Avoiding a Hot Start…

Let there be no doubt about it…the greatest danger that faces your PT6 engine (and consequently, your wallet size) is the potential of a hot start.  So, knowing exactly how to start your PT6 is critical.  Here’s the noteworthy points of an engine start that I recommend you consider when starting your PT6-powered PA-46:

1.) Check battery voltage: If the battery voltage is below 23.5v, stop the sequence and either charge your batteries or get a ground start. On the Jetprop, make sure to check both batteries, ensuring both the individual and collective voltage is over 23.5v.  The Meridian has only one airframe battery.
Possible problems if voltage is below 23.5v:
– Battery Minder inoperative?
– A possible drain on the battery with the battery switch OFF?
– If one battery is low in the Jetprop, someone (possibly maintenance?) used one battery too long for ground checks?
– Loose connections?

2.) Ice deflector – OFF: (Jetprop only) This is generally a minor consideration, but you want to eliminate all variables during the start sequence.

3.) Fuel pump(s) and Ignition – ON: One of the WORST things you can do to your engine is leave the Ignition switch OFF during start, and then engage the Ignition Switch after the Condition Lever is advanced.  The volume of fuel that is dumped into the engine (but not ignited due to the igniters being OFF) will set off a DEFINITE hot start when the ignition is added late.  Think of the PT6 like it is a propane BBQ grill.  If the propane is turned ON your grill with the hood closed and propane fills the grill, the chef will get a “boom!” if he hits the igniter and will probably lose his eyebrows.  Same with the PT6…if the condition lever is advanced, fuel is entering the combustion chamber.  If the ignition is OFF, this fuel is not ignited.  If this excessive fuel meets ignition late in the sequence, the only result will be the same “boom!”, except that a PT6 will cost a LOT more than a BBQ grill.

4.) MOR lever – OFF: A Hot Start will DEFINITELY occur if the MOR lever is in any other position than OFF.

5.) Press the starter switch and notice the voltage drop:  If the battery voltage drops below 17v immediately after pressing the start switch, then the condition of your battery should be suspect.  Usually a drop below 17v indicates the battery(s) will soon need replacement.  If the battery voltage drop goes below 16.5v, I would abort the start.
Possible problems if voltage drops below 18v:
– Battery(s) getting old, possibly needing replacement
– Connection problems?

6.) Allow the Ng to stabilize: Before advancing the Condition Lever, allow the Ng to stabilize and note the speed.  The speed should be above 15% and should not fluctuate much from start to start if the battery voltage is normal.  Most engines will settle at about 16.5 Ng (or so), but the more important factor is noticing a change in speed from start to start, especially notice if the speed decreases. If the engine is hot from being operated recently, allow the Ng to spool and watch the ITT decrease.  When the ITT decreases below 120, then it is safe to advance the Condition Lever.  I’ll often allow the Ng to stabilize for 10+ seconds before advancing the Condition Lever.
Possible problems if Ng is slower than normal:
– Rubbing inside the engine: Extremely expensive…have this checked soon if no other abnormalities are noted
– Starter approaching end-of-life:  The PT6 starter should be overhauled every 1000 hours.
– Starter brushes need replacement: This is a cheap and easy fix, and starter brushes can wear quickly.
– Connection problems?

7.) Watch for Max ITT: Once the Condition Lever is advanced (and you should keep your right hand on the Condition Lever in case you need to shut the engine down), the ITT should dramatically rise and peak before declining sharply to a steady state.  Note the Max ITT.  If the ITT goes above 875F, instantly retard the Condition lever to OFF.  Usually the Max ITT value (if all else is equal) will vary with OAT.  The higher the OAT, the higher the Max ITT.  The lower the OAT, the lower the Max ITT (unless bitter cold weather is present, and then the Max ITT can go up slightly as temps decrease).

On any start sequence, you are looking for any deviation from what is normally seen.  If you see a change, abort the start sequence and troubleshoot.  Make no mistake…the costs of a hot start can be over $100k.  It simply is not worth the risk to “see what happens” or “hope for the best” if a “deviation from normal” is observed.  Solidify in your mind now that you will not let the urgency of the mission coax you into continuing a start that is “not normal”.  It’s simply not worth it.

About Joe Casey

ATP, CFI, CFII, MEI, CFI-G Commercial Pilot - SE, ME, Rotorcraft, Glider US Army AH-64 Pilot and UH-60 Instructor Pilot and Instrument Flight Examiner
This entry was posted in Engine and tagged , , , . Bookmark the permalink.