B-58 Facts – Yaw Damper

A Convair test pilot and engineer were killed when a B-58 lost an outboard engine while flying at Mach-2. This was a test flight to see if the pilot could control adverse yaw using only the B-58’s control stick and without using rudders. For normal operation, the B-58 had a yaw damper system that was supposed to automatically control adverse yaw by applying opposite rudder to stop the yawing motion.

In this test, the aircraft (B-58A 55-664) was a well instrumented test bed with recording devices taking up the entire space in the Navigator’s position. The test engineer, occupied the DSO’s position.  

The test began at Mach-2 at 35,000 feet, with all four engines in full after burner and with the pilot’s feet on the floor. He flipped a switch that instantly failed the right outboard engine. The aircraft side-slipped in the direction of the lost engine and immediately disintegrated. Convair test pilot Ray Fitzgerald and flight test engineer Don Siedhof, the only two crewmembers aboard, were killed instantly. Their bodies were later found, still strapped in their ejection seats. What went wrong with this test? In retrospect, it might have been inadvertently set up for failure.

The test was built around a worse-case scenario. For one, it was run in denser air at 35,000 feet instead of 55,000 feet, which was the Hustler’s normal altitude for supersonic flight. Therefore, the drag on a failed engine and airframe, when entering an adverse yaw condition, would be much greater. Secondly, the aircraft was put in a relatively unstable condition by transferring fuel to the aft tank, creating an aft center of gravity (CG.) Thirdly, the test had been previously conducted at Mach 1.6 with no adverse effects, and subsequent tests were supposed to be run in a stair step approach to Mach-2 by increasing the Mach number in 10% increments. But Convair decided to skip those incremental steps and go directly to the Mach-2 test.

Here’s a photo of 55-664 and what it may have looked like on the afternoon of November 7, 1959, flying in the skies over Oklahoma where the accident occurred.

Three magnetic tape data recorders were on the aircraft when it disintegrated. Two were later found scattered across the countryside. A third was subsequently found but its tape spool was missing. This third tape contained the most critical test data and would have helped the accident board determine the cause of the accident.

The official accident report concluded that there were a number of contributing causes, the main conclusion being a “design deficiency in that the directional restoring moments on the aircraft were not adequate for the test conditions.” There was a complex aerodynamic phenomenon that involved the aircraft’s large elevons as well as a deficiency in the tail fin structural integrity. But that does not tell us a whole lot.

Left unchecked, when an aircraft experiences an adverse yaw condition, (in this case the loss of an engine – let’s say an engine on the right side) it starts a lateral turn/slip toward that engine. In doing so, it also starts a rolling motion. This rolling motion is much more pronounced in swept wing aircraft like the B-58. In this example, the left wing gains lift and the right wing loses lift due to the left wing becoming less-swept than the right wing in reference to the relative wind, causing the aircraft to roll to the right.

So, when Fitzgerald felt that first yawing motion, coupled with the rolling motion, it’s assumed he forced the stick hard left. But in doing so he created more drag on the right side of the aircraft as the B-58’s huge elevons (acting as ailerons) were forced down into the airstream. This additional drag on the right side accelerated the turning and rolling of the aircraft to the right until breakup occurred.

In a paper written in 2011, for the North Texas Chapter of the Society of Flight Test Engineers, Jay Miller sheds additional light on what happened to the ill-fated B-58A 55-664 and its crew.

Miller writes, “Other modifications unique to ‘5664 included numerous skin panels with pressure sensors, strain gauges, and accelerometers attached, and special sensors for ascertaining and documenting ranges of control surface (i.e. rudder and elevon) displacement.’

“As an additional requirement, Fitzgerald had been asked by the engineering department to conduct the asymmetric test with the aircraft’s yaw damper system inoperative. He was also asked to avoid manually responding with rudder to the yaw that was expected to occur following engine spool-down. Level flight was to be maintained using only the control stick…and the aircraft’s massive elevons (working in aileron mode; elevons are dual-purpose control surfaces that serve as both elevators for pitch control and ailerons for roll control).’

“In response to the no-rudder-input request, Fitzgerald had commented to the engineering staff that, ‘It’s against my piloting instincts not to use rudder during a yaw oscillation. In view of this I’ll keep my feet on the cockpit floor to ensure compliance.’”

“Recovered parts of ‘5664 were moved into Convair’s enormous flight test hangar at their Ft. Worth facility and slowly reassembled in an attempt to ascertain the chain of events that had led to its disintegration. At the same time, data on what there was of the recovered magnetic tape was reviewed and a time history of the aircraft’s destruction was recreated.”

Miller notes that at the start of the test, 5664 was in the “max Q” part of its flight envelope. This meant that it was operating in its highest dynamic environment wherein air pressures and associated structural loads were at their respective peaks. This was, in fact, the most difficult environment for the aircraft to fly in…and it was also the environment in which failures would be most pronounced, and thus most unforgiving.’

