Occasional reports from operators and Service Centers citing premature erosion or igniter plug failure indicate a possible misunderstanding of the term Continuous Ignition. To be clear, Continuous Ignition means the ignition system has no defined maximum duty cycle; therefore, it may be operated continuously when necessary to assure safe aircraft operation.
However, igniter plug life is limited by erosion; and the rate of erosion is directly proportional to the number of cycles or the number of times the plug “sparks.”
Therefore, in order to avoid excessive plug erosion, Continuous Ignition should only be used as dictated by the appropriate Flight or Crew Manual. For example, some Aircraft Flight Manuals recommend the use of ignition for all takeoffs and landings, regardless of flight conditions. In addition to this repetitious and predictable use, ignition should also be used:
- During operation in icing conditions.
- Following operations in icing conditions, until accumulated ice has dissipated.
- In heavy rain or turbulence, particularly at reduced power settings.
- At high angles of attack, especially at high altitudes.
The following should provide some insight into ignition system use under normal and unusual circumstances. To put ignition utilization into the proper perspective, a typical operator’s annual use of the engine ignition system can be broken down as outlined below:
Use During - Average Duration in Seconds / Flight
Engine Start - 20
Takeoff - 180
Landing - 300
Total - 500
- Average flight duration = 2 hours
- Average annual flight time = 500 hours
- Ignition utilized during all takeoffs and landings
Therefore: 250 flights x 500 seconds / flight = 34.7 hours per year
Furthermore, the ignition unit (exciter) normally produces a spark rate of 2 sparks per second. However, during the first 15 – 20 seconds of system operation, the ignition unit provides a burst rate of 4 sparks per second to enhance starting and relight. While not startling information by itself, these estimations provide a baseline for analysis of overall annual ignition system utilization and igniter life expectancy.
During certification testing, the TFE731 engine met or exceeded all FAA requirements for water and ice, as well as bird ingestion without the use of Continuous Ignition. However, as added protection against inadvertent power loss during critical phases of flight under extreme conditions – such as takeoffs, departures, approaches and landings where excessive amounts of water, snow, ice, or birds might be encountered – a Continuous Ignition system has been provided and should be turned ON.
As soon as the aircraft is clear of such conditions, the ignition system should be turned OFF after checking that residual ice is no longer present and won’t shed into the engine inlets.
Note: Guidance from the AFM and other OEM documentation takes precedence over this recommendation.
These and other TFE731 Pilot Tips are broad in scope and general in nature. Always consult the Aircraft Flight Manual for specific limitations, procedures and recommendations.