Warbird Information Exchange

I have to ask this after reading the last three pages of this topic,
Is there any discussion about having a separate requirement for aircraft involved in "Mock Combat Operations" or is an all inclusive means of compliance the popular opinion?

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"Hindsight is usually 20% off!"

My understanding is that a T-6 is certified in the same category whether or not it is used in ACM training/mock combat. You would have to track each aircraft for fatigue damage accumulation over its lifetime- this as an aircraft could potentially go from ACM training back into the non-commercial fleet. It would take very sophisticated fatigue monitoring devices that would have had to have been installed when the aircraft was (or attach angles were) new.

As an example, you might repeatedly overload an aircraft to the point just short of the elastic limit (prior to permanent deformation) and add significant fatigue damage without being able to detect it via NDI/NDT/NDE. The very next flight within normal structural limits could initiate a fatigue crack. Conversely, you could fly another aircraft within limits for 30,000 hours with no significant fatigue damage accumulation.

Fatigue damage is a very specialized field because some of the principles are counter intuitive.

Dave Rothenagger (Callsign R+10) wrote an excellent article for the T-34 association that was recently reprinted in the EAA WOA magazine about fatigue life cycles incured during mock BFM and ACM. If you haven't read Dave's article be sure to find a copy of the magazine and avail yourself to this very detailed explaination of the stresses incured during these types of operations. Dave is a T-34 owner and flies Vipers (F-16s) with the Indiana National Guard.

I got a goofy idea; We all pitch in and buy a brand new attach angle from Lance, and have brandon put it in a stress testing apparatus. That way we can determine the life span of the part. Actually we could buy two, and cycle one for occasional aerobatics, and cycle the other for heavy aerobatics.

Annnddd; As Bela suggested check out the DC-3 ADs, and see if any nasty things have happened to them.

there are 2 wing attach angle ADs on the DC-3, here they are :
63-23-01 DOUGLAS: Amdt. 638 as amended by Amendment 39-973.

Applies to all DC-3 series aircraft including military type C-41,
C-41A, C-47, C-47A, C-47B, C-48, C-48A, C-49, C-49A, C-49B, C-49C,
C-49D, C-49J, C-49K, C-50, C-50A, C-50B, C-50C, C-50D, C-51, C-52,
C-52A, C-52B, C-52C, C-53, C-53B, C-53C, C-53D, C-68, C-117A and R4D
series except R4D-8 aircraft, certificated in all categories.

Compliance required as indicated.

(a) Unless already accomplished within the last 400 hours' time in
service after the next 50 hours' time in service after the effective
date of this AD, inspect the wing upper attach angles on both the
outer wing and center section between the front and rear spars for
cracks. Use at least a 4-power magnifying glass. Reinspect at periods
not to exceed 450 hours' time in service from the last inspection.
Replace cracked attach angles before further flight.

(b) At the next wing removal, inspect all wings in the manner
specified in (c), and reinspect at intervals not to exceed 8,000
hours thereafter.

(c) At the time specified in (b), the removed wings shall be
thoroughly cleaned and inspected as follows:

(1) Inspect the center and outer wing upper attach angle doubler for
cracks along the radius of the bent-up flange at the wing attachment
point. Conduct the inspection with a 6-power magnifying glass or dye
penetrant.

(2) Inspect the center and outer wing upper attach angles including
the areas between the attaching bolt holes for evidence of cracks.
Remove all paint to permit inspection with a 6-power magnifying glass
or by dye penetrant.

(d) Replace, before further flight, any cracked parts found during
the inspection set forth in paragraph (c) with new parts per Douglas
Service Bulletin DC-3 No. 262, Reissue No. 1 June 14, 1963, or later
FAA-approved revisions.

(e) The proper installation alignment of the attach angles and
doublers described in (d) shall be maintained. This shall be
accomplished by the use of satisfactory jigs or by FAA approved
equivalent means. Douglas jig fixtures P/N's A652-5110506-1-1F2 and
A652-5110506-1F2 or P/N's C652-5110500-101-1-1F1 and C652-
5110500-101-1F1 or those that meet the criteria of Advisory Circular
AC 39-1, are considered to be satisfactory for alignment purposes.

(f) The parts replaced in accordance with paragraph (d) shall be
inspected in accordance with the requirements of paragraphs (b) and
(c). All uncracked parts which are not replaced shall continue to be
inspected per the inspection procedure of (c) with adherence to the
previously established inspection schedules for these parts.

