From the book:
Beech 18: A Civil & Military History by Robert Parmerter 2004
Appendix K Spar Problems
Among the most dubious records that the Beech 18 holds is having more
Airworthiness Directives (FAA requirement mandating immediate inspection or
modification) issued on it than on any other aircraft. AppK.1 A number of the
ADs involved a wing spar fatigue failure problem, which is described below.
The wing spar fatigue failure problem evolved over 20 years. In 1936, the
original Model 18A was designed as an aircraft of 6,500 lbs. gross weight and
640 hp, with a landing speed of 55 mph. During World War II military versions
were being operated at gross weights of 9,300 lbs. and 900 hp, with landing
speeds of 65 mph. Indications of fatigue or load problems first surfaced as
Beech Service Representatives, assigned to Army Air Base Training Schools, saw
evidence that the hard use AT-11s were being subjected to was taking a toll.
They reported that “the increase in gross weight along with an increase in
landing speed and sinking speed has naturally caused an increase in drag loads
into the truss through the drag leg.” AppK.2 Landing gear problems surfaced
first and dealing with them helped to reduce the loads transmitted to the center
section truss. The post-war D18S and D18C were designed with these problems in
mind and incorporated a new landing gear, heavier center section truss, but also
a higher gross weight.
The civilian spar failure problem surfaced when on December 6, 1947,
D18C-T NC80011, AA-3 crashed while on an aerial mail pick-up flight for All
American Aviation (see Chapter 11, AAA). It was at Wellsburg, WV and wing
spar fatigue failure was found on the aircraft that had a total of 2,324 flight
hours. The CAA issued an airworthiness directive AD 48-16-1 for D18C and
D18C-Ts models requiring an inspection, with a ten power magnifying glass,
looking for fatigue cracks at each 1,000-hour periodic inspection, in an area
“in or near the welds of the outer wing panel front spar lower root fitting.”
AppK.3 During the week of December 20, 1947, Beech service representatives
inspected the 20 D18C and D18C-T examples that were in service in the U.S. and
found no cracks. AppK.4 The one AAA D18C-T had crashed, five Empire Airlines
planes were out of service, and a kit with instructions were sent to Hawaiian
Airlines, to have them check its D18C-T.
Earl Stahl writing in the Summer 1994 American Aviation Historical Society
Journal, noted that “a record of conversations between Beech Aircraft and
Civil Aeronautics Authority engineers suggests the All American D18s were
specified by AAA and designed by Beech for a less demanding mode of operation
than they were ultimately subjected to in both test runs and routine service.
Beech engineering personnel contended it was understood pickups would be made at
130 mph, but they later found (as did CAA engineers) some pickups may have been
made at speeds as high as 200-plus mph. Further, they later estimated, operating
up to eight hours a day between 20 and 1,000 feet altitude, in commonly existing
mountain turbulence, the D18s were subjected to five to ten times as many gusts,
with two to three times the severity of air roughness encountered by average
transport aircraft. The planes manufacturer concluded high speed operations at
low altitude, and possibly, very high-speed mail pickups resulted in more
fatigue and/or loads than had been anticipated based on customer’s
However, Stahl adds, “Captain Harvey Thompson, who piloted all of AAA’s aircraft
types, including scheduled pickup runs in the Twin Beechs, found the D18s to be
responsive, pleasant planes to fly. He questioned the post-accident assertions
that cargo exchanges were actually made at higher than originally intended by
the airline. Normal pickup speed was 140-155 mph, and he believed some
demonstrations may have been made at 170 mph. When the aircraft were phased into
the pickup operation, Beech pilots were said to have flown along to observe the
planned mode of use, so they were believed to be well
aware of typical flying conditions.” AppK.6
In military use, there was a 1948 report from an unnamed foreign government
whose D18S airplanes were subjected to hard use as military trainers. They found
cracks in the center section truss above the lower slide tube socket forging.
AppK.7 Beech designed a bracket to weld on to the socket as a temporary fix and
began tests of a new forging with long legs, which would spread the load over a
wider area. Fatigue tests in 1949 of a truss with the new forging showed that
the service life was increased by about six times over the original D18S design.
