Two fatal accidents that occurred during mandatory stall tests of Hawker Jets have prompted the NTSB to recommend improvements to documentation and training to safely accomplish them. The board is now investigating two fatal accidents involving stall tests on Hawker 800 and 900 series aircraft and, as it sometimes does, decided the information it uncovered during the investigation warranted issuing recommendations before the investigations are complete. The recommendations urge Textron, which now holds the type certificates, to update the information available to pilots and operators on these types of flights. “We are concerned that, due to deficiencies we identified in the information available to airplane owners, operators, and pilots related to the training and procedures needed to safely perform manufacturer-required postmaintenance stall test flights in certain Hawker airplane models, other flight crews tasked to perform such flights may be similarly unprepared,” the board said in its aviation investigation report.
The stall tests are required after maintenance and inspections involving wing leading-edge assemblies, deicing distribution panels, or stall trigger assemblies. The wings have to go back together within very tight tolerances or the stall characteristics of the planes can be seriously affected. The stall tests are done to make sure those characteristics aren’t exhibited in an inadvertent stall. In one of the accidents, the plane rolled inverted and entered a spin during the stall test and corkscrewed into the ground. In the other incident, the plane appeared to enter an unrecoverable flat spin. The full document is below.
Flight performance anomalies are not limited to high-speed airplanes. When performing a full-axis airframe rigging procedure, all wing, stabilizer, and flight control structures are reset to book neutral and cables are rigged for travel and tensions. Once the CG is verified, a flight test is conducted to verify handling and performance throughout the operating envelope. The “aileron snatch” reference in the NTSB report is very real and can be very aggressive at any speed. And it’s not just found on the roll axis.
Airplanes with a history of damage and/or showing signs of major structural repairs and wholesale component replacements should be handled cautiously during these flight tests.
Wow! I shall not be doing stall tests in a Hawker!
Learjets had the same requirement for stall test flights after leading edge wing maintenance. When a 24D I was flying and had over a 1000 hrs in came out of a 12 year inspection it needed a test flight. I had stalled many airplanes before but never a jet at high altitude so I hired a Learjet test pilot who had done several in make and model to perform the test and I acted as SIC. Wow! were my eyes opened! As the PIC slowed and was calling off airspeeds, I was writing them down with his spoken notes. We got the stick shaker at some point and then we suddenly pitched down and went inverted. When you are pointing straight down you are no longer inverted and as airspeed built the PIC stopped the roll and smoothly recovered from the dive after about 5000’. He asked me at what speed did the stick shaker activate and at what speed the nose pitched down. I only had the first speed because I froze at the sudden departure from controlled flight. He noted the airspeed though and said the plane had passed the test. I was sure glad I did not perform the test and never got any airplane close to a stall again outside a simulator.
It sounds like an unsafe design. Is there a specified stall recovery? And did they go through it?
Even a J3 Cub of which it is said “can barely kill you” is unsafe if improperly rigged, set up, AND, you don’t UNLOAD THE WING when it needs unloading.
The Hawker series design in question is not an unsafe design. As with all high altitude civilian swept wing aircraft, they are otherwise mostly very docile airplanes if flown within the certified envelope once they are properly rigged and set up by factory trained personnel. Initial production as well as post maintenance proper rigging and set up is key to docile handling for the consumer. So Pete, rigging and setup, not initial design is the subject at hand.
Consumer pilots not trained and qualified to be part of this post maintenance rigging and set up process should not be asked to do this testing and should refuse to do so if asked even if it means losing your job!
I was part of a factory group which performed these tests both at the factory and post maintenance at factory approved facilities. We were formally trained by studying written material and undergoing formal ground school and flight training in preparation for this work before hand. We never arrived at a test scenario cold, and no two events were ever the same. But, one key aerodynamic principle remained the same regardless of any attitude in which we found ourselves: UNLOAD THE WING! UNLOAD THE WING! UNLOAD THE effing WING!
Whether you’re a consumer pilot or a factory test pilot, if you do nothing else in one of these unusual attitude situations, UNLOAD THE WING! Even if you’re in a J3 Piper Cub which can only barely kill you, UNLOAD THE WING!
I think you’ll find that most aircraft with stick pushers have dangerous stall characteristics and that most transonic aircraft fall into that category. It’s not necessarily unsafe design, it’s just inherent compromise. Degraded stall margins due to damage or ice are a factor for any aircraft, it’s just worse for these high performance aircraft that stall aggressively. Relating to damage such as bent or missing VGs or stall strips, it highlights the importance of thorough preflight inspection and maintenance. Relating to icing, it highlights the importance of vigilance toward atmospheric conditions and maintaining conservative margins during maneuvering. The bizjet world can certainly afford to qualify pilots for post maintenance stall tests, or find designated pilots to perform these flights.
Gotta agree w/NTSB that if you put crews into potential stall, they need training, qual and currency to fly profile/recovery from potential outcomes (stall, post stall gyrations, spin), not just for this family of acft…and perhaps everyone should know how to recover from stall/spin in sim…perhaps not a high likelihood of success in real world, but at least give them the tools.
Suggest emphasis to crews on understanding work performed, potential aggravating factors (as called out: bent vortex gens, surface contamination) loading acft at forward CG limit for better recovery and defining post stall procedure that does not aggravate post stall into spin (typically forward stick, neutral rudder/ailerons, max thrust). Also develop/train spin recovery with emphasis to use post stall procedure which accepts statically stable acft flopping around on its way to nose down until acft recovers or alternatively, spin entry and spin direction are confirmed…with specific parameters ID’d to confirm post stall vs spin emphasizing that anything other than post-stall recovery is pro-spin if not already in a spin…neutral controls while figuring it out defines an acceptable action that could solve the problem even before the crew sort out what just happened.