GA1393

Go back

17th November 2025

Single Column View

Magneto magic

Initial Report

I fly a lot of different types, including a DHC-1 Chipmunk. While it is a real privilege to be able to fly this aeroplane and even aerobat it, its ergonomics set-up is, of course, “of its time” and has many weak points, vastly improved in later machines. One of these is the placement of the magneto switches. In the front cockpit of the aeroplane I fly, they are in the lower left-hand corner of the panel.

Because I jump between types quite a lot, I have developed a very rigorous approach to checklists. And before each flight, I give myself a very detailed briefing of what is where in each particular airframe.

On this particular day, my plan for the Chipmunk was to do a few circuits and some general handling, building up to gentle aerobatics. The aeroplane had been unserviceable for a while, so I wanted to get back into the groove. As usual for me before a first flight on type after a lay-off, I did all the checks three times, first by “flow”, then by following the checklist, and then, finally, by following a somewhat silly mnemonic I use before take-off. The first circuit was uneventful, I landed and taxied off (I never do a touch-and-go in tailwheel aircraft), ran the checks and took off again.

On downwind, I did my usual checks; everything was fine. This was to be a glide approach, so I took the power off and turned a very close base. The aeroplane immediately started vibrating. I focused on flying the aeroplane and landed uneventfully. As I turned off the runway, I looked around the cockpit and noticed that one of the magneto switches was in the down and off position.

I could only surmise that I switched it off by accident, without noticing. I have only one theory as to how this may have happened. In the Chipmunk, I use an RAF-surplus flying suit. Its sleeves are a bit wider than they should be, and it might just be possible that I caught the switch with the opening of my left sleeve. It’s either this or the even scarier thought I had deliberately put the switch into the OFF position for some reason, but my brain had failed to register it. If the former theory is true, the situation would have been very dangerous – what if I’d caught two switches, not one? If the latter is correct, then I should, perhaps, see a doctor. I am only joking, of course, but you can’t rule anything out!

Lessons learned: I now use a rubber band around my suit sleeves, to keep them even tighter around my arms.

CHIRP Comment

As any GA pilot will attest, discovering a magneto switch selected to OFF in flight would be an unnerving experience, particularly when the cause is uncertain. Whether the switch was accidentally caught by a flying-suit sleeve or mis-selected through a momentary cognitive lapse, this report is an excellent example of a “third age” report: one that nobody else could have known about unless altruistically shared. The reporter deserves full credit for their honesty and humour in describing a situation that offers valuable human factors lessons.

Two possible causes were identified. The first – that the sleeve of the flying suit caught and moved the switch – is entirely plausible. Loose clothing can easily interfere with cockpit switches, with potentially serious consequences. It’s a timely reminder to select appropriate clothing for the task. In addition to avoiding garments that might snag or obscure controls, pilots should consider clothing suitable for other risks too. For instance, fire-resistant materials for balloon operations or warm layers for cold-weather flying (especially in the event of a possible forced landing). Long hair or jewellery can also catch on switches, or move controls, or become trapped in a canopy causing injury; passengers should likewise be briefed on suitable clothing for comfort and safety. Threat & Error Management (TEM) isn’t just about the flight plan; it extends to preparation, equipment and environment.

The second possibility is that the magneto switch was mis-selected, either during the pre-flight checks or in flight, and simply not noticed. Despite diligent checking, it’s easy for expectation bias to creep in, leading us to ‘see’ what we expect rather than what is actually set. Checklists are a vital defence against error, but no system is foolproof. For multi-crew operations, challenge-and-response checking provides an extra safety layer; for single pilots, ‘read and point’ is a recognised technique to reduce error probability. Ultimately, action errors – where the correct intention is formed but the wrong action is taken – can never be completely eradicated, but their exposure can be reduced through disciplined routines and continuous awareness. Of note, in 1970 the Chipmunk rear cockpit magneto switches were modified to include a guard, which prevented inadvertent deselection by curious passengers; the front cockpit though wasn’t similarly modified!

In this case, the pilot approached the flight responsibly: aware of not being current on type, mindful of differences between aircraft, and methodical in preparation. Yet, as often happens, human variability still found a way through the defences. Faced with unexpected vibration, the pilot did exactly the right thing i.e. stayed calm, flew the aircraft and landed safely. Perhaps there was more cognitive load at play than realised, with a combination of recency, aircraft differences and tailwheel handling considerations subtly raising workload. Whatever the root cause, it’s likely that magneto switches will now feature more prominently in this reporter’s scan, and that the sharing of this experience will help others stay alert to similar traps.

Key Issues relating to this report

Human Factors Considerations

The following ‘Dirty Dozen’ and Human Factors were a key part of the CHIRP discussions about this report.

Positive HF elements:

Safety/learning culture – proactive self-reporting of an embarrassing but educational incident for the benefit of others. Post-flight reflection and practical mitigation (sleeve restraint).
Preparation and planning – thorough pre-flight briefing and familiarisation, especially after a break on type.
Procedural discipline – consistent use of flow, checklist and mnemonic techniques as error traps.
Composure under pressure – calm handling of the aircraft following unexpected vibration, prioritising flying the aircraft.
Situational awareness recovery – recognition of the potential mechanical and human contributors, reinforcing future vigilance.

Negative HF elements:

Perceptual slip / action error – possible inadvertent or unnoticed switch selection contrary to intention.
Expectation bias – assuming switches were correctly set based on prior checks rather than visual confirmation.
Complacency / overconfidence – routine familiarity possibly reducing perceived threat during checks.
Adverse mental state – cognitive strain from re-familiarisation with type after a lay-off.
Design limitations – legacy cockpit layout not optimised for modern human factors standards, additional awareness and care required.
Single-pilot operations – absence of cross-checking or independent verification allowing slip to persist.