Checking X-Feed during Run-Up

Multiengine aircraft have a feature on their fuel selectors called Cross-Feed (x-Feed).  This feature allows the right fuel tank to feed the left engine, and the left fuel tank to feed the right engine.

Why would you use this?

If you have an engine failure during flight, it is important to keep the aircraft as balanced as possible to avoid control problems.  You can use x-feed to distribute the fuel evenly during flight with only a single engine.  Another reason to use this feature is to extend the range of the aircraft during single engine operations.  Here is an example of how it can be used:

• You lose your left engine and you cannot restart.  You secure the engine and look for an airport nearby, but none are available.  You decide to continue your flight since you are not loosing altitude.  The inoperative engine fuel selector should be set to OFF and the operative engine fuel selector to ON.
• As you fly, you notice the right tank is emptying out faster than the left tank since the left engine is not running.  Using x-Feed, you can cause fuel to be fed to the right engine from the left tank.  So, you move the right fuel selector to x-feed, check fuel flow and set the left fuel selector to OFF.  VERIFY that fuel is still going to the right engine (you should monitor this for at least 30 seconds).  If you think there might be a problem with fuel flow, immediately switch the fuel selector for the right engine back to ON and the left back to OFF.  Continue your flight.
• When you see that the left tank is lower than the right, push the right fuel selector back to ON and the fuel selector on the left should still be in the OFF position.

Note:  Always consult the Pilot Operating Handbook for your aircaft for the exact method of using the x-feed feature.

With this method, you will be able to extend your range as well as keep the aircraft balanced throughout your flight.

Pre-Flight check of X-Feed

Some pilots feel that during run-up it is a good idea to check x-feed to verify that the cross-feed function is working property.  Be aware though, that it will take more time to fully check the x-feed function of the fuel selectors than most pilots are willing to invest in their run-up.  The Seminole POH states that you should check the operation of the cross-feed switches, but this in itself could cause a problem if you don't fully test the flow of fuel after this switch check has been completed.  Here is an example of how I tested this situation myself:

1. During my MEI check ride with Rob Davids (San Andreas California), this discussion came up during the oral portion of my test.  Since my real job is doing research, I decided that Rob and I should investigate this further. 
   • So, after starting both engines, I placed the left engine's fuel selector to Off. 
   • We taxied to the run-up area, did the run-up and prepared for takeoff. 
   • Four (4) minutes after placing the left fuel selector to off, the engine finally began to die from lack of fuel.  WOW!  This means that to really test the cross-feed function of the fuel selectors, you have to sit and wait a minimum of 4 minutes to validate that x-feed is actually working.
2. We then put the fuel selector back to ON for the left engine and had to sit for a minimum of 4 minutes to assure that this setting was still working.
3. During flight, we did another test.  When Rob asked me to show him my engine-out and restart procedures (part of the check ride), we decided this would be a good time to test out just how long the engine will run at normal cruise with the fuel selector off on one of the engines.  So:
   • I set the left fuel selector to Off
   • Started my timer
   • 40 seconds later, the engine stopped

So, at run-up, it takes the fuel in the fuel line on the side of the failed fuel selector six (6) times longer to empty out than during flight at normal cruise.  Wow!

What does this test mean?

The problems with testing x-Feed

There have been personal experiences with testing x-Feed that show, if you don't wait the full amount of time, the test itself can cause disastrous problems.

• During a flight, x-feed was tested on the ground, but the total time needed was not spent in testing to assure test fuel was actually being fed from the cross-fed tanks. 

• The fuel selectors were set back to ON.  During the movement back to ON a cable did not make contact with the valve correctly (in the wing) and unknown to the pilots, the right tank's fuel selection valve got stuck in the off position.

• Since the pilot was ready to go in less than 4 minutes, they took off without fully testing the On position of the fuel selectors.

• At 200 feet above the ground, after takeoff, the right engine stopped and the pilots were not able to start it up again.

• OUCH!

In conclusion

The POH says to check the cross-feed controls during taxi operations.  This means to turn the fuel controls from 'ON' to 'x-Feed' then back to 'ON' (Section 4.25 of the expanded procedures section).  Once you do this, make absolutely sure that the fuel flow has not been stopped because of some sort of problem in the controls.  If the controls feel like they have become lose or too hard to move during the test, perhaps it would be a good idea to not make the flight.  If all feels OK, then wait the extra 3-4 minutes to make sure that the fuel flow is still OK.

If you are in too big of a hurry to not use this procedure, and perhaps not even test the controls, problem may occur during flight especially if you have to use the controls in an emergency situation.  If you check the controls as the POH prescribes but do not wait the full 3-4 minutes to assure adequate fuel flow, you might lose an engine after takeoff.

Statistics show that most multi-engine trips do not result in an engine out condition.  With that said, you will most likely be OK by not testing the cross-feed function of your fuel controls.  And it would even be possible that if you do, you could adversely affect the fuel controls during the test.  The best course of action here?  Do the check, but wait the full time to verify adequate fuel flow.  TAKE THE TIME AND FLY SAFE!

Thanks to the following people for their assistance in the writing of this article:

• Rob Davids, DPE, United Airlines Pilot, Norcal Aviation Services

• Keith Breton, CFI, CFII, MEI, Ahart Aviation Services

• Jordan Miller, CFI, CFII, MEI, SkyWest Pilot