Expert: James Bell - 6/22/2009

Question
Hello James,

How big a concern is the possibility of a spark during aerial refueling?  Assuming it's a concern, how is the electrical potential between the tanker aircraft and the receiving aircraft equalized prior to opening the spigot?  I'm assuming that there must be some metal-to-metal contact between the boom and the coupler before any fuel flows.  Is that correct?  Lastly, is there any was for the boom operator to confirm that the two aircraft are at the same potential before letting the fuel flow?

Thanks,
Justin

Answer
Justin -

You ask a very good question.  You remind me of another Justin I once flew with ... my boom operator during my deployment to the Middle East ON September 11, 2001.

Despite what you may have seen in the Harrison Ford blockbuster AIR FORCE ONE, planes just don't go boom like that.  Yes, there is metal to metal contact, but the nozzle that fits in the receptacle locks into place with hydraulically powered locks.  As these two pieces come into contact, they "bond" to each other (as opposed to grounding).  Just like if you generate static electricity and grab someone's arm, you shock them.  But if you hold onto their arm and try and generate more static electricity, you can't shock them because your two bodies are now bonded together.  Any static electricity flows between the two of you without generating a spark.

Once the nozzle is locked into the receptacle, the design of the boom opens a poppet valve at the end of the boom that will allow fuel to flow.  This connection also completes an electrical signal telling both refueling systems in both aircraft that they are now in "contact."  Only after this contact signal has been received does the system turn on the fuel pumps and open the actual fuel valves that allows fuel to flow.  None of this can happen until the aircraft are physically connected and "bonded."

As far as the one scene (in said Harrison Ford movie) where a spark ignites the spraying fuel, the moves up the boom and blows the tanker aircraft up ... simply can't happen.  Refuelings of aircraft of that size take place at altitudes of 25,000 feet or higher (the air is pretty thin up there), and at over 300 MPH.  Try holding a cigarette lighter out the window of your car at even 55 MPH and try and get it to light.  Even if you did, the flame would travel with the wind ... not against it.

All aircraft are equipped with static eliminators.  They are little barbs that are on the trailing edge of all wing and tail surfaces.  If you look at the following picture, you can see them ...

http://64.41.64.113/home/pictures/oef-08.jpg

On the back end of the F-16's wings (and just barely visible on the back of the tail), just inboard of the missiles at the tip, you will see three small protrusions no bigger than soda straws.  Those are the static eliminators.  As electricity moves over the skin of any aircraft, it moves with the wind.  It keeps moving back until it can't move back anymore.  Installing these static eliminators on the trailing edge gives the electricity a path to reach a point where they can be harmlessly discharged back into the air.  Every plane has these on the trailing edge of their wings and tail surfaces.

I hope this answers your question.  If not, or if you have any more questions, don't hesitate to ask.

Sincerely,
James Bell