its findings, the NTSB made safety recommendations to the FAA, the General Aviation Manufacturers Association, and the National Business Aviation Association. There should be automatic alerts to activate ice protection systems in turbofan airplanes requiring a type rating, and that are certified for single-pilot operations and flight in icing conditions, the NTSB said. The NTSB recommended that NBAA “develop enhanced pilot training guidelines pertaining to risk management in winter weather operations, including the use of ice protection systems and adherence to checklists, with special emphasis given to deficiencies in pilot performance identified in this accident, and make the results of this effort available to the community of pilots who fly these airplanes.”

For further information, search the NTSB website for case number NTSB/AAR-16/01.