I went to see the simulation.xml file for answers and I saw these lines:

Click to reveal..
<Failure weight="1.0" masterdisable="n">
<FailureRule SystemID="engine_one" average_hrs="90" damageAmount_pct="31" dump="y"/>
<FailureRule SystemID="engine_one" average_hrs="120" damageAmount_pct="70" dump="y"/>
<FailureRule SystemID="engine_one" average_hrs="160" damageAmount_pct="5" frequency_secs="111" dump="y"/>
<FailureRule SystemID="engine_one" average_hrs="200" damageAmount_pct="5" frequency_secs="25" dump="y"/>

<FailureRule SystemID="engine_two" average_hrs="90" damageAmount_pct="31" dump="y"/>
<FailureRule SystemID="engine_two" average_hrs="120" damageAmount_pct="70" dump="y"/>
<FailureRule SystemID="engine_two" average_hrs="160" damageAmount_pct="5" frequency_secs="114" dump="y"/>
<FailureRule SystemID="engine_two" average_hrs="200" damageAmount_pct="5" frequency_secs="24" dump="y"/>

<FailureRule SystemID="coolant_reservoir" average_hrs="100" damageAmount_pct="4" frequency_secs="32" dump="y"/>
<FailureRule SystemID="coolant_reservoir" average_hrs="95" damageAmount_pct="50" dump="y"/>
<FailureRule SystemID="oil_reservoir" average_hrs="150" damageAmount_pct="5" frequency_secs="50" dump="y"/>
<FailureRule SystemID="oil_reservoir" average_hrs="95" damageAmount_pct="50" dump="y"/>

Short of the devs releasing an update to the failures, I think it might be worth a try to untangle these numbers and experiment a bit. Unfortunately, I don't have much free time and the only way to find out if adjusting some numbers really works would be to play over time. Since you wouldn't know right away if it's working or not like my DM MOD.

So, I'll give it a shot at explaining what I think everything means, but I could be wrong.

Click to reveal..
Each Plane's DM has an engine_one plus/minus an engine_two. So, engine_one is definitely in all planes. All planes also have an oil reservoir, but only some have a coolant reservoir. However, I doubt these are what will cause engine fires, since it's not set up that way in the DM.

So, I've narrowed it down to adjusting the values for engine_one. Here are my observations:
-System ID is obvious
-average hours and damageamount_pct are repeated with different values. I wonder if increasing the average hours would make the engines last longer or maybe lowering the damageamount_pct to have the same effect.

Note that if this is successful, then you won't get as many engine failures, but the tradeoff is that you won't see as many fires either. However, the oil and coolant can still fail as usual.