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ACRO E-mail Archive Thread: [IAC-L:278] response to proposed AD ammendment

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ACRO E-mail Archive Thread: [IAC-L:278] response to proposed AD ammendment



                


Thread: [IAC-L:278] response to proposed AD ammendment

Message: [IAC-L:278] response to proposed AD ammendment

Follow-Up To: ACRO Email list (for List Members only)

From: "Dr. Guenther Eichhorn" <gei at head-cfa.harvard.edu>

Date: Wed, 02 Jul 1997 17:53:36 UTC


Message:

  
Hi,

Following is my response to the NPRM.  I have included a part
about excluding the S-2A from the AD because of the lower power
and weight of the engine.

Guenther

************************************************************

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


                


© Dr. Günther Eichhorn
Retired
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