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[aeroscout]

iChris !

 

Or anyone else who wants to add a reply. Specifically regarding the Allison 250-C18 turbine engine, I was under the impression that during the start sequence that the fuel nozzle directs start fuel initially into the combustion chamber, then at some rpm or fuel pressure or some combination, secondary fuel is introduced. This makes sense to me as I always note 2 separate TOT peaks, the second being lower than the first. I have had some starting problems. Out of nowhere this engine started hung starting around 30% N1. Derichment adjustment is called for, and in addition some troubleshooting points to the fuel nozzle. Regarding the fuel nozzle, I was also under the impression that the fuel nozzle had a slide mechanism in it that shifted from primary fuel to allow secondary fuel. Am I correct in any of my impressions, or do I need a new level of understanding ?

[iChris]

iChris !

 

Or anyone else who wants to add a reply. Specifically regarding the Allison 250-C18 turbine engine, I was under the impression that during the start sequence that the fuel nozzle directs start fuel initially into the combustion chamber, then at some rpm or fuel pressure or some combination, secondary fuel is introduced. This makes sense to me as I always note 2 separate TOT peaks, the second being lower than the first. I have had some starting problems. Out of nowhere this engine started hung starting around 30% N1. Derichment adjustment is called for, and in addition some troubleshooting points to the fuel nozzle. Regarding the fuel nozzle, I was also under the impression that the fuel nozzle had a slide mechanism in it that shifted from primary fuel to allow secondary fuel. Am I correct in any of my impressions, or do I need a new level of understanding ?

 

That's a correct impression...

 

The engine should light off and begin to accelerate. Acceleration will be proportional to the rise in compressor discharge pressure (Pc) until speed enrichment occurs at approximately 45 percent N1 (Second TOT Peak). When N1 reaches approximately 63 percent RPM, the fuel control will start to govern and maintain idle RPM.

 

The fuel control has a start dercehment assembly, which incorporates a bellows for sensing Pc pressure. During the initial stages of starting, when Pc is low the metering valve is toward the minimum flow stop. As Pc pressure increases the derichment bellows will allow the metering valve to move away from the minimum flow stop. With the start derichment device, low turbine outlet temperatures can be maintained immediately after light off while the basic acceleration schedule can be richened so there will be no tendency toward stagnation before idle RPM is attained.

 

The dual orifice fuel nozzle has a metering valve with three positions, which result in no fuel, primary fuel flow, and primary and secondary fuel flow. The position of the metering valve is determined by fuel pressure acting to open the valve & spring acting to close the valve. The valve opens to primary fuel flow at 30 psi. The valve moves from primary fuel flow only to primary and secondary fuel flow at 150 psi.

Edited February 18, 2013 by iChris

[aeroscout]

Thanks iChris ! Very helpful and informative, as usual.

Just for further clarification, is it the case that once the throttle is moved to idle and fuel is introduced that further manipulation of the throttle will have no effect until the engine reaches idle speed ?

[iChris]

Is it the case that once the throttle is moved to idle and fuel is introduced that further manipulation of the throttle will have no effect until the engine reaches idle speed ?

 

It does have an effect. If you were to move the throttle pass the idle stop, fuel flow would increase. Moreover, if you went far enough pass the idle stop you’ll set yourself up for a hot start. You most remember fuel flow during the start is a function of four parameters, any single one or combination we affect fuel flow.

 

1. Gas Producer speed (N1)

 

2. Fuel control lever position (controlled by twist grip)

 

3. Fuel pump discharge pressure (P1)

 

4. Compressor discharge pressure (Pc)

 

During normal operation, after you move pass the idle stop toward full open, the Power Turbine Governor comes into play (teaming up with the Fuel Control unit) with two additional parameter to control rotor RPM.

 

1. Regulated air pressure (Pr)

 

2. Governor Reset pressure (Pg)

 

Note with caution (Bendix FC): In the field sometime you may find yourself with a low battery that will not allow for a normal start within TOT limits. However, by manipulating the throttle passed the idle stop and back, modulating the throttle, you’ll be able to safely start the engine. This technique can also be used to get pass some hung-start issues in the field.

 

Modulating the throttle on start is normally only done with the CECO FC.

 

This is information only and not recommended for most pilots. The results if not done correctly are possible hot-start or engine failure. In these cases there’s an underlying problem that’s being ignored to get the aircraft out of the field. Prior experience required.

Edited January 27, 2017 by iChris