(g) Whenever wings are being replaced after modification per (d);
whenever spar butt plates on the center and outer wing, the
compression angles on the center wing or the waffle plates on the
outer wing are being reworked or replaced; or whenever one outer wing
is substituted for another, the following tolerances shall be
maintained:

(1) Compression angles attached to the corrugations and stringers and
the spar cap butt plates of the center wing must be held in plane
with the wrap around doublers on the attach angle to within plus
0.004 inch/minus 0.000 inch.

(2) Waffle plates attached to the stringers and the spar cap butt
plates of the outer wing must be held in plane with the wrap around
doublers on the attach angle to within plus 0.006 inch/minus 0.000
inch.

NOTE-The tolerances ranges set forth in (g)(1) and (g)(2) will allow
a flush to 0.010 interference between the compression angles and
plates when the wing is installed. This interference fit assures the
most effective distribution of loads across the joint and the maximum
service life.

(Douglas Service Bulletin No. 262, Reissue No. 1, June 14, 1963, or
later FAA-approved revisions, covers this same subject.)

Amendment 638 effective December 6, 1963. This Amendment (39-973)
becomes effective April 16, 1970.

66-18-02 DOUGLAS: Amdt. 39-264 as amended by Amendment 39-972.

Applies to All DC-3 series aircraft including Military Type C-41,
C-41A, C-47, C-47A, C-47B, C-48, C-48A, C-49, C-49A, C-49B, C-49C,
C-49D, C-49J, C-49K, C-50, C-50A, C-50B, C-50C, C-50D, C-51, C-52,
C-52A, C-52B, C-52C, C-53, C-53B, C-53C, C-53D, C-68, C-117A and R4D
series except R4D-8 aircraft, certificated in all categories.

Compliance required as indicated.

(a) Unless already accomplished within the last 400 hours' time in
service, within 50 hours' time in service after March 15, 1963,
inspect the wing lower attach angles of both the outer wing and
center section between the front and rear spars for cracks between
the attach bolt holes. Use at least a 4-power magnifying glass.
Reinspect at intervals not to exceed 450 hours' time in service from
the last inspection.

(b) Unless already accomplished in accordance with AD 39-24-01 and AD
52-22-03, or AD 63-04-01, (or AD 63-04-01 as amended by AD Card No.
63-20), the following modifications must be accomplished within 500
hours time in service after March 15, 1963, except that airplanes
being presented for an initial airworthiness certificate must be so
modified before that certificate will be issued:

(1) All outer wing lower surface doublers, P/N's 570602-206 and -207
or P/N's 5115200-206 and - 207 shall be replaced with new doublers
fabricated from 0.072 material in the manner described in Douglas
Service Bulletin DC-3 No. 262, reissued June 14, 1963.

(2) All outer wing lower surface doublers, P/N's 570602-208 and -209
or P/N's 5115200-208 and - 209 shall be replaced with new doublers
fabricated from 0.064 material in the manner described in Douglas
Service Bulletin DC-3 No. 262, reissued June 14, 1963.

(3) All outer wing lower surface attach angles shall be replaced with
new angles fabricated for proper installation with the new heavier
doublers in the manner described in Douglas Service Bulletin DC-3 No.
262, reissued June 14, 1963, except that aircraft modified to
incorporate doublers and attach angles described in Douglas Service
Bulletins No. 220 and No. 146, respectively, are satisfactory until
parts must again be replaced per (d).

(4) The wing center section lower surface shall be modified by
incorporating witness holes, installing new wrap around doublers
attached to the wing skin and new attach angles as described in
Douglas Service Bulletin DC-3 No. 262, reissued June 14, 1963, except
that modifications made prior to the effective date of this amendment
in accordance with Douglas Drawing 5406787 "D", "E" or "F" are
satisfactory until parts must again be replaced.

(5) Certain airplanes have been reworked in accordance with Service
Sketch 624, which incorporates a lower center wing attach angle
doubler, between front and center spars, which is 3/8 inch wider than
standard in order to accomplish Service Bulletin No. 262. This rework
is satisfactory. In such cases, rework in accordance with Service
Sketch 624 must be accomplished at each subsequent replacement of the
wing attach angles and doublers.

(6) The proper installation alignment of the attach angles and
doublers described in (b)(1), (2), (3), and (4) shall be maintained.
This shall be accomplished by the use of satisfactory jigs or by
FAA-approved equivalent means. Douglas jig fixtures P/N's
A652-5110506-1-1F2 and A652-5110506-1F2 or P/N's C652-
5110500-101-1-1F1 and C652-5110500-101-1F1 or those that meet the
criteria of Advisory Circular AC 39-1, are considered to be
satisfactory for alignment purposes.

