Imagine flying, along with 20 or so fellow aircrew, in an Air Force E-8C Joint Surveillance Targeting and Attack Radar System (JSTARS) jet for a mission to track down insurgents planting roadside bombs in Iraq or Afghanistan. You’ve just taken off from your base in Qatar but before you can go scan the ground for bad guys with the plane’s powerful AN/APY-7 radar, you’ve got to refuel from a waiting KC-135 tanker since the E-8’s ancient Pratt & Whitney JT3D engines burned way too much gas taking off on a hot Middle Eastern day.

The E-8 you’re flying in is a converted Boeing 707 passenger jet that was built in 1967 and flew in airline service for decades before being purchased by the Air Force and refurbished for military use in the 1990s.

Approaching the tanker, all is going smoothly until the two planes hook up and fuel starts flowing into the JSTARS. You hear a “loud bang throughout the midsection of the aircraft.” This freaks everyone out enough for the pilot to immediately stop the refueling to check the aircraft for damage or malfunctioning systems. Finding none, the pilot brings the jet back into contact with the tanker and as soon as fuel starts flowing between the two jets, the E-8C begins to shudder as “another series of loud noises and vibrations” are “heard and felt throughout the aircraft.”

As this is happening, the KC-135’s boom operator, lying on his couch underneath the aft-belly of his jet, sees vapor and fuel pouring out of the JSTARS. Something is very wrong. The tanker crew tells the pilots of the JSTARS what they’re seeing and the E-8’s crew sees the same thing as they look out the windows in the aft of their jet; fuel is streaming out of “at least two holes in the left wing, just inboard of the number two engine.”

The pilot immediately brings the jet back to its base in Qatar. Once on the ground, mechanics find that the number two main fuel tank has been ruptured, “causing extensive damage to the wing of the aircraft.” How extensive? $25 million dollars worth of damage extensive.

What caused this potentially fatal and incredibly expensive accident to one of the United States’ biggest spy planes? A contractor accidentally left a plug in one of the fuel tank’s relief vents during routine maintenance.

This actually happened on March 13, 2009 and the story above is taken directly out of the Air Force accident investigative board’s report on the incident.

Yup, a civilian contractor inadvertently left the test plug inside the jet’s fuel tank when the plane went in for Programmed Depot Maintenance (PDM) — which may have been at the Air Force’s E-8 PDM depot, the Warner Robins Oklahoma City Air Logistics Center — according to the accident report.

“The PDM subcontractor employed ineffective tool control measures,” reads the document. Tool control measures; you know, the absolutely basic practice of accounting for the exact location of every tool that is used to work on an airplane once that work is finished. Wow. Just, wow.

The contractor’s mistake caused a “near catastrophic fuel tank over-pressurization” and $25 million in damage during that aerial refueling session, the report states. Luckily, no one was hurt.

The report goes on to say, “The PDM subcontractor failed to follow Technical Order (TO) mandated procedures when employing the fuel vent test plug during PDM. Due to the relatively short period of time between take-off and [aerial refueling], the [mishap crew] did not have the opportunity to burn a substantial amount of fuel from the number two fuel tank which could have allowed the “dive flapper” valve to open after the tank’s excessive air pressure decreased to the point where the flapper valve would open. This explains why this mishap did not occur during [aerial refueling] conducted between the time the [mishap aircraft] left the PDM facility and the time of the mishap.

A colleague tells me that Air Force Chief of Staff Gen. Norton Schwartz said this morning that one JSTARS will be retired, it may well be this bird.

Here are some pictures of the damage to the wing. Note how it doesn’t look that bad from outside. Once you see the inside however, it’s another story. If that’s not enough to convince you that fuel tank over-pressurization caused by a forgotten plug is a big deal, check this out.