Why Were KC-135 Tankers Exploding?

A KC-135A Stratotanker refueling aircraft en route from K.I. Sawyer Air Force Base in Marquette, Michigan, to Chicago O'Hare International Airport suddenly exploded at 13,700 feet. All four crew members of the aircraft, along with 23 passengers, members of a C-130 crew being transported from K.I. Sawyer to O'Hare, perished in the accident.

A four-engine turbojet, KC-135 Stratotanker was the military counterpart to the venerable Boeing 707. The aircraft platform was developed in the mid-1950s and established an extensive service record in both military and civilian operations. 

Sadly, this was not the first KC-135 to mysteriously explode---nor would it be the last. A similar accident occurred near Centenera, Spain, on June 3, 1971, when a KC-135 returned to Madrid-Torrejón AFB after a refueling mission over the Mediterranean Sea. At 17,000 feet, the plane suddenly disappeared from radar. All aboard were killed. Unfortunately, much of the wreckage could not be recovered, limiting investigators' ability to figure out what caused the explosion.

In the last three decades of the 20^th century, at least three other similar accidents resulted in KC-135 tankers being destroyed in explosions either in the air or on the ground. How could it be that these reliable Air Force tankers were suddenly exploding without warning?

On the evening of the 1982 accident, fog and rain had been observed in the Greenwood area, approximately 35 miles northwest of Chicago's O'Hare International Airport. Was it possible that lightning struck the aircraft, leading to a fire that ignited the fuel tanks and caused an explosion?

While lightning strikes are potentially dangerous events, lightning strikes to aircraft are not uncommon, so planes are designed to be able to withstand and dissipate the powerful electrical strikes. Most aircraft are made with aluminum skins. Metals like aluminum are excellent conductors of electricity, so when a lightning bolt strikes the surface of an aircraft, the metal skin effectively channels the electricity away from the interior of the aircraft and safely dissipates the charge. Composite materials used on newer aircraft are not effective conductors of electricity, so they are embedded with wire mesh, metal strips or other devices to safely redirect and dissipate the electricity and prevent serious damage to the aircraft. FAA regulations stipulate strict requirements for the design, construction and maintenance of fuel tanks and fuel systems to ensure they are also protected from lightning, in addition to the protection provided by the aircraft surface.

Midair collisions between aircraft almost always result in catastrophic accidents, causing the aircraft involved to break apart or explode. Other aircraft were flying in the airspace around the accident, as the northern Illinois site is located in airspace near one of the world's busiest airports, Chicago O'Hare International. In fact, there was one pilot flying a Piper Seminole, a small twin-engine model, at the time people witnessed the explosion. However, there were no reports of any unusual radar targets or missing aircraft that night. The lack of any unusual radar activity would also rule out other possibilities, such as UFOs.

Could a missile attack have taken down the KC-135? While the 1971 accident occurred in foreign airspace, the accident in Greenwood was located in a rural Midwestern area, so enemy fire would be unlikely. Errant friendly fire would be unlikely as well because there are no military practice ranges nearby. 

Moreover, two of the other KC-135 accidents actually occurred on the ground in military locations. On Sept. 20, 1989, two of seven crew members were killed in an explosion that occurred while the crew was shutting down the KC-135A aircraft at Eielson AFB in Alaska. Six maintenance personnel of the Wisconsin National Guard were killed in Milwaukee, Wisconsin, on Dec. 10, 1993, while working on a KC-135R.

Given the tragedy of 9/11 and other terrorist activities in recent decades, the possibility of a bomb, especially in an accident involving such violent detonation, seems a logical explanation. However, that scenario may be more probable on civilian transport aircraft. With the multitude of security protocols involved, it seems unlikely that any of the military crew and passengers would have the motive, opportunity or inclination to bring an explosive device onto a military aircraft at a military base. Besides, it's beyond improbable that a bomber would target the same kind of aircraft at locations many thousands of miles part. 

The possibility of an inadvertent detonation of military weapons loaded onto the aircraft would not be out of the realm of possibility for aircraft on combat missions. However, the KC-135 is not a bomber or fighter aircraft. It is an unarmed tanker whose mission is to refuel other aircraft that do carry live ordnance.

Unfortunately, the KC-135s in the accident aircraft were not equipped with "black boxes" containing voice and data recordings to help investigators determine a definitive cause. Based on the type of damage found in the wreckage of the Greenwood accident, Air Force investigators determined that the probable cause of the inflight breakup was an over-pressurization under the cargo floor, an area where the fuel tanks are located and fuel vapor could have accumulated and been ignited. Similarly, evidence of over-pressurization was observed in the forward, aft and upper fuel tank areas as well. 

Evidence from a similar accident on Oct. 4, 1990, at Loring AFB in Limestone, Maine, which killed four crew members, led to the discovery of a problem with the hydraulic-powered air refueling (AR) pumps. In the Loring accident, a fuel pump in the rear aerial refueling storage tank overheated to at least 1,400 degrees, causing vapors in the empty tank to ignite and explode. While insufficient evidence exists to determine the exact cause of the Greenwood explosion, given the similarities in the damage to that aircraft with the others, it is likely that it suffered a similar fate.

In all these KC-135 accidents, the explosions occurred at a time late in the flight (on descent to land, approach or on the ground post-flight) when the fuel tanks were likely to be empty or nearly so, and fuel pumps had been running for an extended period of time, creating the ideal conditions for an explosion.

After the Loring accident, crews were directed to keep at least 3,000 pounds of fuel in the tank. Liquid fuel is less volatile than vapor and, therefore, less likely to explode. Furthermore, in the KC-135 hydraulic system, fuel helps cool the hydraulic fluid and pumps. 

Even without that specific fuel pump issue, empty tanks containing vapors are an explosion risk if any heat or ignition source is present. Investigators believe a faulty boost pump wire probably triggered the KC-135 accident in Spain. 

More recently in the civilian world, on July 17, 1996, TWA Flight 800, a 747-100, exploded shortly after takeoff from JFK International Airport after worn electrical wires sparked to ignite the vapor in the empty center tank of the airplane, killing 230 passengers and crew. Since that accident, it has become common practice on newer aircraft to fill empty fuel tanks with an inerting substance, such as nitrogen, to prevent residual fuel vapor from igniting. 

Since the discovery of the fuel pump problem, changes in personnel training, operational procedures and maintenance have avoided additional explosions of KC-135. Those changes, as well as new safety technologies, such as inerting systems and better designs of fuel and hydraulic systems on new aircraft, have prevented additional tragedies and saved many lives. 

Do you want to read more Mysteries of Flight articles? Enjoy "Why Were P-38s Falling From The Sky?" here.