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 specifications.” AppK.5

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 truss. AppK.9
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 thereafter. AppK.12

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 some concern
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 illustrated.
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 these inspections.
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. AppK.17

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). The
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. Jourdan.

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. 1950): 4.
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 1, 1975.
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