Daytona Beach, FL, USA
N106ER
PIPER PA28R
An airline transport pilot and a private pilot were performing commercial pilot checkride maneuvers, which included a touch-and-go landing. After the landing and during the climb, the airplane's left wing separated near the wing root and the airplane collided with terrain. The airplane's radar-derived ascent and heading profile following the touch-and-go maneuver was consistent with the airplane maneuvering in a manner that wouldn't reasonably be expected to produce airframe loading that would result in an in-flight structural failure. The airplane was operating under the maximum allowable gross weight and within its center-of-gravity limits at the time of the accident. According to automatic-dependent surveillance broadcast data, the airplane's radar target was last recorded at an altitude of 900 ft mean sea level (msl) and a groundspeed of 80 knots. Witnesses within 1/4 mile of the accident site reported that the airplane climbed "normally" on a westerly heading when they saw the wing separate from the fuselage. Metallurgical examination of the accident airplane's left-wing main spar lower cap found that it exhibited fracture features consistent with fatigue through more than 90% of the cross-section, reducing its residual strength capabilities almost completely. The examination identified three fatigue cracks in the left-wing main spar lower cap (see figure 1). One fatigue crack initiated near the lower forward corner of the outboard forward wing attachment bolt hole (designated as bolt hole LC-1) and propagated forward to the forward edge of the cap. A second fatigue crack initiated near the lower aft corner of bolt hole LC-1 and propagated aft toward the lower aft wing attachment bolt hole (designated as bolt hole LD-1) and up the cap web almost to the edges. A third fatigue crack initiated near the lower aft corner of bolt hole LD-1 and propagated aft almost to the aft edge of the cap. Fatigue cracks also originated in the forward and aft doublers from both sides of the bolt holes (see figure 2). Figure 1. Close view of the fracture surfaces in the left-wing main spar lower cap. Unlabeled arrows indicate fatigue origin areas and dashed lines indicate approximate fatigue boundaries Figure 2. Close views of the fracture surfaces in the aft (left) and forward (right) doublers. Unlabeled arrows indicate fatigue origin areas and dashed lines indicate approximate fatigue boundaries A small fatigue crack was also identified in the accident airplane's right-wing main spar lower cap; however, that crack had not progressed to the point of failure. Examination of the left-wing main spar of another Piper PA-28R-201 owned and operated by the same operator as the accident airplane; manufactured in the same year with a serial number two previous to the accident airplane; and with similar hours, cycles, and operational exposure found a similar fatigue crack that initiated near the lower forward corner of hole LC-1. The accident airplane was manufactured about 10 years before the accident and had been used solely for flight instructor and commercial pilot training by the flight school operator. The airplane had accumulated 7,690.6 hours and 33,276 landing cycles before the accident flight, which equates to 4.33 landing cycles per hour of flight time. According to operator personnel, the commercial flight lesson times averaged 1.4 to 1.8 hours; most of that time the airplane was being operated in the airport traffic pattern, performing takeoffs, landings, and power-off 180º maneuvers. While in the airport traffic pattern, the airplane operated between 1,000 ft agl and ground level. On longer flights, necessitating the need to operate in the practice area, maneuvering typically took place between 2,500 and 3,000 ft msl before returning to the airport. A review of the operator's airframe discrepancy log for the life of the airplane and flight crew safety reporting system for the 24 months preceding the accident found some reported flap extension overspeed, gear extension overspeed, and hard landing events. In each case, the events resulted in airframe examinations during which no defects were noted by maintenance personnel. Interviews with flight instructors did not yield any safety of flight issues or critical airframe or loading exceedances caused by pilot operation that would be expected to precipitate cracking. The operator's maintenance program alternated between 50-hour and annual inspections that included airframe manufacturer-recommended inspection items. The annual inspection detailed in the airframe manufacturer's maintenance manual includes a specific line item to inspect the wing attachment bolts and brackets that was included in the operator's inspection. There were no specific instructions from the airplane manufacturer or additional steps from the operator for a detailed inspection of the wing main spar lower caps in the area where the fracture occurred. A review of the maintenance logbook for the entire life of the airplane found no indications of airplane damage, major repairs, or significant nonstandard maintenance concerning the wing structure. About 31 years before this flight training accident, in response to a pipeline patrol accident in which the left wing separated from a PA-28-181 airplane, the airframe manufacturer issued Service Bulletin (SB) 886 recommending a visual and dye penetrant inspection of the wing main spar on certain PA-28, PA-28R, and PA-32 model airplanes. The initial and recurrent inspection intervals recommended by the SB were calculated based on an accounting of the affected airplanes' lifetime usage, which the SB categorized into four usage classes: normal, severe, extreme, and unknown. Based