Pilot suspects static electricity in Citation X pitot failures

 - October 28, 2009, 12:41 PM

Static electricity may cause pitot static probes to fail, according to a Cessna Citation X owner-pilot who survived a simultaneous failure of all three pitots during a flight earlier this year. Kirill Minovalov, a Russian entrepreneur and private pilot, was flying in stormy weather conditions when the incident happened; he managed to land safely at Moscow’s Domodedovo Airport.

Minovalov, 38, was quick to take advantage of Russia’s economic reforms in the early 1990s. In 1994 he formed Bank Avangard and still serves as its president and CEO. He was urged by the Russian Business Aviation Association to discuss the serious safety incident and spoke exclusively to AIN.

Seven years ago, Minovalov bought his first aircraft, a Dassault Falcon 10, and he has since logged 2,500 hours as a pilot in that airplane and the Citation X. He also owns an AgustaWestland A109 Grand, in which he has 350 hours.

On July 27, Minovalov was at the controls of the German-registered Citation X. Built in 2003, the airframe had logged nearly 2,400 hours and 1,660 landings.

Due to thunderstorms in the Moscow area, air traffic controllers placed the airplane at a rather high altitude, 39,700 feet, over the villages of Sukhotino and Skurygino (22 miles south of central Moscow). At around 1 a.m. the crew was cleared for descent.

The initial descent was rapid, at 5,500 feet per minute, and in between two thundercloud fronts, 25 nm to the right and 40 nm to the left. At around 34,500 feet the Citation X entered heavy turbulence.

A minute later Minovalov and his German copilot saw light shining on the other side of the windshield. “For the first time in my flight experience I saw an electric charge; it was shining brightly like welding [torches]. A golf ball in size, it was sitting on the shroud of the windshield airflow system,” Minovalov recalled. Apparently, a huge amount of static electricity had amassed on the airframe. Its protruding parts, such as the shroud and, quite possibly, pitot probes, were carrying high charges of electrical current, Minovalov surmised.

Thirty seconds later the pitot heater fail left warning light came on. The crew was not too concerned about this since the speed indication remained accurate. But after another 30 seconds the speed-measurement system suffered a complete failure. All three pitot static probes were apparently producing incorrect readings.
The captain’s and copilot’s indicators gave an airspeed reading of some 40 knots, while the standby instrument was on zero. Pitot heater fail right and pitot heater fail sb warning lights came on. The pilots also got an audio low-speed warning at this point.

The incorrect speed reading caused the yaw damper to disengage and issue the warning yd fail–lower. Other aircraft systems responded to the false readings. For example, mechanical limitations for rudder and aileron deflection adjusted to the wrong speed reading.

The Rolls-Royce AE3007C1 engines produced enough thrust, but their full authority digital engine controls (Fadec) reacted to the erroneous speed reading by activating reverse [reversion] mode and generating a fadec rev adc l-r message.

The cabin pressurization system appeared to be fooled by the wrong speed reading. Both pilots sensed bad pressure settings with their ears; it was rather uncomfortable, but bearable. Later Minovalov discovered that control of cabin pressure and pitot heat in the Citation X is linked.

“Having accepted the controls, I felt the yoke and pedals had wrong gearing for
the actual speed, requesting very careful handling,” explained Minovalov. A sharp deflection of hydraulically boosted control surfaces might have led to entering dangerous flight regimes or airframe structural damage.

To their credit, the captain and copilot acted in unison, sharing a common view about how to cope with the situation. The crew preferred “vertical speed” mode to “flight level change” in following the flight directors and ATC commands. They focused on keeping airspeed within the safety corridor while descending at some 2,500 feet per minute, once heading, gyroscopic horizon, barometric altitude and descent rate readings seemed correct, and the radio and flight directors remained intact.

“We flew through darkness sparked by lightning flashes all around, no visual reference to the ground,” Minovalov recalled. “There was a strong feeling the airplane had been flying too slowly, and it was tempting to add thrust. But doing so may have resulted in [exceeding] Vne [never exceed speed].”

The crew asked air traffic controllers to keep them informed of their groundspeed. “I did not know what the wind was, but, having information on speed from the ground, I could figure out that, despite the feeling of too low a speed, the airplane in fact flew rather [more] fast than slow.” When the airplane was at 27,900 feet,
the air traffic controller reported ground- speed of 397 knots.

