History of the Flight
The pilot, flying from Manchester International Airport to Halfpenny Green, reported that approximately 30 to 35 minutes into the flight, the radio and GPS displays began to flicker and eventually failed. Despite radio difficulties, the pilot was in contact with Shawbury Radar, then unexpectedly with London Information, before the radio failed completely. To verify his position, the pilot orbited, set carburettor heat to HOT, and descended to 2,800 feet. When opening the throttle to maintain height and speed, the engine did not immediately respond and ran roughly. Believing he had both an electrical and engine fault, and being near Otherton Airfield, he decided to make a precautionary landing.
Arriving overhead at approximately 2,000 feet, the pilot selected Runway 25, the longest option. He was aware the grass runway might be insufficient but saw a suitable field beyond. He made a normal powered approach configured for a short field landing. On base leg, the engine again responded slowly to throttle input. The aircraft touched down at the threshold, but halfway along the runway the pilot realized he could not stop due to the downhill slope. Considering a go-around, he judged the M6 motorway ahead and poor engine acceleration made it inadvisable, so he continued. The aircraft travelled 20 to 30 feet into a recently ploughed field, where the nose landing gear collapsed and the aircraft stopped.
Description of the Electrical System
The Piper PA-28-181 was equipped with a 28-volt direct current electrical system, including a 75-amp alternator and a 24-volt battery connected to a busbar powering all electrical equipment except the starter motor. The battery and alternator were controlled by rocker-type switches (Battery Master and Alternator) on the overhead panel. Alternator and Low Voltage warning lights were in the annunciation panel in front of the pilot. The Alternator light illuminated when no current came from the alternator; the Low Voltage light illuminated when alternator output was lost and the system drew from the battery alone. A digital ammeter on the instrument panel indicated alternator current.
Engineering Investigation
After repair, the maintenance organisation performed engine ground runs and tests of the electrical generation system using the same components fitted during the accident flight. The engine, electrical, and warning systems all performed satisfactorily.
Comment
The pilot provided a candid account, noting that although he could not recall the warning lights being illuminated, he thought he might not have selected the alternator switch to ON during pre-start checks. If the alternator was off, electrical equipment would run from the battery, which would discharge after approximately 30 minutes, consistent with the loss of GPS and radio 30 to 35 minutes into the flight. The unexpected response from London Information may have occurred because the radio was preset to 124.75 MHz; as battery voltage decreased, insufficient power likely caused the radio to stop, but when other equipment shut down, enough power returned for the radio to default to that frequency.
Regarding engine performance, carburettor heat set to HOT enriches the fuel/air mixture, reducing engine power and adversely affecting acceleration. If the pilot advanced the throttle rapidly, the carburettor accelerator pump would add more fuel to an already over-rich mixture, causing slow acceleration and possible rough running.