on the SB criteria for the normal usage class, which included normal flight training operations, the accident airplane was not due for an initial inspection for another 23,000 flight hours (the SB recommended an initial inspection of the wing main spar at 30,600 hours). Considering the absence of anomalies in materials or construction of the examined wing spars from N106ER and N104ER, the disparity between the failure time of the accident wing spar and the inspection threshold time in the SB likely derives from the differences between normal usage (as analyzed after the pipeline patrol accident that prompted the SB) and the actual flight training usage experienced by the accident airplane. The Piper PA-28R-201 and other similar airplanes were type certificated under Part 3 of the Civil Air Regulations, which only required that design details avoid stress concentrations; thus, this airplane model and others similar to it was not subject to the more robust fatigue analysis requirements under Title 14 Code of Federal Regulations (CFR) Part 23. In 2005, the FAA published Advisory Circular (AC) 23-13A with information and guidance for performing fatigue, fail-safe, and damage tolerance evaluations of metallic airplane structures developed under Part 23 regulations. AC 23-13A provides flight and ground load information for various types of flight operations. AC 23-13A includes detailed information on the flight gust and maneuver loads, load factors for taxi movements, and landing sink rate data that can be used to determine flight and ground loads for the various types of operations for single-engine airplanes. These flight and ground loads drive the aging and fatigue of metallic airplane structures. The NTSB's investigation referenced this information to evaluate the flight and ground loads that the accident airplane was likely exposed to over its operational life in the flight training environment. This evaluation found that the maneuver loads experienced by the accident airplane were likely similar to the maneuver loads for basic instruction published in AC 23-13A, which are slightly higher than for personal use. The gust loads in the accident airplane flight training environment could also be higher than in the personal use environment (in the personal use environment airplanes are typically operated at higher altitudes, and gust loads are generally more severe and frequent at lower altitudes for any given airspeed). However, the gust loads for basic instruction are presented as identical to personal use in AC 23-13A. Loads associated with taxi movements for the accident airplane were likely similar to those of other operations other than agricultural application as shown in AC 23-13A. Landing impact loads for the accident airplane were likely higher and more frequent than other types of operations. Sink rates for basic instruction use as published in AC 23-13A are higher than that of other uses including the pipeline survey use and normal use analyzed for SB 886, and higher sink rates correspond to higher landing impact loads. The NTSB concludes that, due to flight training maneuvers, significant operation at low altitudes, and frequent landing cycles, the accident airplane (and its sister airplane in the operator's fleet) likely experienced landing, gust, and maneuver loads that were more severe than expected for training aircraft when SB 886 was developed. Therefore, the low-altitude flight training and frequent landing environment likely resulted in the accident airplane accumulating damaging stress cycles at a faster rate than a personal use airplane. After the accident, 16 airplanes from 4 flight schools (including ERAU) were examined using eddy-current inspection of the outboard attachment holes. Times in service ranged from 2,777.5 to 10,301.5 hours, and estimated landing cycles ranged from 8,841 to 39,000 cycles. Among the inspected airplanes, a crack was found in one wing of ERAU airplane N104ER, which had a time in service of 7,660.7 hours and 33,288 landing cycles. No additional cracks were detected in the remaining airplanes, including four airplanes with higher estimated landing cycles. However, due to the limited sample size and the expected variability in fatigue crack initiation times, the absence of detected cracks in airplanes with more cycles and/or hours than N106ER and 104ER does not eliminate the risk of fatigue cracking due to loads associated with a more severe training environment. The location of the fatigue cracking observed on the accident airplane's left-wing main spar lower cap would have prevented visible detection from the interior or exterior of the airplane. Evidence of the presence of a crack would only have been visible after cracking had begun in the doubler and grown past the bolt head to a significant length. The only reliable method to detect the fatigue cracking in the wing main spar, as installed, would have been a nondestructive inspection procedure, such as a high frequency eddy current bolt hole inspection. The FAA issued a notice of proposed rulemaking 8 months after the accident proposing to require such an inspection for all Piper PA-28 series airplanes, except the PA-28-201T and PA-28-236 model airplanes; all Piper PA-28R model airplanes; and all Piper PA-32-260 and PA-32-300 model airplanes. In the NTSB's February 15, 2019, comments to the FAA, we expressed our support of the proposed AD's inspection requirements but urged the FAA to reexamine the proposed AD's applicability to certain airplanes based on airplane usage. As of the date of this report, the FAA has not published an AD for inspection of the accident airplane type and other similar types, especially those operated in the flight training environment.
