STAY AHEAD OF TROUBLE. Looking at multiple NTSB investigations into pilot failure to activate pitot heat, a pattern emerges: Carelessness often leads to disaster.
In an ideal world, once the probable cause of an accident is identified, there never will be an accident like it again. The NTSB would figure out everything leading to the accident, and armed with that knowledge, aviators would have an easy time preventing the same thing from happening again. Unfortunately, it’s not that simple in the real world, where we often see repeats of the same probable causes.
No one has devised a foolproof way to prevent pilots from failing to maintain airspeed, continuing VFR into instrument conditions, or neglecting checklist items. For example, the NTSB spotlighted neglecting to turn on pitot heat 35 years ago, and then again 18 years ago: In a July 1992 special investigation on PA46 accidents, the NTSB examined PA46 pilots’ failure to use pitot heat while operating in freezing instrument meteorological conditions. Several months ago, the Safety Board again focused on failure to turn on pitot heat as a safety concern.
In November 2009, the NTSB determined that an accident involving a turboprop Piper PA46-500TP Malibu Meridian, in which all three occupants were killed, was caused by the pilot’s failure to activate the pitot heat as called for on the checklist. This resulted in erroneous airspeed information due to pitot tube icing. At 8:15 a.m. on June 28, 2007, the PA46 suffered an in-flight breakup and crash near Wellsville, Mo. The IFR flight had originated from Spirit of St. Louis Airport, near Chesterfield, Mo., at about 7:50, and was destined for Buffalo Municipal Airport, near Buffalo, Minn.
At 7:11, the pilot contacted flight service to file an IFR flight plan and get an abbreviated weather update. The briefer told the pilot that there was heavy rain and thunderstorm activity in Missouri along the planned route of flight. The pilot said that he had onboard radar for weather avoidance.
At 7:50, the pilot contacted St. Louis Departure Control, and two minutes later, was cleared to climb to 4,000 feet. The controller advised of light to moderate precipitation three miles ahead of the aircraft. The pilot was approved for a northerly course deviation for weather avoidance. At 7:53, the pilot was cleared to climb to 10,000 feet. The controller then advised of additional areas of moderate and heavy precipitation, gave the pilot information on the location and extent of the weather areas, and suggested a track to avoid them. The pilot responded that he saw the same areas on his onboard radar. Radar data showed that the airplane flew northwest, then turned toward the west. Subsequently, the flight was handed off to Kansas City Center.
A minute after the flight was cleared to climb to FL230, a new controller relieved the current controller. This controller made no transmissions to the PA46, and was relieved by a third controller at 8:06. That controller radioed the pilot to ask whether he had been given a clearance to deviate from the assigned heading. The flight’s radar track showed that the airplane turned to the left. The pilot responded, “We’ve got problems.” The controller asked if the pilot was declaring an emergency, and made several other attempts to contact the flight. The pilot didn’t respond, and radar contact was lost. None of the three Kansas Center controllers had given the pilot any weather information during the time they were handling his flight. Ground and aircraft observers located the wreckage at around 10:40 a.m.
The 44-year-old pilot held a private pilot certificate with airplane single-engine land and instrument ratings. He had taken flight training for the PA46-500TP along with a flight review and instrument refresher from a commercial provider. At the time of training, the pilot reported 1,000 hours of total flight time, 300 hours of instrument flight time and 50 hours of turbine flight time.
The POH listed 127 KIAS as the airplane’s maximum operating maneuvering speed (Vo). The limitation associated with Vo stated, “Do not make full or abrupt control movements above this speed.” The POH listed 188 KIAS as the airplane’s maximum operating speed (Vmo). The limitation associated with Vmo stated, “Do not exceed this speed in any operation.”
The switch to turn on pitot heat is located on the right overhead switch panel. The static source ports aren’t heated; if they ice over, selecting the alternate static source is supposed to alleviate the problem. According to the POH, the airplane’s annunciator panel was equipped with an amber caution light that displayed “pitot heat off,” which “indicates the pitot heat has not been selected on.” There also were two red warning lights, one for each of the left and right pitot heads. The red lights come on in the event of a pitot heat failure, such as a burned-out heating element. The POH’s “before takeoff” checklist calls for “pitot heat on.”
