Ignition Test Switch – Good, Bad, and Ugly

The Good:

Redundant (Left/Right) ignitions have been a bedrock safety feature since the early days of aviation.  Most experienced pilots can recount at least one flight that was completed on a single magneto.  It’s the best example of aviation’s redundant safety principal at work.

Redundant ignitions are controlled by a cockpit Left/Right/Both switch that allows pilots to test and/or isolate ignitions.   Pre-flight run-up tests are standard procedure.  In-flight testing is done as needed and appropriate.  Pilot training provides a level of familiarity and proficiency with both tasks.

The Bad:

The premise for dual ignition safety is largely based on the ability to test and then isolate, turn off, a faulty ignition.  If the test protocol is inhibited, for whatever reason, the premise and protection that redundant systems provide is undermined.

The only test tool pilots have is the cockpit ON/OFF switch.  Turning ignitions OFF in-flight, under any circumstances, is an occasion for pilot alert.  Training and practice go a long way, but most pilots will admit to some degree of unease with even routine in-flight ignition tests.  This can range from:

  1. I test on a regular basis, but I wait until I’m over an airport and have lots of altitude.
  2. I can do in-flight tests, but it’s not something I enjoy. I don’t do it often.
  3. I don’t do in-flight tests unless there is a compelling need. Ground testing is sufficient.

Regardless of which one you do or don’t see as problematic, they all point to a larger question.  Can the pilot perform this test in a real-world emergency?  Routine test anxiety is trivial, compared to pilot stress (workload) in urgent conditions.  Stress will escalate with new and less experienced pilots.

The Ugly:

When the ignition test routine falls short, it does so in significant and predictable ways.  Every single flight has at least one interval of heightened exposure.  It’s best described with an example.

At takeoff and landing, altitude and airspeed are in short supply.  If engine power lags (cause unknown), the pilot will commence emergency procedures that, at some point, will call for ignition testing (Left/Right).   With test switch in hand, the pilot now considers the 50/50 odds he may be about to turn OFF his only/last working ignition – if only for a moment.  Do I have enough airspeed?  Do I have enough altitude?  Should I check something else first?   Never mind the answers; there may not be time for questions.  Even experienced pilots struggle with this 50/50 proposition.  All pilots know:

  • Hesitation – can be catastrophic.
  • Failing to test/isolate a faulty ignition – can be catastrophic.
  • A momentary loss of ignition risks, already minimal, airspeed – and can be catastrophic.

In these situations, all pilots face the same odds.  Training and practice do not change the basic equation that is (quite literally) hard-wired into this ignition test.

Another complication – Ignition may not be the root cause of the crisis.  But the pilot doesn’t know that and is no better off from an ignition test (stress and hesitation) standpoint.   If the ignition test is delayed, so too are all items lower on the emergency checklist – where the root cause may reside.  Bottom line – it’s a complicated and stressful situation.

This problem is universal.  Every piston engine ever built with traditional left and right ignitions has the same exposure.   At its core, this testing dilemma is a tools problem – not a training problem.

 E-MAG Series 200T Tandem with SafT Switch (Patent Pending)

E-MAG’s Tandem uses a single cockpit “SafT” switch, that turns both left and right ignitions from ON to “ON”.   It does not turn either ignition OFF.   Each E-MAG Tandem has two separate logic control circuits – the core of any electronic system.  One circuit provides full Variable control firing, and the other has Fixed firing.  The SafT switch simply selects Variable or Fixed firing circuits in both left and right ignitions.  The SafT switch does not replace traditional Left/Right/Both (ON/OFF) controls – it compliments them.  The implications for flight safety are profound.  E-MAG Tandem gives pilots a much needed simple/quick/effective first response for most (ignition related) threats to engine operation**.

Above all else, the SafT switch is designed to be simple.  It does not require special study, skill, or real-time analysis from the pilot.  Since it’s an ON/ON control (vs. ON/OFF) there is no “wrong” setting.  Knowing this, pilot stress (workload) and hesitation related to SafT switch testing is drastically reduced.  SafT testing can be done anytime (run-up, taxi, climb, or cruise).

Ignition controls:

  1. Existing cockpit Left/Right/Both switch, wiring, and operation is unchanged.  To review – when p-lead is grounded, via the switch, plug firing is disabled. When p-lead is open (float), ignitions are “hot”.
  2. SafT switch FIXED firing is the default selection any time p-lead wiring is open (float). FIXED position sensors and FIXED position firing circuits are enabled.
  3. SafT switch VARIABLE firing is enabled by applying a 100 ohm resistance across the existing p-lead. Variable control uses separate engine position sensors and separate VARIABLE logic circuits.  The SafT switch (1 SPDT) wiring consists of a short extension from the existing kill switch (p-lead) terminals to any convenient cockpit location.

This simple switch configuration preserves fail-safe functionality.  If the kill switch or p-lead wire fails (open), the ignitions defaults to ON.  If the SafT wire, resistor, or SafT switch itself fails (open), the ignition defaults to FIXED firing mode.

Both FIXED and VARIABLE firing circuits have benefit of redundant power supplies:

  1. Aircraft electrical bus.
  2. E-MAG’s internal alternator.
  3. Dual installations are triple redundant (alternators in Left, Right, and bus power to both).


**Maintaining engine operation and lessening pilot workload are the primary objectives:

  • Interruptions in power regulation and high voltage elements (plugs, plug wires, coils, and drivers) are fail-safe. Issues in these areas might cause a unit to fire weak, or fail to fire at all, but they don’t cause the ignition to fire chaotic.  In a redundant left/right installation, a single point failure is automatically covered by the other ignition.  No operator action is required. Engine operation is not jeopardized.
  • Interruptions due to incorrect or chaotic plug firing can be said to fail-unsafe. Proper firing from the other ignition does not provide a remedy.  Engine operation is jeopardized.  The faulty ignition MUST be isolated to ensure engine operation. Fail-unsafe sources can be grouped as follows:
    • Sources that are resolved by the SafT switch (Variable to Fixed).
      • Errors in position sense (receiver).
      • Errors in CPU processor.
      • Errors in program code (latent factory defects, or corruption in the field).
      • Errors in subordinate sensors (MAP sensor, MAP plumbing).
      • Errors in configuration memory (incorrect or lost data).
    • Error sources not resolved by the SafT switch. Must be isolate by kill switch.
      • Mechanical elements. Series 200T has minimal mechanical exposure.
        • Main Shaft (supported by two sealed bearings)
        • Signal Gear
        • Engine drive train and accessory drive (elements outside the ignition itself).
      • The above is a simplified risk summary based on a single-point failure. We are mindful of the countless ways the real-world can complicate and humble the best laid plans.  Series 200T and the SafT control is not a guaranteed fix all solution.   Rather, we might think of it as an enhanced/improved fix most-all tool.
      • Engine performance will vary as you move between Fixed and Variable modes. Variable control firing can enhance engine efficiency (fuel consumption and power).  Exactly how much will depend on a number of variables that are best revealed by in-flight sampling after installation.  Continuation of basic engine operation is the primary  objective.  Changes in efficiency, is something the pilot can evaluate and address in the relative calm of stable ongoing operation.