We are deep into convective weather season in the Northern Hemisphere and accident investigators are beginning to tally losses. Many weather-related accidents will undoubtedly be classified as inflight loss of control (LOC) incidents.
Typically we think of weather-related control-loss accidents as those involving VFR pilots who stray into instrument meteorological conditions (IMC). While this accident type accounts for a lion’s share of general aviation weather accidents, many incidents involve relatively experienced, appropriately licensed instrument pilots flying sophisticated airplanes.
A cautious pilot must consider many variables before launching into weather — the system’s severity, the pilot’s experience with weather, the airplane’s equipment and performance capability, and the pilot’s comfort in type, skill level and physical/mental/emotional status. All of these considerations shift from day to day — sometimes from hour to hour. All should be factors in the go/no-go/wait-awhile decision.
Consider theinvestigation into the Jan. 12, 2013, loss of Piper Meridian (PA-46-500T) N5339V. The six-seat, high-performance aircraft, powered by a single Pratt & Whitney Canada PT6A turboprop and operated by a Utah bank, was destroyed and its professional pilot and two executive passengers killed when it crashed shortly after departure from Cox Field Airport (PRX) at Paris, Texas.
The 49-year-old Commercial pilot had filed an instrument flight plan from PRX to Austin, Texas, Executive Airport (EDC). IMC prevailed for the business flight, which was conducted under FAR Part 91.
Investigators determined that prior to the flight the pilot had obtained a weather briefing via DUAT that contained surface observations along the route of flight and AIRMET information. The briefing also included convective weather advisories, a convective outlook, the area forecast, pilot reports, radar summary and winds aloft information.
The area forecast was issued at 0445 and valid at the time of the accident. The forecast was for overcast ceilings at 1,500 ft. MSL with tops at 6,000 ft. MSL; visibility between 3 and 5 mi. in light rain and mist; and isolated thunderstorms with cumulonimbus tops to 35,000 ft. MSL.
A stationary front was located in the Paris area. Associated with that front were scattered showers, brief heavy precipitation, dropping temperatures and dense fog. No SIGMETs were valid for the Paris area at the time of the accident. However, a weather advisory was issued at 0731 for an area in northeast Texas in which the accident site was located. The advisory was for low instrument conditions with areas of ceilings below 500 ft. and/or visibility below 1 mi. in mist and fog.
AIRMET Sierra was issued at 0845 and valid at the time of the accident. The advisory forecasted IMC along the accident route of flight with ceilings below 1,000 ft. and visibility below 3 mi. with precipitation and mist.
A review of ATC communications revealed the pilot was issued an IFR clearance at 0844 from Paris to Austin. The flight departed a few minutes later. (At 0855, Cox Field weather was reported as wind from 120 deg. at 5 kt.; visibility, 10 mi.; overcast clouds at 400 ft.; temperature, 16C; and dew point 15C.
Shortly after takeoff, about 0850, the pilot contacted the Fort Worth Center. A controller issued a Mode C transponder code and current altimeter setting.
At 0852:59, a controller advised the pilot he was 5 mi. south of Paris and to confirm his altitude. The pilot responded that he was nearing 5,000 ft.
At 0853:28, the controller instructed the pilot to climb and maintain 16,000 ft., and the pilot acknowledged.
At 0853:33, the controller advised the pilot to contact Fort Worth Center on another frequency, but the pilot did not acknowledge the instruction and there was no further communication with him.
A witness was standing behind his shop around 0900 about a quarter mile west of the accident site when he first heard the airplane. He said it sounded “funny” and was “very loud,” unlike other airplanes he had heard fly over the area.
The witness then looked up and saw the airplane exit the low cloud layer. The airplane’s nose was pointed down toward the ground about 80-85 deg. and was “spinning out of control” to the right. It made two revolutions before it impacted the ground and exploded.
The witness said the engine was operating and sounded “loud.” He said there was a low cloud layer but was not sure of how high the base of the clouds was above the ground. It was not raining or foggy at the time of the accident.
A second witness, who was working outside about a half-mile southeast of the accident site, said he heard the sound of an airplane engine overhead just before 0900. The witness said he looked up toward the noise, but was unable to see the airplane due to low clouds and fog. The witness said the airplane’s engine revved up and down about three times before the engine noise just stopped.
Another witness was working outside of a natural gas power plant about 2 mi. northwest of the accident site, when he heard the sound of an engine “back firing” about four or five times. It was between 0830 and 0900. At first the witness thought the noise came from within the power plant but realized later that day that it was the accident airplane’s engine he had heard. The witness described the sound as being similar to the sound of the power plant’s turbine engines when they “flame out.” The witness described the weather as “foggy and drizzly.”
