AIR TRACTOR AT-502A, Maintenance Section

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Maintenance Page 08 Air Tractor, Inc.
March 14, 2018 AT-502A/502B/504
Troubleshooting
Troubleshooting on the PT6A engine is not that difficult, once you get acquainted with the engine. The
most important thing you can do as an operator is keep an accurate record of the engine parameters. All
troubleshooting starts with the information you will have on that little tablet. Maintenance personnel will need
to know in what way the engine parameters have changed. As you become more experienced with the PT6A
engine, all these parameters will come to have more meaning. Until all these things begin to come together,
consult qualified personnel.
Starting Procedures
Starting the PT6A turbine engine is not complicated, but it is the best opportunity you will have to
damage your engine if the proper procedures are not followed. A battery cart will be a good investment. You
can take two truck batteries wired in series for 24 volts, put them in a cart for ease of movement, and
connect them to cables with a plug compatible to your Air Tractor. It is also convenient to wire a trickle
charger to a female socket like your airplane has and you can keep the batteries plugged in when not in use
so they will always be ready when needed.
Before you begin the starting procedure, make sure all the levers and switches are in their proper
positions. The throttle against the idle stop, the start lever in the cut off position, the propeller in feather, and
the ignition switch in the middle position. Hot batteries or the battery cart will spin your engine between 18%
and 20% Ng. NEVER TRY TO START AN ENGINE TURNING LESS THAN 15%. Once you have obtained
an Ng speed above 15%, turn on your ignitors, wait for the Ng speed to peak, then bring the start lever to the
RUN position. Start counting 1001, 1002, 1003, etc, your engine should ignite before you reach 1005. If it
does not, let off the start switch, pull the start lever back to cut off, and investigate. In any case, allow the fuel
in your engine time to drain. You do not want to start an engine with a lot of excess fuel inside. Most engines
will “light” around 1003. There will be a pause in most engines around 35% Ng, then a continuation on to
*50% to *52% Ng. The ITT will usually peak around 45% and be coming down as the engine reaches its idle
speed. Engines with the 10-4 fuel nozzle configuration will start well under the red line, (usually more than
100
o
C) while the 7-7 configuration will start somewhat hotter. In no case should an engine normally exceed
the red line on a start, although the manual allows temperatures above the red line for 2 seconds.
Once the engine is above 50% Ng, release the starter switch. You may leave the ignitor switch in the
up position since the ignitors are OFF as soon as the start switch is released. Turn on the generator. Right
after a start, especially without a battery cart, your generator will be carrying a heavy load while recharging
the batteries, and can drag an engine down. If your Ng speed begins to fall below *50% when you turn on the
generator, you should advance the power lever as required to maintain a Ng speed above *50%.
Starting an engine that is turning slow with weak batteries is asking for a disaster. During the start
process, if anything seems questionable, pull the start lever back to the cut-off position with the starter still
engaged and allow the starter to run till the ITT has fallen to safe limits. As I stated at the beginning of this
section, starting the engine is not hard, but you have to do it right or you will find yourself in a trap.
* These are values for the PT6A-15AG/-34AG. For the PT6A-45R/-60AG/-65AG/-140AG expect values of
56-58%.