A 16-year-old boy and an 80-year-old man were killed on Friday 16 October when a light aircraft crashed north-east of the village of Drymen in Stirling, Scotland. Police Scotland received a report of the crash at around 4.45pm local time. Emergency services attended and both occupants were pronounced dead at the scene; their next of kin have been informed1. The Air Accidents Investigation Branch (AAIB) has opened a safety investigation and sent a team to the site2.

What is known about the flight

Drymen lies on the eastern side of Loch Lomond, around 20 miles north of Glasgow52. According to BBC Scotland, the aircraft was a Piper Tomahawk II, a two-seat trainer, registered to Leading Edge Flight Training in Renfrew, near Glasgow, on the Civil Aviation Authority's aircraft register3. The Independent, citing the BBC, gives the registration as G-LFSU5. Police and the AAIB have not published the type or registration themselves.

Online flight-tracking data reported by the BBC and The Guardian show the aircraft took off from Glasgow Airport at 4.17pm and stopped transmitting at 4.37pm, a few miles north-east of Drymen34. Earlier that day it had left Cumbernauld Airport at 11.46am and flown 15 minutes to Glasgow. At around 1pm it set off on a two-hour flight north-east to Newport-on-Tay, opposite Dundee, and back to Glasgow, where it spent just over an hour on the ground before its final flight3.

Gavin Birch-Williams, managing director of Glasgow Airport, said: "We are aware of an incident involving a light aircraft that departed from Glasgow Airport. This is a deeply tragic event, and the thoughts and heartfelt sympathies from all at Glasgow Airport are with the friends and families affected"3.

What the authorities say

Police Scotland said the AAIB had been informed and that enquiries were ongoing to establish the full circumstances. Temporary airspace restrictions, including for drones, are in place over the Drymen area and are likely to remain for some time to allow the AAIB to work. Police reminded the public that flying in restricted airspace is a criminal offence1. On Saturday officers were still at the scene near Old Drymen Road and had cordoned off a gate leading to the hill where the crash appears to have happened3.

The AAIB said on Saturday morning: "A team has travelled to the accident site near Drymen, north of Glasgow, to make enquiries and gather evidence as part of the process of conducting a safety investigation"2. Neither the police nor the AAIB has commented on a possible cause. Police have not released the names of the two victims and have not said who was flying the aircraft or what the purpose of the flight was1.

The Tomahawk as a trainer

The Piper PA-38-112 Tomahawk is a two-seat trainer with fixed tricycle gear, a T-tail and a Lycoming O-235 engine8. Piper built more than 2,400 of them at Lock Haven, Pennsylvania, between 1978 and 1982, and like the Cessna 150 and 152 the type was widely used for primary flight training6.

Its stall and spin behaviour has been debated for decades. A Piper engineer later told the US National Transportation Safety Board (NTSB) that soon after deliveries began, owners and operators complained that the aircraft rolled abruptly and unpredictably at the stall. In 1979 Piper modified the wing with two additional stall strips, small triangular blocks on the leading edge that control where the wing stalls first6. The FAA made the retrofit mandatory for earlier aircraft in 1983 through Airworthiness Directive 83-14-08, "to standardise and improve stall characteristics". The kit came with a supplement to the flight manual and revised airspeed indicator markings7.

In 1997 the NTSB found that between 1985 and 1994 the Tomahawk had a higher fatal stall/spin accident rate than the Cessna 150/152: an estimated 0.34 to 0.75 such accidents per 100,000 flight hours, against 0.10 to 0.13 for the Cessnas. The board also found no documentation that all the required stall tests had been flown during certification in 1977, and made five recommendations to the FAA, including new flight tests on at least two production aircraft6. The FAA then told operators that slow flight and stall training in the Tomahawk should be flown at or above the minimum altitude the flight manual gives for spin training. In 2012 Piper added a caution to the handbook: start slow flight and stall manoeuvres high enough to fully recover by at least 4,000 feet above ground level, as a margin against an inadvertent spin8.

None of this history says anything about the crash at Drymen. Whether handling, a technical problem, the weather or anything else played a part is not known, and that is exactly what the AAIB will investigate.

How the AAIB investigates

For a field investigation like this one, the AAIB sends a small team of at least two inspectors, drawn from operations, engineering, recorded data and human factors. The site phase usually takes two to three days, or up to a week for complex sites. Inspectors document the wreckage and ground marks, take fuel samples and talk to witnesses while their memories are fresh. For light aircraft, which rarely carry flight recorders, they also collect phones, tablets and GPS units that may hold useful data. The wreckage is then usually taken to the AAIB's base at Farnborough for detailed examination9.

That second phase can take several months for a light aircraft accident. If an urgent safety issue emerges, the AAIB publishes a Special Bulletin. A draft report goes out to the parties involved for a 28-day consultation, and most field investigation reports are published within 12 months of the accident9. Reports appear on the AAIB website and in its monthly bulletin, published on the second Thursday of each month. The investigation exists only to prevent future accidents, not to apportion blame or liability10.