Investigators reviewed data from terminal Doppler weather radar (TDWR). At 0856 and 0857, the airplane was operating in an area of developing rain showers and updrafts. These updrafts and rain showers were then observed over the accident site after 0900. The upper air sounding from Fort Worth revealed that the meteorological environment around the accident site supported developing rain showers with vertical updraft speeds as high as 62 kt. possible.
The airplane crashed upright in an open pasture approximately 9 mi. southwest of PRX on a heading of 128 deg. It remained relatively intact except for the horizontal stabilizer/rudder, which had separated from the airframe and was found about 30 ft. behind the main wreckage. A post-impact fire consumed a majority of the cockpit, right wing and fuselage.
Examination of the airplane revealed that the flaps and landing gear were fully retracted. Flight control continuity was established for all major flight control surfaces from the surface to the cockpit. Elevator trim continuity was also confirmed. The elevator trim tab was found in the 8-deg. nose-down setting. Continuity of the autopilot system could not be established due to impact and fire damage.
The backup attitude indicator (electric) was disassembled and rotational scoring was found on the pendulous vane and on the interior of the pendulous vane housing. The backup airspeed indicator needle was frozen at 110 kt.
The four-blade propeller assembly remained attached to the engine; however, two of the blades had separated the hub. These blades were found imbedded in the ground directly below the hub. Each of the blades exhibited leading edge nicks and chord-wise scratches. The propeller spinner displayed impact damage and rotational deformation.
Examination of the engine revealed damage consistent with the engine operating at the time of impact. No mechanical malfunctions or failures were observed that would have precluded normal operation of the engine.
Investigators collected the recorded ATC radar data. The airplane was first observed on radar at 0852:22. It was on a southwesterly heading at an altitude of 4,200 ft. About 36 sec. later the airplane reached an altitude of 4,700 ft. and a ground speed of 249 kt.
At 0853:46, the airplane had climbed to 5,100 ft. and slowed to a ground speed of 214 kt. It then entered a descending right-hand turn.
At 0853:58, the airplane was at an altitude of 4,800 ft. and a ground speed of 202 kt.
At 0854:22, the airplane continued to turn right and climbed to 5,200 ft. and slowed to a ground speed of 115 kt.
The last radar return was received at 0854:34. At that time, the airplane was at 4,500 ft. at a ground speed of 110 kt.
The pilot’s logbooks indicated he had accrued a total of 2,365.7 hr.; of which, 126.9 were in the same make/model as the accident airplane. The pilot also had logged a total of 118.3 hr. operating in actual instrument conditions and 86.3 hr. in simulated instrument conditions. He logged 57.3 hr. in the last 90 days and 4.3 hr. in the 24 hours prior to the accident.
An autopsy determined the pilot died of injuries sustained in the accident. The toxicological testing was negative.
Ultimately, the NTSB determined the probable cause of this accident was, “The pilot’s encounter with convective weather, which resulted in a loss of airplane control.”
If there’s a lesson to be taken from this incident, it’s simply to be on the pessimistic side when evaluating the potential impact of convective activity expected along your route. A young, developing cell can often present sharp edge gusts every bit as upsetting (literally) as a mature cell.
|SIGMET and CWSU Advisory|
|No SIGMETs were valid for the accident site at the accident time.|
|A CWSU Advisory (CWA) was valid for the accident flight in northeastern Texas and was issued at 0731 CST. The CWA advised of LIFR (see below) conditions with areas of ceilings below 500 feet and/or visibility below 1 mile in mist and fog.|
|FAUS21 KZFW 121330|
|ZFW1 CWA 121331|
|ZFW CWA 102 VALID UNTIL 121500|
|FROM 35N TXK-ELD-50SW GGG-30NW ACT-TTT-45NE TTT-35N TXK|
|AREA OCNL LIFR CONDS. CIGS AOB 005/VIS LCL AOB 1SM BR/FG.|
|AIRMET Sierra issued at 0845 CST, and valid at the accident time, forecasted IFR conditions for the accident site with ceilings below 1,000 feet and visibilities below 3 miles with precipitation and mist.|
|WAUS44 KKCI 121445|
|_DFWS WA 121445|
|AIRMET SIERRA UPDT 2 FOR IFR AND MTN OBSCN VALID UNTIL 122100|
|AIRMET IFR . . . OK TX AR TN LA MS AL AND CSTL WTRS|
|FROM 40S LOZ TO GQO TO 50SW PZD TO 40W CEW TO 50SE SJI TO 50SSE|
|LEV TO 120SSW LCH TO 80E BRO TO 30W BRO TO 30S LRD TO DLF TO|
|40SSW SJT TO 30S MLC TO RZC TO 40S LOZ|
|CIG BLW 010/VIS BLW 3SM PCPN/BR. CONDS CONTG BYD 21Z THRU 03Z.|
|19 Low Instrument Flight Rules - Refers to the general weather conditions pilots can expect at the surface. LIFR criteria means a ceiling below 500 feet agl and/or less than 1 mile visibility.|