Flight Sequence
On 26 April 2013, at about 17:25 local time, a Cessna 172 (registration HB-CPL) departed from runway 33 at Bex aerodrome (LSGB) for a private VFR circuit to practice speed control on final. The pilot, the sole occupant, climbed to 2800 ft, turned left to join the downwind leg, and descended to 2200 ft. On final approach, full flaps were selected and the propeller set to fine pitch. The pilot noted that the approach speed was too high but continued.
The aircraft touched down with the nose wheel, bounced, and touched down again. The pilot then decided to perform a go-around and increased power. According to the pilot, the engine was "a bit slow to pick up." At about two-thirds of the runway, the aircraft became airborne with the flaps still extended. It climbed to approximately 30 m above ground and flew over the last third of the runway at low speed, gaining little altitude.
The pilot turned slightly left to overfly a wooded area at the end of the runway. Shortly thereafter, the stall warning sounded; the pilot pushed forward on the control column and observed a high-voltage power line parallel to the runway. The left wing lower surface contacted the upper cable (an earth conductor equipped with orange marker spheres). The aircraft rolled to the right, and the propeller struck phase conductor #1, located 3.7 m below the earth conductor. An electrical arc occurred, damaging the conductor, the propeller, and the left wing lower surface. Circuit breakers interrupted the current fractions of a second later.
The damaged phase conductor #1 broke and, in a whiplash effect, wrapped around the right main landing gear leg, causing the aircraft to yaw clockwise. The conductor broke again, and the aircraft fell into trees, severing phase conductor #2. Large branches broke, slowing the descent. The aircraft struck the ground with a nose-down attitude; the nose gear leg collapsed and folded under the fuselage.
The pilot, uninjured, switched off the master switch and magnetos, unfastened the seatbelt, and removed the headset. The left cockpit door was jammed, so the pilot exited through the right door and walked to the main building of a nearby kennel. A flight instructor standing at the edge of the runway had observed the go-around and noted an apparent lack of power (estimated engine speed 1800 RPM), extended flaps, very slow airspeed, and minimal climb. He immediately called emergency services. The aircraft was destroyed; no fire occurred.
Aircraft and Pilot Information
The aircraft was a 1956 Cessna 172, a four-seat, high-wing, metal monoplane with fixed tricycle landing gear. It was powered by a Franklin six-cylinder air-cooled engine rated at 180 HP, driving a McCauley two-blade metal constant-speed propeller. The empty weight was 1598 lb; maximum takeoff weight was 2200 lb. The estimated weight at the time of the accident was 1900 lb, and the center of gravity was within limits. The aircraft was equipped with Madras Air Service Super Tips wing tips (STC SA2075WE) for improved low-speed flight. The last 100-hour inspection was performed on 12 March 2013 at 3587.12 hours time since new.
The pilot, a Swiss citizen born in 1950, held a Private Pilot Licence (PPL(A)) first issued on 12 July 2012, with a single-engine piston (SEP) land class rating valid until 31 July 2014. A Class 2 medical certificate with a limitation "shall wear multifocal lenses" was issued on 13 November 2012, valid until 13 November 2013. Total flight time was 139 hours 48 minutes, with 6 hours 49 minutes in the preceding 90 days; time on type was 35 hours 51 minutes, with the same 90-day amount.
Throttle Control Peculiarity
The throttle could be operated in two ways: by screwing it in or out (slow power changes) or by pressing a central button (8 mm travel) for rapid linear adjustment. Releasing the button by 3–4 mm re-engaged the screw mechanism. The pilot, his instructor, and a previous user of HB-CPL confirmed that when power was increased rapidly from idle, the Franklin engine tended to hesitate or run irregularly. To avoid this, they would stop the linear movement or reduce power before advancing the throttle more slowly.
Meteorological Conditions
The weather was influenced by a trough extending from Scandinavia to the Bay of Biscay. A cloud band about 400 km wide stretched from northern Spain to the North Sea. Foehn winds from the south partially cleared clouds in the lower Valais and Lake Geneva area. In the Chablais region, local pressure patterns due to foehn caused northwesterly winds near the ground, while winds aloft were from the south. Near Bex, winds were heterogeneous with shear. Reported conditions: 1/8 stratocumulus base 4700 ft AAL, 5/8 altocumulus base 9000 ft AAL, visibility 25 km, wind 330° at 6 kt gusting to 12 kt, temperature 18 °C, dew point 12 °C, QNH 1010 hPa. The sun was at azimuth 258°, height 30°, in daytime. A flight instructor estimated surface wind as 330° at 5–8 kt with no significant phenomena. The pilot described the environment as calm.
Cause
The Swiss Transportation Safety Investigation Board determined that the accident was due to a collision with a high-voltage line after a go-around attempt following a misapproach.