Deep stall accident in Utah
July 1991
A Long-EZ was involved in an accident in Utah recently that resulted in serious back injury to the pilot who was flying solo. This pilot was a relatively new private pilot with only a few hours in type. While attempting to cut a roll of toilet paper, this pilot managed to get the airplane too slow, with too much angle of attack and the airplane apparently entered a "deep stall" condition. The pilot did not recover from the deep stall condition, and the aircraft descended in a flat attitude (75 to 85 degrees AOA), striking the ground slightly nose high with very little forward speed. The pilot suffered serious back injuries and the entire aircraft bottom and landing gear were heavily damaged.
There were a number of eye witnesses to this accident and our investigation leads us to suspect that the aircraft was being flown with a CG that was well aft of the published aft limit. This aircraft also was not equipped with vortilons.
If you are currently flying a VariEze, a Long-EZ or a Defiant and you are not positive of your aircraft's center of gravity, ground your aircraft until you have conducted an accurate weight and balance using calibrated balance beam scales or calibrated load cells. Do not bet your life on bathroom scales. You must not fly your aircraft unless you know exactly where your CG is. Do not fly a Long-EZ or VariEze without vortilons. In addition, due to the variance in aircraft shapes, and indeed, airfoils shapes possible in a homebuilt aircraft, we would strongly recommend that you conduct a stall test at least 10,000 feet above the ground while wearing a parachute. This will clear the stall envelope on your particular aircraft which, as we have said, may not be identical to the RAF prototype or to anyone else's aircraft. If you see any sign of an unusual or uncommanded pitch up or any hesitance in nose down control power when at full aft stick, go to full power and full forward stick immediately and recover! If your aircraft hangs in a high sink condition, rock it out with ailerons and rudder. using maximum available engine power. Ballast your aircraft to a more forward CG and retest. If you do not want to take the risk of doing this stall test program, @ at least, limit your flying to mid or forward CO.
This particular accident and injury pointed again to the advisability to modify the LB-9 plywood bracket that supports the landing brake actuating weldment. This was called out as a mandatory change in July 1981, CP29, page 7. We have noted that few builders have made this modification. We would like to reiterate this requirement and add an additional change as shown in the sketch below. Cut away the entire lower portion of the LB-9 bracket as shown and remove the lower piece and discard it. Cut out a piece of 1/4" thick birch plywood (firewall material) approximately 8" wide and 9" long. Bevel the edges and flox it onto the forward face of the front seat bulkhead, centering it over the LB-9 bracket. Lay up four (4) plies of glass BID over the entire piece of plywood lapping onto the front seat bulkhead a minimum of 2" all around.
This change is mandatory and should be completed before next flight. Also, strongly consider the use of the energy-absorbing Tempafoam cushions for both seats. Now, this may seem ridiculous to modify your airplane in order to protect yourself from a full-blown deep stall crash that on a normal airplane would be fatal. However, we continue to be surprised at the protection provided by the EZs composite structure and we always take the conservative approach to increase safety as much as possible.
THE FOLLOWING IS AN ANALYSIS OF THE UTAH ACCIDENT
The Utah accident involved a deep stall, flat descent (angle of attack of about 80 degrees). The fact that the pilot survived and that a slower-than-expected sink rate occurred (confirmed by video tape evidence of the last 2.3 seconds of descent) presents somewhat of a dilemma. We are baffled as to why this can occur. A similar phenomena has been experienced during several deep stall accidents with the Velocity aircraft. All were survivable and one went into water with the pilot experiencing no injury at all! (See article in July '91 Sport Aviation.)
The Utah Long-EZ had a wing-loading of about 12.2 lbs./sq. ft. and, considering all its area, including the wings, strakes, cowl and fuselage, a "flat-plate loading" of about 9.2 lbs./sq. ft. (1150 lbs. divided by 125 sq. ft.). A basic calculation of the predicted rate-of-sink in a flat descent would use a flat-plate drag coefficient of about 1.2and would predict a sink of about 4820 ft. per minute or 80 ft./sec. This would definitely be survivable.
Using two different methods, we have calculated that the Utah Long-EZ probably had a drag of about 2.8 times that predicted by simple flatplate theory, i.e. a co-efficient of about 3.3. This results in an energy at impact of only about 1/3 that which would result from the "calculated prediction" sink of 4820 ft./min. Here's the two methods:
1) Analysis of the video tape shows a sink rate of about 48 ft./sec. (2900 ft./min.). This required measuring the size of the airplane image and may be off as much as 30%. The post-crash video data show the rate of drift of dust from impact. Comparing this rate of drift of dust (wind was about 20 knots) to the rate of sink of the airplane (on video) confirms the approximate 48 ft./sec. estimate.
2) Assuming a 48 ft./min. descent, the main landing gear would absorb 18 ft./sec. before the fuselage strikes the dirt - this is a relatively accurate calculation knowing the gear's stiffness and strength. Absorbing the remaining 30 ft./sec. over a total deflection of approximately 6.7" (cushion, plus fuselage, plus dirt), results in an average deceleration of about 25 G with a peak deceleration of about 40 G. Considering thesupport and attitude of the pilots back, this is consistent with the injuries he sustained. An 80 ft./sec. descent would result in a fatal 150+ G impact of the spine.
Both these methods are very rough but (along with the deep stall accident experience with the Velocity) they tell us that an unusual phenomena is occurring. It is likely that a large, trapped vortex forms above the aircraft. It's relatively easy to see how this could increase the drag by 25 to 50%, but it makes no logical sense that it could increase drag by a factor of 2.8 - this would require the airplane to decelerate a column of air that is more than 3 times the size of the airplane! What is even more baffling is the report (not confirmed by us) that the Velocity aircraft sinks at less than 1500 ft./min. (15 knots!). If that were true, it would have to have a "flat-plate" drag coefficient of about 12!! (A totally illogical result). We suspect that the Velocity and Long-EZ have similar drag coefficients and that the cushion of water landing provided the difference in pilot injury.
The Utah pilot had one thing going for him, he was sitting on seat cushions fabricated from TempaFoam an excellent impact absorber.
CONCLUSION: What can we learn from this accident? First of all, don't just jump into someone's homebuilt airplane and go flying. Insist on seeing a current weight and balance and discuss any possible "quirks" the airplane may have with the owner.
Do not let peer pressure tempt you to fly beyond your experience or capability. Cutting a roll of toilet paper requires absolute knowledge of your aircraft without referring to the instruments. You will be looking over your shoulder for the toilet paper ribbon for most of the flight which requires some acrobatic experience at least. This is not a sport for neophytes. If a VariEze or Long-EZ is not equipped with Vortilons on the leading edges of the wings, do not fly it!