“With the virtually instantaneous loss of thrust suffered by the no. 4 engine, the resulting asymmetric drag immediately skewed the aircraft to an angle of 3.2 degrees to the right of the direction of flight. As residual jet fuel flowed into its combustion chamber, the no. 4 engine momentarily remained functional. For just a second, ‘5664 stabilized, but at the very point where it should have begun to recover, a second yaw oscillation – probably induced by Fitzgerald’s elevon input – pushed the angle of divergence even further to the right…to an untenable 12 degrees off center. Without any kind of corrective rudder action, and a vertical tail that – because it was bending in an ever more severe arch toward the right side of the aircraft – had limited ability to provide a correcting force, all remaining directional control was lost.”

“By now, the vertical tail aerodynamic loads, later estimated to be in excess of forty tons, had exceeded their maximum sustainable limits. The skins and longerons between fuselage stations 13.0 and 15.0 began to buckle…and then fracture. As they failed, the entire vertical tail began a rapid separation from the aircraft’s core components. Consequently, all signals between the second station transmitter system and aft transmitter antenna ceased…as the wiring between them had separated.’

“At this point, the entire tail assembly blew away from the forward two-thirds of the aircraft. Moments later, the eleven intermediate hinges that attached the rudder to the vertical fin failed. When the top and bottom hinge brackets gave way, the rudder separated completely. In the meantime, the remaining components, including the forward fuselage and crew, had yawed to an angle of 30 degrees to the line of flight. The entire assembly was still supersonic at this point, though slowing rapidly. Structural and aerodynamic loads were enormous. The fuselage now began to break in two at about bulkhead 10. Concurrent to this, a transverse break occurred in both wings, going to the aft inboard corner of each main gear well. In a matter of seconds, the main wing panels broke downward and aft and separated from the forward fuselage section containing Fitzgerald and Siedhof. The right wing departed with the aft center fuselage section still attached, and the left wing flew off on its own. The engines, three of which were still functioning, now separated from their attachments and continued forward along separate parabolic paths.’

“As the fuselage began to break-up, the ventrally-mounted MB-1 fuel/bomb pod rolled to the right of its attachment points, separated, and began to break into several major pieces.’

In Miller’s recounting of the accident, the engineering team had asked that the yaw damper be made inoperative. However, there is some question whether or not Fitzgerald actually turned off the yaw damper prior to the test.

John Watson, one of Convair’s engineers, recently informed me of the following,

“I was in the Automatic Controls Group at Convair. My memory tells me the limited authority yaw damper was not turned off for the test. Years later I was in charge of designing, installing, and flight testing (including the Mach 2.0 engine failure test) a larger amplitude, triple redundant yaw damper as part of the overall flight control system. The damper servo used a dual input valve.”

In a subsequent Email John wrote to me,

“The B-58 was a feet-on-the-floor controlled aircraft. Adverse yaw during turns was minimized by an aileron-to-rudder interconnect that was both mechanical and electrical (through the yaw damper). The yaw damper had inputs from the yaw gyro, the lateral accelerometer, and the aileron signal. Supersonic flight without all the dampers operational was only allowed under special test conditions. The subject engine failure test was not one of those conditions. The pilot was instructed to “control” the reaction to the simulated engine failure with the aileron control with both feet on the floor with all dampers on. There was no reason to turn the yaw damper off. Continued operation at high Mach nos. was not permitted. The lateral acceleration was fed into the yaw damper at high Mach nos. to supplement static directional stability. There was no evidence to show that the dampers were not on. There were gross errors in aerodynamic stability derivative data supplied to the flight control system that was the primary cause of the fatal accident.”

If the yaw damper had been left on it might account for the initial stabilization of the yaw at 3.2 degrees, prior to Fitzgerald’s movement of the stick. But on or off this could be a moot point, because the dynamic stresses encountered by the aircraft may have been just too much for a yaw damper with limited capability to handle.

As a result of this accident the aircraft was restricted to Mach 1.6 until a triple redundant yaw damper, with larger capacity to control yaw, was developed and installed. It’s important to note that there was only one yaw damper. The “triple redundancy” comes from three separate sensors. All three sensors had to agree for normal operation. If one sensor failed to agree with the other two a caution lamp came on and the pilot would have to come out of afterburner and decelerate to subsonic flight. However, the yaw damper would remain in operation using input from the two sensors that were in agreement. If all three sensors disagreed with each other a red warning lamp would come on and the system would disengage.

With the installation of the “Triple Redundant Yaw Damper” system, crews were once again allowed to fly at Mach 2.

If you’re interested in seeing how fast an aircraft can become uncontrollable in an adverse yaw situation click on the following link. It shows a B-57 fatal accident when the pilot decided to go-around on a single engine. In my book, I describe my experience with a pilot on his solo flight in an RB-57 when we lost an engine.

https://www.youtube.com/watch?v=cHXxhmpwCxo&t=51s

Copyright © 2017 by Colonel George Holt, Jr.

Copies of my book, “The B-58 Blunder – How the U.S. Abandoned its Best Strategic Bomber”  are available on Amazon.com