The new forging was incorporated on all new commercial D18S models starting in
1949. Additional tests in 1949 used C18S aircraft from the CAA fleet modified
with oleo drag leg struts in place of solid struts and these showed a reduction
in shock and fatigue loads transmitted to the main truss. Oleo Drag Leg Struts
became an option on D18S models after c/n A-532 (September 1950). AppK.8 A CAA
AD, (AD 50-28-1) was issued requiring the oleo drag legs be installed on
any C18S or AT-11 aircraft, which had weld repairs made to the center section
The 1951 USAF contract for 900 C-45G/H models called for oleo drag legs to be
included, as did U.S. Navy SNB-5 production orders after 1950. The USAF issued a
technical order on September 1, 1954: Inspection and Reinforcement of Center
Section Spar on T-7, T-11, C-45A, C-45B & C-45F. It described the inspection
requirements and reinforcement actions in order to “prevent the failure of the
center section main spar truss” on the noted aircraft. It called for inspecting
the center section spar truss cluster welds with a magnifying glass. If cracks
in the cluster welds were found, one flight at minimum gross weight and in VFR
conditions was authorized to an Air Material Area Depot for modification. If no
cracks were found, it could continue in use “subject to daily inspections until
cracks were found or until the next 500 hour inspection,” at which time the
reinforcement would be carried out. AppK.10
On February 9, 1960, a former AAA aircraft, D18C-T NC80363 crashed in heavy
turbulence near Alpine, TX. The lower left wing spar failed from a fatigue
crack. On July 16, 1964, G18S N9423Y BA-569 of the Arizona Public Service
Company was flying in severe turbulence, near Toadlena, NM, when a wing
separated in flight and it crashed, killing all four aboard. AppK.11 A fatigue
failure of the center section left lower spar cap at the tip of the landing gear
outboard slide tube cluster gusset was found. As a result, three ADs
(64-21-1, 64 21-2 and 64-21-3) were issued in September 1964, all requiring
an X-ray or magnetic particle inspection of the center section lower spar cap
areas on each side of the landing gear truss cluster and at the side of the
fuselage, for aircraft having accumulated 1500 hours, and every 500 hours
The first AD applied to C18S and AT-11 models and also included a magnetic
particle inspection of truss clusters in the nacelle area. The second AD applied
to Super 18 models and the third AD was for D18S and C-45G/Hs, and included a
magnetic particle inspection of lower slide tube clusters at 1,000-hour
intervals. Aircraft, to which the first and third ADs applied, could have the
Beech shock absorbing oleo drag leg installed, in place of the standard
solid-type rear drag leg, and that would exempt them from the recurring X-ray or
magnetic particle inspections. If cracks in the inspection areas were found,
reinforcement plates could be carefully welded in place and this too could
exempt them from recurring inspections.
On December 17, 1964, D18S N57G suffered wing spar damage in turbulence, flying
an air taxi passenger flight near Muskegon, MI, but landed safely. There was
that the three recent ADs should also include a magnetic particle inspection of
the wing fitting area of the lower spar cap, so that was added as AD 66-13-1
on May 17, 1966.
On August 15, 1966, at Anchorage, AK, N445 C18S operated by Shaw Flight Service
crashed with four fatalities. A fatigue fracture next to the landing gear slide
tube cluster caused an inflight wing separation. The accident brief noted that
the aircraft had been properly maintained and that there were no weather
problems on the flight. App.K.13 On February 28, 1967, C-45H N830K crashed and
burned due to fatigue failure of the steel lower spar cap, eight inches outboard
of the wing hinge fittings, at Wilmington, DE. This prompted AD 67-8-2,
issued on March 11, 1967, which added an outboard wing panel inspection,
including X-rays of wing stations 102 and 111, lower spar cap, for all
Beechcraft 18s manufactured before D18S, c/n A-441 (July 1948). AppK.14 On April
28, 1967, at Acworth, GA, E18S N918X suffered a fatigue fracture in the lower
left wing spar and crashed. Concern grew since both this E18S and the Shaw C18S,
had recently complied with the inspection directives.
The National Transportation Safety Board (NTSB) began operations on April 1,
1967. Its first aviation recommendation was issued on May 5, 1967 as a result of
the Beechcraft 18 wing spar failures in the Anchorage, Wilmington and Acworth
accidents. It called on the FAA to consider the need to subject Beechcraft 18s
to more intensive inspections before further flight. The FAA agreed with that
recommendation and issued an AD the same day. The FAA grounded 2,200 U.S.
