Dr. Guenther Eichhorn 20 Hamilton Rd., #203 Arlington, MA 02174 Federal Aviation Administration, Central Region Office of Assistant Chief Counsel Att. Rules Docket No. 97-CE-17-AD Room 1558 601 East 12th St. Kansas City Missouri 64106 2 July 1997 Dear Sirs: This letter is in response to the NPRM modifying AD96-09-08 R1, Amendment 37-9691 requiring periodic inspection and/or modification of specific fuselage longerons in Pitts model S2 Aircraft. Please consider the following information: 1. There is a considerable difference in speed, power, and weight between the models S-2A and S-2B. The cracked longerons have ONLY been found on the S-2B models. Because of the lower weight and power of the engine and the lower speeds, the S-2A is subjected to much less stress in the longeron area of concern during snap rolls and hard landings. I believe that the AD should not apply to these lower powered aircraft. 2. Under the section "Actions Since Previous Rule", The following statement is made: Upon further investigation of the incidents, further analysis and testing show that hard landings and snap roll maneuvers in excess of +6 and -3 gravity("g") force limits cause enough stress and fatigue to crack the upper longerons. This condition, if not corrected could result in failure of the airframe and possible loss of control of the airplane. The stress analysis performed by Aviat Aircraft confirms that excessive stress to these longerons occur during a snap roll performed at excessive airspeed - causing +6 "g" to be placed on the airframe during a maneuver which twists the structure. No data have been obtained which substantiate that the fuselage structure will fail in this area when subjected to forces in excess of +6 "g" when it is not being twisted (as in a snap roll.) Furthermore, no data substantiate that damage will be caused to the longerons by forces in excess of -3 "g"s when the fuselage is not simultaneously being twisted. In order to achieve a "g" force in excess of +6 while performing a snap roll in a Pitts Special, the maneuver would have to be initiated at an airspeed of greater than 160 mph. In order to exceed -3"g"s, this aircraft would have to initiate a snap roll at greater than 130 mph. This is 20 mph faster than the limitations for this maneuver already placarded in the aircraft. It is virtually impossible to reach these "g" levels if the aircraft enters a snap roll at the recommended airspeeds. As mentioned above, t is significant that no failures have been found in the model S2A. This is most likely because this aircraft does not have the horsepower to sustain the airspeeds that will cause damage to the fuselage structure during a snap roll. The Pitts Special model S2 is a proven design that has withstood the test of time for over twenty years. It is well known among pilots who fly this aircraft that the one maneuver that will damage a Pitts is snap rolling it at higher than recommended airspeeds. The fact that there have been so few longeron failures in these aircraft attest to the fact that this problem is not the result of exceeding the +6 -3 "g" envelope while performing aerobatic maneuvers (which, in practicality, happens frequently) but that it must be related to a maneuver which is done in these aircraft relatively infrequently. There are not too many pilots who will routinely do a snap roll in their Pitts Special at better than 160 mph, or generate +6 "g" while landing. I therefore would make the following proposals: AD96-09-08 R1,Amendment 37-9691 be further amended to: 1. Not include the lower powered models (e.g. the S-2A) 2. Make the frequency of longeron inspection every 100 hours after the initial inspection. 3. If a placard is to be placed in the aircraft, it should reflect that the aircraft should not be subjected to "g" forces of greater than +4.5 or -2.5 WHILE PERFORMING A SNAP ROLL and that the fuselage structure should be inspected for damage if a "g" factor of +6 is exceeded during landing. Thank you for your time, Guenther Eichhorn