(c) After the accumulation of 7,500 hours' but before 8,000 hours'
time in service and after the accumulation of 11,500 but before
12,000 hours' time in service following any modification prescribed
by (b) the outer wings shall be removed, affected areas must be
thoroughly cleaned and inspections must be conducted as follows:

(1) Inspect the center wing skin and outer wing attach angle doublers
for cracks along the radius of the bent-up flange at the attachment
joint. Conduct the inspections with at least a 6-power magnifying
glass or by dye penetrant;

(2) Inspect the center and the outer wing attach angles including the
areas between the attaching bolt holes for evidence of cracks. Remove
all paint to permit inspection with at least a 6-power magnifying
glass or by dye penetrant; and

(3) Make visual inspections through the witness holes in the center
section at the front and center spars for evidence of cracks.

(d) The modifications specified in (b) shall be reaccomplished within
each 16,000 hours' of service after the original modification unless
an inspection per (c) reveals no cracks and the reinspections are
conducted at intervals not to exceed 2,000 hours' of service.

(e) If the maintenance records cannot verify the time in service
since the modifications prescribed in (b), one of the following must
be accomplished:

(1) Accomplish the modifications at the time and in the manner
prescribed in (b); or

(2) Conduct the inspections prescribed in (c) within 500 hours' time
in service after March 15, 1963, and thereafter reinspect per (c) at
each 2,000 hours' time in service until the modifications per (b) are
again accomplished.

(f) Any cracked parts found during the inspections required by (a),
(c) and (d) shall be replaced with new parts before further flight.
These new parts shall then be inspected within the inspection periods
specified in (a) and (c). All uncracked parts that are not replaced
will continue to be inspected at the periods previously established
in accordance with (a) and (c). Uncracked parts that are not replaced
after 16,000 hours' time in service shall be reinspected at intervals
not to exceed 2,000 hours' time in service.

(g) Whenever wings are being replaced after modification per (b);
whenever spar butt plates on the center and outer wing, the
compression angles on the center wing or the waffle plates on the
outer wing are reworked or replaced; or whenever one outer wing is
substituted for another, the following tolerances shall be
maintained:

(1) Compression angles attached to corrugation and stringers and the
spar cap butt plates of the center wing must be held in plane with
the wrap around doubler on the attach angle to a plus 0.004
inch/minus 0.000 inch.

(2) The waffle plates attached to the stringers and the spar cap butt
plates of the outer wing must be held in plane with the wrap around
doubler on the attach angle to a plus 0.006 inch/minus 0.000 inch.

(3) Tolerances of a maximum of 0.010 inch interference to a maximum
of 0.010 inch clearance between the compression angles and waffle
plates and between the spar cap butt plates, are acceptable if
previously accomplished per AD 58-12-01. The tolerances set forth in
(g)(1) and (2) shall be used for all attach angle installations
subsequent to the effective date of this AD.

NOTE: These new tolerances will allow a flush to a 0.010 inch
interference between the compression angles and plates when the wing
is installed. This interference fit assures the most effective
distribution of loads across the joint and the maximum service life.

(h) Upon request of the operator, an FAA maintenance inspector,
subject to the prior approval of the Chief, Aircraft Engineering
Division, FAA Western Region, may adjust the repetitive inspection
intervals specified in this AD to permit compliance at an established
inspection period of the operator if the request contains
substantiating data to justify the increase for such operator.

(i) FAA-approved revisions to Douglas Service Bulletin DC-3 No. 262,
reissued June 14, 1963, are also considered satisfactory in complying
with this AD.

This supersedes AD 65-27-02. Amendment 39-264 effective July 26,
1966. This amendment (39-972) becomes effective April 16, 1970.

This would help us to determine a life based upon some highly controlled conditions. I think the problem is that we really don't know what stress levels the T-6s flying ACM are operating at. It depends on the instructor, the pilot, and the previous (unknown) history of the aircraft.

The DC-3 case is similar, yet quite different. I don't think that there is any suggestion that the DC-3s are being operated outside their limitations. This is why the cracks grow slowly enough that you can space the inspections so far apart.

Thanks for posting that Matt (the DC-3 AD). Makes
for interesting reading.

Next question:

Has an outer wing panel ever failed on a DC-3 in flight?

I realize the operational considerations are completely
different than the T-6 (nobody in their right mind would
loop, roll and/or hammer a DC-3 airframe with excessive
Gs), but the design is quite similar...

Since I'm writing, on all the castings I've restored so far
for the SNJ, a 10x glass either showed the part was in
pristine condition or not. Upon stripping layer upon layer
of paint, the non-usable stuff is/was instantly identifiable
as such with the naked eye (no need for a 10x glass
there!).

Bela P. Havasreti