HW Elec/Fuel Question/Anomoly

[Reigate]

HW POH 7-21 indicates that when Main Bus power is unavailable, Feed Pump power is lost.

HW POH 7-30 contains a WARNING that an engine stoppage will occur if no feed pump is operating.

If GEN A fails, GEN B will take over to supply the engine and the battery will no longer be charged and will start to deplete. Rotax has designed the system such that the engine will continue to run until fuel is exhausted regardless of the battery state of charge, which was no doubt a regulatory requirement for certified aircraft. Now, if you accept that the Sling statements in the POH are correct, then once the battery is depleted, the feed pumps will stop running and result in an engine failure. I have checked the circuit diagram in the current HW Construction Manual, and it shows that the Feed Pump that is controlled by the Main Pump Switch is only powered by the main bus, which concerns me for obvious reasons. Now, if power to that Feed Pump was powered by HIC A instead, then the engine wouldn't fail should the battery be depleted.

The Sling engineers are fairly talented, so I'm probably missing something and would appreciate it if someone would have a look at the above and point out where my logic is wrong?

[lutorm]

I'm not familiar with the HW, what is the "feed pump"? Is this another pump that's different from the Rotax fuel pumps on the firewall? Because those are powered by the engine directly.

[Reigate]

The HW has a 6 litre header tank on the back if the firewall which supplies the main fuel pumps. The two feed pumps are located between the fuel selector and the header tank and their purpose is to keep the header tank full. Without a feed pump running, no fuel will be pumped into the header tank and the engine will eventually stop when the tank runs dry.

[Reigate]

To anyone currently with a HW equipped with a header tank (I’m not sure if the early SN’s were equipped with the tank), would someone like to try the following as my aircraft is currently in the build process in SA otherwise I would perform it myself.

1. Start the engine;
2. Switch on FEED PUMP 2;
3. Switch off the MASTER SWITCH; and
4. Monitor the header tank fuel level.

If the fuel level in the header tank decreases, then we have a potential safety issue which needs to be addressed. A simple fix would be to transfer Feed Pump 1’s power source from the main bus to HIC A.

Certified electronic ignition systems are required to be able to run the engine without any ships power and while our aircraft are not certified, I can see no reason why the same standard should not apply when it would appear to be a fairly easy fix to implement.

[lutorm]

The HW has a 6 litre header tank on the back if the firewall which supplies the main fuel pumps. The two feed pumps are located between the fuel selector and the header tank and their purpose is to keep the header tank full. Without a feed pump running, no fuel will be pumped into the header tank and the engine will eventually stop when the tank runs dry.

Huh, that sounds weird. You'd think there would be sufficient head pressure from the high wing tanks for fuel to make it into the header tank. Is the header tank vented to atmosphere or sealed? Does the return go into the header or back to the wing tanks?