Older jets like Offutt's half-century-old RC-135s are notorious for leaks in the hydraulic systems. The hydraulic systems allow the movement of an aircraft's moving parts such as flaps, landing gear, ailerons and rudder. The RC-135s have two separate hydraulic systems, known as "left" and "right," which each control different parts. The two systems also back each other up.
The backup systems have served their purpose. Failing hydraulics have caused at least 26 in-flight emergencies on 55th Wing jets since 2012. Nine other times, hydraulic problems prompted pilots to abandon flights without declaring an emergency.
On April 28, 2017, while returning to Okinawa at the end of a mission, the co-pilot of an RC-135V Rivet Joint saw a light flicker warning that a pump in the left-side hydraulic system wasn't working. Shortly thereafter, a gauge showed that the hydraulic-fluid level had dropped to one gallon, less than one-third of its normal level. Soon a new warning light showed that a landing-gear door was unlocked, and a crew member saw that the right landing-gear door was wide open. The crew extended the gear manually and slowed the aircraft for the approach. The pilot declared an emergency and landed safely, greeted by emergency vehicles.
Former 55th Wing pilot Robert Hopkins III, a historian of Air Force tanker and reconnaissance aircraft, said the hydraulic system is like a house's plumbing. Hydraulic lines, carrying fluids, are embedded deeply in the framework of a jet just as pipes run through the walls of a house.
"Over time, you're going to develop a leak — because it happens," Hopkins said.
Leaks may not be easy to find, he said. And hydraulic lines are no easier to replace than the plumbing in your home.
"If you don't know where the leak is, you have to rip out the walls," Hopkins said.
Sometimes hydraulic fluid drips onto the brakes, causing them to heat up, smoke or even catch fire on landing. Back in 2012, a Rivet Joint had to evacuate its crew on the ground after landing at Offutt or Lincoln four times in 3½ months because of hydraulic fluid leaking onto the brakes.
The last C-135 variant involved in a fatal crash was a KC-135 tanker version that plunged to Earth from 20,000 feet in 2013, after taking off from an air base in Kyrgyzstan. The chain of events that led to the crash began with "rudder-hunting," a phenomenon in which the rudder — a large, movable piece like a sail on the jet's tail — begins moving in flight.
The rudder's purpose is to help control the aircraft in flight, But it is dangerous if it moves on its own, without the pilots controlling it. In the early 1990s, two commercial Boeing 737s — a United Airlines jet in Colorado Springs, and a USAir plane in Pittsburgh — rolled over and spiraled into the ground following an extreme rudder movement called a "hardover," killing everyone on board.
The autopilot on modern jets, including Offutt's RC-135s, includes a "yaw damper," an electronic control that corrects for unexpected rudder movements in flight and prevents the plane from slipping or skidding during a turn.
If it's not checked, rudder-hunting can escalate into a dangerous condition called "Dutch roll," in which the aircraft gyrates wildly, waggling its wings and wagging its tail. It can stress the plane beyond its limits and cause it to break up in flight. That's what happened in the Kyrgyzstan crash, when the crew tried to use the rudder to counteract the motions instead of simply shutting off power to the yaw damper and flying the plane by hand.
Aircraft incident reports show that more than 20 flights of 55th Wing aircraft were interrupted by serious rudder or yaw damper problems since 2012, including 13 declared in-flight emergencies.
One of the Wing's three workhorse TC-135 training aircraft cut short flights six times in 2013 and 2014 because of unexpected rudder movements. Three times the pilots declared emergencies.
Maintenance teams have had difficulty solving the recurring rudder problems on one of the Wing's two WC-135 Constant Phoenix jets. It has experienced at least eight in-flight rudder incidents since 2013, both before and after a major overhaul.
On Oct. 23, 2013, the pilot cut short a mission after the plane experienced "significant" yaw on a flight out of Offutt, and an emergency was declared after what the pilot described as "violent left and right yaw" during a flight from Japan to Alaska the following August.
Then, two months later, the plane experienced repeated Dutch roll during a test flight that apparently followed attempts to repair the rudder system. Three times in the summer of 2016, pilots noted problems with the plane's rudder pressure, resulting in two aborted flights and one declared emergency. It happened again as recently as October 2017.
After a flight on Oct. 26, 2016, the pilot of an RC-135W reported to mechanics that the aircraft exhibited "severe oscillations" after turbulence, which became "very violent and rapidly progressed."
And one of the Wing's three valuable RC-135S missile-monitoring aircraft was very nearly put out of action April 28, 2017. During a flight in the Midwest, the plane's rudder deflected sharply and suddenly as the jet cruised at 33,000 feet. At first, the pilot blamed rough air. But, after a second, even more violent episode, the pilot ended the mission and returned to the base.
As the plane touched down on the runway, a crosswind pushed the rudder hard to one side. The right inboard engine struck the ground, and the pilot struggled to keep the plane from running off the runway into the grass field. The plane spent 17 days grounded for repairs.