HMU Solenoid valve test on EMB120

[WmuBronco]

Hey, was wondering if any of you EMB120 (Andy N) pilots could answer this question...

What is the reasoning for closing the the bleed on the perspective engine and retesting the HMU solenoid de-enrichment valve after the Nh fails to remain above 50% on the initial test?? I know you are robbing air from the engine, hence a lower drop in Nh, but isn't this just possibly covering up a stuck solenoid??

THanks

 

[sabreliner]

Above 50%? That's interesting. At ASA when we did the HMU solenoid valve test, we turned off the EEC and noted the % Nh drop. It couldn't exceed10%. If it did then the valve was most likely stuck (never saw it stick.) On our A/C it didn't matter what the actual Nh ended up at as long as the drop was less than 10%. (Which, if everything was working right the Nh wouldn't get near 50% unless something was wrong anyway.)

If you are flying for SkyWest, maybe it has to do with the higher altitudes they operate the airplane at? Then, having the bleed on could get you below 50% maybe just because you are working the engine harder with no electronic governing of the engine. That's my best guess.

 

[Chairman]

IF the HMU fails the test an drops below 50% the reason that you are tuning of the bleed air is that the HMU uses a certin amout of bleed air P3. By turning of the bleed you are no longer loosing that bleed air to the PACs and allowing more bleed air to the HMU. If it still fals below 50% then the fuel scheduling is to low to sustain rated power. What is left out of the procedure is that if it fails but works with the bleeds closed you must do a bleeds closed takeoff.

Hope that helps.

 

[WmuBronco]

Thank you, got it