Two airmen died after their Bulgarian Air Force L-39ZA crashed during a training exercise at the Third Air Force Base on September 13. The names of the dead are Major Petko Dimitrov, 51, and Lieutenant Ventsislav, 28. Both were instructor pilots. The crash occurred shortly after midday while low-altitude maneuvers were being carried out at the airbase as part of a rehearsal for an air show that had been scheduled for September 14 but was canceled by order of Acting Defense Minister Atanas Zapryanov. The death of the pilots was confirmed at a short press conference at the air base by Acting Prime Minister Dimitar Glavchev, Minister Zapryanov, Deputy Chief of Defense Lieutenant Tsanko Stoykov and the Commander of the Bulgarian Air Force Major General Dimitar Petrov.
By reviewing available footage widely available online and in the media, an educated guess can be made that the cause of the accident was pilot error, a consequence of the Air Force's overall organization, management and aviation culture.
Technical malfunction of the catapults?
From the video, the DRM team believes that the pilot applied the control stick to the aircraft until the last moment in order to increase the angle of attack to exit the steep descent.
Ejecting requires a pilot to release stick control to act on the designated instrument with both hands to perform an ejection, resulting in a free fall of the aircraft. We think it is clear from the video that the pilot did not stop trying to regain control.
“When the decision to eject is made, the pilot grasps the dual feed handle (ejection handles) with both hands, squeezes the release handles and pulls them continuously upward until the seat is ejected from the cockpit. The remainder of the ejection operation, up to the deployment of the main parachute, is fully automatic.” - L-39C aircraft technical manual (T.O. 1T-L39C-1).
We note that the observation has the possibility of being wrong, since the textbooks for this aircraft say that when increasing speed, the fighter often independently tries to get out of a dive.
“During acceleration the aircraft is stable and has no tendency to overturn. When the speed increases, the aircraft tries to exit the dive.”
About losing control
Without relying on the black box data, there is no way to know the exact speed and altitude of the aircraft to make inferences about potential pilot errors. Therefore, we will derive extremely basic and indicative minimum speeds from the airplane textbooks.
At all altitudes, the minimum permissible speed is 200 km/h, at which speed the aircraft is sufficiently stable and controllable. During the performance of aerobatics, the pilot must avoid flights with an instrument speed of less than 200 km/h. This is especially important when performing vertical figures. In the case of a speed of less than 200 km/h, excessive pulling of the control lever should be avoided.
Minimum instrument speed (overload speed with a G-factor equal to 1) with flaps and landing gear retracted when the engine is idling is 180 km/h.
When the aircraft reaches a speed that is 5-10 km/h higher than minimum, aircraft shake and stick vibration (from the ailerons) occur.
When a speed of 160-165 km/h is reached and the stick is pulled all the way back, the aircraft normally lowers its nose and goes into parachuting mode with a gradual increase in speed to 200-220 km/h.
Indicative instructions for performing “looping” are also given:
At the start of the maneuver, an instrument speed of about 650 km/h is needed, maintaining about 4G load on climb. At its highest point, the aircraft must not have a speed lower than 200 km/h.
Achieved altitude should be on the order of 1 km to 1.4 km. When exiting the loop, the speed should be 650 km/h.
We clarify that these are extremely basic values, and pilots actually monitor many more indicators when performing this type of maneuver, especially at extremely low altitude.
The DRM team believes that there are two risk factors that contributed to the accident: too low speed and too low altitude. The fatal risk is the height, since the plane in the last moments before crashing started to come out of a dive, that is, it regained maneuverability by accelerating to the most likely necessary 220 km/h instrument speed.
But the most important question is who made the plan for this maneuver and why was it given to an L-39 training aircraft that has a thrust to weight ratio below 1.0? This type of maneuver would be unnecessarily dangerous even for a MiG-29 fighter with a ratio higher than 1.0, unless it performed a longer climb to have a buffer height in case of error.
Using the L-39 for figures of such a low height is, according to the DRM team, an absolutely unnecessary risk. A full review of the timeline from the incident to the planning of this figure and whether it was even pre-planned with the necessary parameters is needed. What are the reasons for taking these risks? Air Force culture, too high expectations, years of management mistakes, or other factors?