Beechcraft 18s (except for Model 18s with fewer than 1,500 hours and H18 models
above c/n BA-722). Owners were informed by telegram that AD 67-16-1 would
require both X-ray and magnetic particle inspections of the lower spar cap at
the inboard tips of the wing hinge fittings, on each side of the center line of
the nacelle and at the side of the fuselage. The National Transportation Safety
Board (NTSB) began operations on April 1, 1967. Its first aviation
recommendation was issued on May 5, 1967 as a result of the Beechcraft 18 wing
spar failures in the Anchorage, Wilmington and Acworth accidents. It called on
the FAA to consider the need to subject Beechcraft 18s to more intensive
inspections before further flight. The FAA agreed with that recommendation and
issued an AD the same day. The FAA grounded 2,200 U.S. Beechcraft 18s (except
for Model 18s with fewer than 1,500 hours and H18 models above c/n BA-722)
Owners were informed by telegram that AD 67-16-1 would require both X-ray
and magnetic particle inspections of the lower spar cap at the inboard tips of
the wing hinge fittings, on each side of the center line of the nacelle and at
the side of the fuselage. AppK.15 The 25 H18 models above c/n BA-722, were
exempt because they had the wall thickness of the lower spar cap increased from
.095 inches to .120 inches. In June, several amendments were issued to the AD,
allowing aircraft not yet inspected to operate an additional 50 hours with a 500
lbs. reduction in gross weight; and as long as they avoided turbulence. In
addition, the interval between repetitive inspections was increased from 200
hours to 500 hours.
In June 1968, the FAA issued an advance notice of proposed rule making,
involving an airworthiness directive that would require either inspection of the
whole lower spar cap or reinforcement of it with a spar strap kit however, no
such AD was issued. As X-rays were taken to satisfy the earlier ADs, problems
arose with the quality and the interpretation of the X-rays. Some owners found
that when the new X-rays that supposedly showed cracks, were compared with those
taken by Beech when the aircraft was new, there was no difference, there were no
cracks. On the other hand, “in many instances X-ray plates were made,
interpreted, and given to the owner or operator with the assurance that the
aircraft was airworthy. However, when the plates were evaluated by FAA
engineering personnel, or by Beech Aircraft Corporation, the plates showed that
cracks existed, or were
suspected, or the quality of the plates was such that a satisfactory
interpretation could not be made.” AppK.16 The FAA recognized those problems,
and ADs issued after 1972 showed that the FAA was paying attention to the
details of improving the accuracy of X-ray testing.
AD 71-11-5 was issued on May 28, 1971, and it required four X-ray and
magnetic particle inspection areas on the lower spar cap be added to those
required in AD 67-16-1. Additional inspection doors in the lower wing skin were
AD 72-8-5 was issued on April 13, 1972, which provided diagrams to
identify spar stations and inspection sites as well as adding X-ray procedures
and techniques. It also listed the approved spar straps and Beech repair kits.
On June 22, 1972, near Cleveland, OH, E18S N42A, on an air taxi cargo flight,
crashed when the left wing failed and folded up due to a fatigue crack.
AD 72-16-1 was issued on August 4, 1972, which required a surface
inspection of the lower spar cap at wing stations 73 and 81, within 25 hours,
and thereafter at 100-hour intervals. The wing now needed to be flexed during
AD 72-20-5 was issued on September 29, 1972, which required applying a
load to the wing during X-rays and flexing the wing during surface inspections.
On April 19, 1973, E18S N310WA flying for Air Iowa, crashed near Davenport, IA,
killing all six aboard. A pre-existing fatigue crack in the lower spar cap at
wing station 81 caused the failure of the right wing. AppK.18 AD 72-20-5
was amended on May 7, 1973 to require that at five stations on the front
spar lower cap of the center section, either Beech repair kits be installed or
an approved spar strap be used to reinforce the center section of the wing spar.
AppK.19 The spar straps that were approved were designed to reduce the stress in
the existing wing structure; and in the event of a failure in the existing
structure, to provide another load path so that the wing wouldn't fold. An
additional requirement was that the last two sets of X-rays had to be sent to
the FAA for audit, and a special surface inspection at wing stations 73 and 81
had to be done within 25 hours, and at 100-hour intervals.