The shining ball of light outside remained visible for a minute or two. “I cannot recall exactly when it vanished since I was preoccupied with smooth handling and keeping safe speed, while the siren screamed alarm. But the ball was certainly on for quite a while,” Minovalov said.

The Citation flew in turbulence for some four minutes, getting out of it at about 19,500 feet. Back in stable air, the crew checked the circuit breakers. Then Minovalov tested the system by pressing the yaw damper button. Everything went back to normal and the yd fail-lower light went off. In a split second the system displayed the correct speed reading.

Then Minovalov turned on the autopilot, which also worked. Some warning messages remained on the screens for another couple of minutes and others disappeared one by one. Pitot heater fail lights went off midair, and fadec rev adc l-r at landing. As the airplane taxied to its parking position, no warning lights were illuminated.

The next day essential information about the case–including files from flight recorders–was passed to FlightSafety International and Cessna.

Not an Icing Incident

At the request of the manufacturer, the pilots discussed the incident with Cessna over the phone. It was hard for them to convince Cessna that the incident had not involved icing. “I frequently fly from Moscow, sometimes to Innsbruck [in Austria], and so I know very well what icing is like,” insisted Minovalov.

In his view, the symptoms were different from what would have happened if the pitots had been iced up. The speed readings on all three indicators dropped simultaneously, while icing normally causes smooth changes depending on the altitude. All three pitots failed within a split second and started operating again in tandem. What’s more, there was no rain and the ambient temperature outside the aircraft was -15 degree C.

Minovalov is convinced that static electricity was the root cause of the pitot failure. He believes that a static electricity charge can create a plasma that affects the air flow. Rather than being stuck, the pitot static probes were not getting a rush of air flow, which resulted in all of them having the same incorrect speed reading from the pressure measured on both ends of the tube. Incoming airflow was unable to create a difference in pressure since the plasma prevented it from entering the pitot probe.

Minovalov and his crew were back flying in the Citation X the next day. He said that he had complete faith in the aircraft.

“What happened the previous night was merely a natural phenomenon,” he said. “Static electricity charge and plasma build up in certain weather conditions and naturally disappear when ambient conditions change, leaving aircraft systems intact. In a way, it is like icing: the substance builds up and disappears, and the airplane is fully operable again.”

Soon after the incident the Citation X underwent scheduled maintenance, and all systems were found to be working. Neither the manufacturer nor aviation authorities required any additional work on the airplane.

“After that flight, I began my own research into what happened, this natural phenomenon. I also read much about Air France Flight 447 [the June 1 crash of an A330-200 into the Atlantic Ocean],” he told AIN. “In my view, we experienced the same phenomenon. We both flew in heavy turbulence. The French crew also radioed about static electricity charge. It could be nothing else but them actually seeing a shining ball. There is some evidence that the A330-200’s engines changed mode; in my case the Rolls-Royces went into reversion. Reportedly, the other airplane had also had the yaw damper and autopilot disengaged. It seems to me that the chain of events and the flow of failures were much the same in their case and mine.”

However, there are differences between the two incidents. Unlike the Airbus over the middle of the Atlantic Ocean, the Citation flew over land in the vicinity of airports, with sufficient ATC support. “Seemingly, in my case it was a lot more comfortable, morally and psychologically,” Minovalov admitted. “Besides, it was easier to maintain safe speed on descent than in level flight at high altitude.”

Importantly, the Cessna has a yoke, not a sidestick like the A330. If the speed reading is incorrect, the flight-control system’s gearing in pitch, yaw and bank channels may become inadequate and cause greater difficulties for handling.

It is important to note that speeds, descent rates and altitudes of the Citation X’s troubled flight were typical, much the same in everyday use by many fast jets. This makes Minovalov believe that the same incident could happen again.

The hypothesis that a static electricity charge can create plasma that blocks airflow is something that has previously been considered by pilots, but evidently no one else has yet reported simultaneous failure of all pitots.

Cessna responds:
This appears to be an isolated event. We have had no other reports of this type in any of the 305 Citation Xs, which have accumulated some 1.3 million flight hours to date. Extensive maintenance was done on the airplane in an effort to determine the cause of the incident; however, to date we have been unable to determine the probable cause. We have obtained several system components from the aircraft, have subjected them to extensive testing and have not been able to duplicate the event. We are keeping the FAA fully informed concerning the scope of our investigation and our progress.