MNT Tech:
Concerning development of simulation for hot starts – I have done this before on a FSI sim, and here are some thoughts that might assist you:
First, you should get together with your instructor team and determine exactly what it is that the instructors want to accomplish with this particular malfunction. Do they want the malfunction to have characteristics such that the pilot should abandon the start attempt (cut off the fuel), then continue to crank the engine? Or, do they want to present a starting scenario in which the start temperatures are higher (for cause) than what is normally observed, but don’t exceed the published limits? In the latter case, it would be appropriate for the student to continue the start, most especially if the lesson plan setup puts the aircraft at a remote location where no ground power or maintenance services are available.
I’m not familiar with your aircraft or engine model, but I am very familiar with the PT6A-27, so, I’ll use limitations for the -27 in this discussion – you can easily apply the concepts to your aircraft by modifying the limitations to match your aircraft’s AFM and engine MX manual.
There is an enormous amount of confusion and misunderstanding amongst pilots of PT6 powered aircraft about what exactly constitutes a “Hot Start”. So, let’s first define a Hot Start as being a start sequence where one of the published limitations
that are applicable during the starting phase of engine operations are exceeded. This definition does not include starts wherein the starting temperatures are much higher than normal (usually for cause), but do not exceed the published limitations that are applicable to starting conditions.
For example, in the case of the -27 engine, the starting limitations permit temperatures of up to 1090° provided that:
1) the temperature drops below 980° within 2 seconds, and;
2) the temperature continues to drop to below 925° within the following 8 seconds.
The illustration below – taken from the engine maintenance manual - depicts these limits.
As you can see, temperatures of up to 925° for the entire duration of the start are acceptable. Acceptable does not mean desirable, nor does it mean common, ‘acceptable’ simply means that the
starting temperature limitations have not been exceeded.
Only a minority of pilots are aware of these starting limitations, because unless the aircraft is fitted with an electronic display system that automatically adapts the coloured green, amber, and red instrument markings to suit the phase of operation of the engine (starting, idle, take-off, etc.), the only limitations that the pilot sees are those applicable to take-off, cruise, and (perhaps) MCP. The picture below shows the T5 gauge on an aircraft fitted with a -27 engine. There is no indication of the starting temperature limits, other than a single red radial mark at the 1090° position. The colour coding on the face of the gauge corresponds to the temperature limitations that are applicable in conditions
other than starting. The top of the green arc is the limitation for climb and cruise, and the top of the amber arc is the limitation for take-off.
Normally, if the battery is in reasonable condition or if an external power cart is used, the highest temperature encountered during any portion of the start will not result in a temperature higher than the green arc depicted on the gauge. This is coincidental. As a result, the pilots have a habit of thinking that “green is good” so far as start temperatures are concerned, and if the peak start temperature results in a gauge reading in the amber or red region, that must indicate a caution or warning condition applicable to the start. That is not correct.
For sake of this discussion, let’s describe any start with a peak temperature greater than the green arc on the T5 gauge as being a “warm” start, with “warm” meaning higher than normal, but not exceeding starting limitations.
As you are probably aware, there is a direct and (almost) linear correlation between peak starting temperature and the stabilized NG achieved prior to introduction of fuel. So, before you even begin to model the malfunction, make sure that the engine library is written such that it models this behaviour. A stabilized NG of 22% prior to fuel introduction (correlating with a good quality external power cart) should give a peak start temperature of (for example) 625°, a stabilized NG of 18% (correlating with a fully charged battery) should give a peak start temperature of perhaps 675°, and a stabilized NG of 12% (correlating with a seriously discharged battery) should give a peak start temperature of about 900° to 1,100°. Don’t forget to ensure that your battery library reflects the current drain that the first start takes out of the battery – the second engine should achieve a stabilized NG of about 1% less than the first engine if both engines are started sequentially from the battery alone. Thus, using the battery only for starting, the peak starting temperature of the second engine should be slightly higher than the first.
There can be many causes for a warm start. The most common is lower than average NG prior to introduction of fuel. Let’s presume that you write the code such that achieving a stabilized NG of 14% prior to fuel introduction (correlating to a battery in poor condition, or a partially discharged battery) gives a peak starting temperature of 800°. That’s easily within starting limitations, but certainly much higher than pilots will see on a daily basis. This is why (and where) you need to talk to your instructors and find out what behaviour they want to see from the pilot. If your instructors foresee a scenario where the student is briefed that the aircraft is at an out station with no maintenance capability, it’s late at night, and an ice storm is forecast to come through in 2 hours… and, oh, by the way, the battery is quite discharged as a result of (whatever – talking on the radio, passenger getting sick before engine start and needing to be offloaded, etc.), then the logical pilot action would be to anticipate that the stabilized NG prior to fuel introduction will be much lower than normal, and consequentially, the start will be much warmer than normal, peaking at 800° or so. That’s not a problem, and in the case of the -27 engine, doesn’t even need to be reported (the AFM and MX manual suggest that starting temperatures greater than 850° be investigated for cause, although in this scenario, the cause is well understood before the start is even commenced). This type of scenario presents a ‘warm start’.
On the other hand, let’s say your instructors want to present a scenario in which the starting limitations are well and truly exceeded. This might be caused by a fault in the ignition system that allows 5 to 8 seconds worth of fuel to pool in the combustion chamber prior to ignition. In such a case, there will be a huge spike in T5 following light-up, and it would be realistic to expect that the T5 temperature indication might peak above the 1090° limit, or, it might rise up to 980 and then hang there for longer than the 2 seconds allowed. This type of scenario would present a true “Hot Start”, that being defined as starting temperature limitations being exceeded. The expected student response would be to shut off the fuel and continue to crank the engine (assuming that is what the AFM for your aircraft states should be done).
So, to summarize, before you can write the code to deliver a realistic “Hot Start”, you first have to make sure that your normal code varies peak starting temperature in relation to what stabilized NG is achieved prior to fuel introduction, and you then have to get your instructors to tell you exactly what the learning objective of their “Hot Start” scenario is. Once you have the engine giving good simulation of starting temperatures relative to air density and stabilized NG prior to fuel introduction, you can create a set of conditions that will give a true “Hot Start”. But… don’t forget that simulation is theatre, and good theatre requires realistic presentation. It would not be realistic to have the engine spool up to normal NG, and light up in the normal time frame after fuel is introduced, then hammer the student with a starting temperature higher than limitations. You need to build in a credible reason for the hot start. That could be delayed ignition. It could also be a result of fuel pooled in the bottom of the engine following a prior aborted start attempt… but doing it that way requires the instructors to set the stage in their briefing (by explaining that the prior start attempt did not result in light-up and was abandoned). Personally, I think it’s easier (and more realistic) to use delayed ignition.
You could also set a low NG threshold that causes the peak starting temperature to exceed limitations. For the -27 engine, pilots are advised to not introduce fuel unless they achieve 12% stabilized NG. So, you could write your code such that the engine only stabilizes at 11% NG, and if the student then introduces fuel, the starting temperatures then exceed the published limitations.
Have fun with your project…