AD 73-18-4 was issued on September 10, 1973, and required the reinforcement
or replacement of the outer wing spars within 24 months (by September 10, 1975).
pre-war Model 18s were also added to the AD. On October 16, 1973 near Thompson,
Manitoba, C-45H CF-OWU belonging to Ilford Riverton Airways had a wing fold up,
causing it to spiral down into a swamp, killing the pilot. AppK.20
AD 75-27-09 was issued on September 27, 1975, amended in January and May
1976, **and amended again in August 1980. It is the AD in effect in 2003, and
calls for all Beechcraft 18 models to have the center section spar and outer
wing spar X-rayed in about 30 different places at 1,500 hours, time in service,
and at each recurring 1,500 hours. In addition, visual and either magnetic
particle or penetrant inspections are required at multiple spots, at 11
different wing stations. These include places on the lower spar cap, such as
welds, splice plates, and gussets. The spar strap must be removed and an upward
force exerted on each wing during the X-rays. This AD also established a new
limited repair station rating, “Limited Airframe - Beech 18 Series Aircraft -
Wing and Center Section Spar X-ray Inspections,” needed in order to perform
X-ray inspection of the Beechcraft 18. AppK.21
Tom Buschke of Quality Assurance Industries has X-rayed over 650 different
Beechcraft 18s since 1963; some freighters as many as 15 times. He finds that
spar cracks are not a serious problem today, but that corrosion on the inside of
the lower spar is a concern. He is
especially concerned about corrosion that may be hidden in the spar of a 40-or
50-year-old Model 18 that has never been X-rayed, because it is still within the
1,500 hour limit. AppK.22
In 2003, the FAA is considering a new spar AD, which would change the 1,500 hour
“time in service” interval, to an interval based on a specific number of years
between X-ray inspections. A set number of years between manufacture and the
first required X-ray, is also being considered.
Spar Strap STCs
(for full details see Chapter 11 under each Strap Manufacturer)
Aerocon Spar Strap
STC (SA962EA) was issued in September 1973 for an Aerocon 73, fail-safe wing
spar kit to meet the requirements for a center section and outer wing panel spar
strap for any Beechcraft 18 model. It is still held in Canadian Aerocon’s name.
Aerospace Products Spar Straps
STC (SA1192WE) center section spar strap was originally granted to Aircraft Tank
Service, Burbank, CA, and was acquired by Aerospace Products Inc (API), North
Hollywood, CA. It also held STC (SA3021WE) for AT-11/SNB-1 models, (SA3010WE)
for C18S models, and kits (SA3009WE and SA3229WE), for the other post-war Model
18 types. In 2003, Vintage Aircraft bought the five API spar strap STCs.
Aircraft Tank Service Spar Strap
The first center section (only) spar strap reinforcement, STC
(SA1192WE), was granted before July 1966. Acquired by Aerospace Products Inc.,
and then in 2003, by Vintage Aircraft.
Airline Training Spar Strap
STC (SA814SO) was developed by Airline Training (ATI) for pre-war Model 18s,
wartime production models and post-war Model 18s. Now held by Aviation
Fabricators, Clinton, MO.
American Turbine Lifetime Spar,
STC (SA1533WE) center section spar strap was issued in May 1967. Obtained by
Hamilton Aircraft in March 1971.
Dee Howard Spar Straps
STC (SA832SW) issued on August 30, 1967. To strengthen the whole wing, STC
(SA895SW) for the outboard wing must also be installed. Both are now held by
Sierra Industries Inc., Uvalde, TX.
Hamilton Spar Straps
STC (SA1533WE) center section strap was obtained from ATE in March 1971. May be
used with outboard wing spar extension STC (SA2737WE), issued in August 1973.
Both still held by Hamilton Aviation. For AT-11s, Hamilton strap STCs (SA1581SW)
and (SA1582SW) were used and these are now held by Sierra Industries, Inc.,
Uvalde, TX. Hamilton STC (SA2000WE) was a full-span spar strap now held by John
L. Osborne of Osborne Tank and Supply, Victorville, CA.
Jourdan Spar Straps
STC (SA643CE) wing center section strap was concealed entirely inside of the
skin with no ridge across the bottom of the airplane. A later strap, STC
(SA1206CE), reinforced the outer wing spar. Both are still listed to Ray M.
Vintage Aircraft Spar Straps
The Stockton, CA, company bought the five API strap STCs in January 2003 (see
above). Owner Taigh Ramey notes that he currently has two kits available and
hopes to have more available in 2004. These straps are the only new straps that
are still available.
Appendix K Endnotes
AppK.1 “A.D. Logs: A Better Idea,” Private Pilot , March 1977: 52.