According to a Piper representative, the data for similarly equipped Meridians showed an increase in the electrical load of approximately 13 amperes when the pitot heat is turned on. The pitot heat system is designed to cycle itself on and off at a 30-second rate to prevent overheating while the airplane is on the ground with the pitot heat switch selected on. While in flight, the pitot heat operates continually when turned on.
Investigators removed chips from the airplane’s glass-panel avionics, which record data used to generate panel displays. The data revealed that the outside air temperature dropped from 24 degrees C at the surface to zero degrees C at 8:09:41 and 15,900 feet MSL. The pilot’s airspeed data dropped from 142 KIAS to zero at 8:10:45, and the copilot’s airspeed dropped from 140 KIAS to zero at 8:10:51.
The airplane’s climb rate decreased as the airspeed data dropped to zero, and at 8:11, the airplane started a left turn. Airspeed, vertical speed and altimeter data validity bits switched to a fail state. A red “X” is flagged over displayed parameters that have failed validity bits. The pilot’s PFD data was in the flagged fail state between 8:12:13 and 8:12:42. The copilot’s PFD data was in the flagged fail state between 8:10:55 and 08:11:03 and again between 8:11:25 and 8:11:43. The data showed that the airplane’s engine was operational and that there was no reduction in power as the aircraft started the left turn. At 8:12, the data showed that the airplane had achieved a 250-knot groundspeed and then sustained two vertical accelerations of about 5 G’s. The airplane subsequently descended and rolled during its descent. Review of MFD data didn’t reveal any evidence of changes in the plane’s electrical load, which would have been expected if the pitot heat was operating.
In 1975, the NTSB reported on the December 1, 1974, crash of a Northwest Airlines Boeing 727 near Thiells, N.Y. The Safety Board found that contrary to standard operational procedures, the flight crew hadn’t activated the pitot heat. The airplane had departed John F. Kennedy International Airport on a ferry flight to Buffalo, N.Y. The three crewmembers, who were the only people on board, were killed. The airplane was climbing to FL310; it stalled after climbing above FL240. Earlier, the airplane had been climbing at 2,500 fpm with a 305-knot airspeed, according to the flight data recorder. As the altitude increased above 16,000 feet, the airspeed and climb rate displayed on the cockpit indicators began to increase. The first officer, who was the flying pilot, remarked, “Do you realize we’re going 340 knots and I’m climbing 5,000 feet per minute?” The second officer responded, “That’s because we’re light.”
Soon, the rate of climb exceeded 6,500 fpm, and the airspeed reached 420 knots. The first officer said that he couldn’t reduce the climb rate or slow the 727. The captain told him to “just pull her back. Let her climb.” Then, the stall-warning stick-shaker activated, and the airplane started to buffet. The first officer said the buffeting was because they were close to the airplane’s critical Mach speed as shown on the airspeed indicators: “I guess we’ll have to pull it up.” A high-speed buffet is caused by the formation of a shock wave on the airfoil surfaces and turbulent separation of the flow aft of the shock wave. But there also is buffeting associated with a stall.
The captain commanded, “Pull it up.” Two seconds later, the airplane began to descend, making a rapid turn to the right and reaching a descent rate of 15,000 fpm. The crew issued a “mayday” call, and advised controllers they were descending through 12,000 feet in a stall. At about 3,500 feet MSL, part of the left horizontal stabilizer separated.
NTSB wind tunnel tests of the pitot heads found that when exposed to liquid in freezing conditions with the pitot heat inoperative, a thin film of water flowed into the pressure port and out of the drain hole. Then one to two inches of ice formed over the pitot’s pressure inlet port. Ice also blocked the drain hole.
Peter Katz is editor and publisher of NTSB Reporter, an independent monthly update on aircraft accident investigations and other NTSB news. To subscribe, write to: NTSB Reporter, Subscription Dept., P.O. Box 831, White Plains, NY 10602-0831.