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We have a problem on a 500c.

Fuel filter warning light illuminates & gen drops off line,(brushes not sticking & only 3/32" worn from new) will re cycle no problem, might happen once a month.

Filter spotless, diaphragm switch for filter tests OK + exchanged, the only common point seems to be dimmer box.

All plugs checked, wiring shook moved, seems imposable to induce fault.

Any Suggestions appreciated

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Is it possible the anti-ice airframe fuel filter is intermittently becoming clogged with ice?

 

The following applies to a Hughes 369...

 

LIST OF MODELS AFFECTED

369HS Helicopter Serial No. 0101S and Subs

369HE Helicopter Serial No. 0101E and Subs

NOTE: This Kit is approved for installation in helicopters equipped with Allison

250-C20 engine only.

 

Altitude limitations:

If ground temperature prior to flight is 5°C or less, flight operations are

limited to 12,000 feet pressure altitude maximum.

 

If ground temperature prior to flight is above 5°C, flight operations are limited

to 14,000 feet pressure altitude maximum, but not to exceed limitations

of Basic RFM Section II.

 

Anti-Ice fuel Additives:

Installation of the Anti-Ice Airframe Fuel Filter eliminates the need for

anti-ice fuel additives in the fuel (i.e., Prist).

Placards:

 

ANTI-ICE FUEL FILTER INSTALLED

For ground temperature of 5°C or less, max ALT 12,000 ft.

For ground temperature greater than 5°C, max ALT 14,000 ft.

 

The Anti-Ice Airframe Fuel Filter is designed to filter out ice particles and

other solid contaminants from the fuel prior to entering the engine fuel system.

Installation of the filter will delete the requirement for use of fuel containing

anti-ice additives.

 

The filter unit is mounted on the aft face of the firewall ahead of the engine

fuel pump inlet and contains a 10 micron, 500 square inch disposable filter element.

A pressure sensing switch in the filter body will illuminate a cockpit caution

light and activate the start pump when the fuel differential pressure across the

filter drops to a preset level. If the filter becomes clogged, a by-pass valve in

the filter body will open allowing unfiltered fuel to flow to the engine fuel

pump.

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We have a problem on a 500c.

Fuel filter warning light illuminates & gen drops off line,(brushes not sticking & only 3/32" worn from new) will re cycle no problem, might happen once a month.

Filter spotless, diaphragm switch for filter tests OK + exchanged, the only common point seems to be dimmer box.

All plugs checked, wiring shook moved, seems imposable to induce fault.

Any Suggestions appreciated

Latent failure of components of the Dimming Unit can cause false caution and/or warning light indications. As an example, a shorted or open diode will cause the false illumination of multiple caution/warning lights or the inability to press-to-test one or more caution/warning panel lights.

 

Case in point is one 500C with a type A instrument panel (six light panel). The aircraft was reported to have an intermittent problem were the generator dropped-off line and the Eng Chip, Fuel Low, Fuel Filter, and Gen Out lights all illuminated. It was also noted that the problem seemed to relate to the end of the fuel cycle were the fuel was below about a quarter tank.

 

The problem was found to be a shorted diode in the dimmer unit (CR11, see diagram). With that diode shorted a normal Fuel Low ground input at pin AA would also place a ground input at the cathode of the other diodes in the array and simultaneously illuminate the Gen Out, Eng Chip, and Fuel Filter lights.

 

It was just the normal first indication of a Low Fuel light coupled with a failed diode in the Low Fuel light circuit. The generator never dropped-off line, verified by the amp meter. Neither was there any Engine Chip or Fuel Filter problem. The Low Fuel light was the only true indication

 

It seems you're picking up an unwanted ground/short before or after the dimmer unit. Most likely you'll find a bad dimmer unit or wiring issue. Note the Gen Out and Fuel Filter input lines to the dimmer are adjacent pins on both the Grimes and Autronics dimmer. Also note any recent upgrades, installs, or repair work to the instrument panel.

 

 

fromMD500CMaintManual.jpg

Edited by iChris
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  • 3 weeks later...

iChris

My thoughts exactly, had one AP 40 = years teling me it was reverse current relay or induced voltage in the filter switching circuit, had studied diagrams & felt he was blowing smoke.Your explination is exactly the same as my conclusion

Thanks

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