AppK.2 Paul E. Allen, “Summarized History of the Model 18 Beechcraft Truss and
Landing Gear Drag Leg,” 20 Oct. 1949 (Beech Inter-Office Communications): 2.
AppK.3 “Airworthiness Directive Summary,” CAA, 1 Jan.1949: 68.
AppK.4 “D18C Wing Inspection in Accordance with the Service Bulletin,” Beech
Aircraft Corporation, Dec. 1949.
AppK.5 Earl F. Stahl, “Treetop Mail Pt. II,” American Aviation Historical
Society Journal 39, 2 (Summer, 1994): 122.
AppK.6 Stahl, “Treetop Mail Pt. II,” 122.
AppK.7 Paul E. Allen, 4.
AppK.8 Paul E. Allen, 4.
AppK.9 “Model 18 Oleo Drag Leg,” Beechcraft Service News 4, 1 (Oct.
AppK.10 “USAF Technical Order TO 1C45-43, Inspection and Reinforcement of Center
Section Spar on T-7, T-11, C-45A, C-45B, and C-45F,” 1 Sept. 1954, 1.
AppK.11 Tom Warner, a former Beech salesman who was familiar with the Arizona
Public Service pilot, believes that this pilot was careless in his care of the
airplane. For example, the pilot would show off by taxiing in to the ramp, spin
the G18S around and then let momentum back the airplane into a parking spot. Tom
Warner, interview with the author, 14 Oct. 1994.
AppK.12 Different intervals between inspections applied to aircraft depending
on: the model, whether their landing weight was over or under 9,000 lbs., if
they had reinforcing plates welded onto the center section, or if they had oleo
drag legs installed.
AppK.13 On Jan. 5, 1967, Air Taxi Company D18S N2045D and its nine occupants
crashed at North Shrewsbury, NJ, following takeoff from Red Bank, NJ, when the
port engine failed (possibly due to carburetor icing) according to the NTSB
preliminary accident brief. However, Aviation Week & Space Technology
magazine’s May 15, 1967 issue, noted that the crash was being investigated for
the potential failure of the elliptical lower spar cap in the outboard wing
panel. However, the final NTSB report made no mention of spar or wing problems.
AppK.14 A brief March 20, 1967 telegram (in Raytheon vaulted archives) from
Brazilian authorities to Beech requested technical assistance following an
accident in which a wing folded on a Beechcraft 18 and all aboard were killed.
The rest of their Beechcraft 18 fleet (assumed to be the military fleet) was
grounded. The Brazilian Air Force bought 20 Super 18s (6 E18S and 14 H18 models)
in the previous 12 years but it also operated some older model 18s. Of the Super
18s, 19 can be accounted for by the time their service was over. Only E18S/U-45
FAB 2880 BA-266 is unaccounted for, but it’s not certain that it was the one
that crashed. No further information has been found.
AppK.15 “Beech 18 Grounding Disrupts Schedules,” Aviation Week & Space
Technology, 15 May 1967: 41.
AppK.16 “FAA Advisory Circular #145-2,” 21 Apr. 1976: 1.
AppK.17 It added station 61 to the surface inspection of the lower spar cap,
eliminated X-ray of wing station 43 and substituted magnetic particle or dye
penetrant methods at stations 43 to 45.
AppK.18 An account in “Used Airplane Guide,” Aviation Consumer, 15 Apr.
1981: 6-7, notes that this April 19, 1973 Air Iowa crash “was the critical event
that precipitated the big 1975 Airworthiness Directive” because of the death of
“several VIPs from Washington” who were on board. Among the six on board were
four ordinary citizens, the pilot and President of Air Iowa, Charles Nixon, and
William Hodgson, a member of the board of directors of Bandag Inc., an
international truck tire company based in Iowa. There were no Washington VIPs
involved. Quad-City Times, 20 Apr. 1973: clipping. AppK.19 For stations
73 and 81 the reinforcement kits must be installed within 600 hours, time in
service, from May 7, 1973, while for stations 32, 57 and 64 it must be done
within 2000 hours, by kits or center section spar strap, but no later than May
AppK.20 “Synopses of Aircraft Accidents, Civil Aircraft in Canada,” Aircraft
Accident Investigation Division, CA.1.2495, Box 1975 #5, #C30139.
AppK.21 “FAA Advisory Circular #145-2,” 21 April 1976: 1.
AppK.22 Tom Buschke, telephone interview by the author, 31 Apr. 2001.
copyright Robert Parmerter 2005