HISTORY OF FLIGHTOn April 4, 2018, about 0953 eastern daylight time, a Piper PA-28R-201, N106ER, collided with terrain following an in-flight separation of the left wing near the wing root during climb after a touch-and-go maneuver at Daytona Beach International Airport (DAB), Daytona Beach, Florida. The airline transport pilot and private pilot were fatally injured, and the airplane was destroyed. The airplane was registered to and operated by Embry-Riddle Aeronautical University (ERAU) under the provisions of Title 14 Code of Federal Regulations (CFR) Part 91 as an instructional flight. Day visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed for the local flight. The private pilot was conducting a practical test to obtain a commercial pilot certificate with an airplane single-engine land rating, and the airline transport pilot was acting as a designated pilot examiner (DPE). Radar and voice communication data provided by the Federal Aviation Administration (FAA) and automatic-dependent surveillance broadcast (ADS-B) data provided by the operator revealed that the private pilot began the training flight by departing runway 25L about 0927. The airplane then turned southbound and climbed to about 3,000 ft mean sea level (msl) at a groundspeed of 100 knots. The airplane flew about 14 miles to the south of DAB, and several climbing and descending turns occurred between 1,300 and 3,000 ft. About 0940 the airplane leveled off at an altitude about 2,200 ft and airspeed of 112 knots on an easterly ground track before turning northbound toward DAB. The airplane tracked parallel to the coastline about 5 miles inland at 2,200 ft and 115 knots before descending gradually to enter the DAB traffic pattern. About 0940, the private pilot contacted the DAB south arrival radar controller and advised he was inbound to DAB for "closed traffic" (that is, he stated his intention to perform successive operations in the airport traffic pattern) and had automatic terminal information service "Yankee." The controller issued a transponder squawk code, which the private pilot acknowledged. About 1 minute later, the controller informed the pilot that the airplane was in radar contact and assigned the pilot the ROSE 25 arrival. (The ROSE 25 arrival is used by local aircraft inbound to DAB from the south.) At 0945:00, the pilot advised the DAB tower controller that he was on the ROSE 25 arrival. The DAB tower controller instructed the pilot to follow a Cessna Skyhawk and to maintain 1,500 ft, which the pilot acknowledged. About 0947, the tower controller cleared the pilot to descend to the traffic pattern altitude and join the downwind leg while continuing to follow the Cessna. About 0948, the DAB tower controller cleared the pilot to land on runway 25, which the pilot acknowledged, stating "we're uh cleared for uh closed traffic." The DAB tower controller responded that the pilot was "cleared for the option"—that is, at the pilot's discretion— which the pilot acknowledged. The pilot subsequently performed a touch-and-go maneuver on runway 25. During the airplane's initial climb after the touch-and-go maneuver, the DAB tower controller advised the pilot at 0951:44 to continue upwind until advised when to turn crosswind. At 0952:28, the controller instructed the pilot to squawk one two zero four, which the pilot read back. At 0952:47, the pilot asked the DAB tower controller, "…can we turn crosswind." The controller replied, ",,,negative, continue upwind," which was not acknowledged. There were no further communications from the accident airplane. According to ADS-B data, the airplane's radar target was lost about 0953; it was last recorded about 2 miles beyond the departure end of runway 25L at an altitude of 900 ft and a groundspeed of 80 knots. Witnesses within 1/4 mile of the accident site reported the airplane climbed "normally" on a westerly heading when they saw a wing separate from the fuselage. The wing appeared to "float" down and the airplane descended in a steep spiral before it impacted a field. PERSONNEL INFORMATIONThe pilot, age 25, held a private pilot certificate with ratings for airplane single-engine land and instrument airplane. He was issued an FAA second-class airman medical certificate on June 17, 2016, with no limitations. Review of the pilot's logbook revealed that he had accrued 218.2 total hours of flight experience of which 26 hours were in the accident airplane make and model. According to FAA records, the DPE, age 61, held an airline transport pilot certificate with ratings for airplane-single engine land and airplane multiengine land. He held a flight instructor certificate with ratings for airplane single-engine, airplane multiengine, and instrument airplane. The DPE was issued an FAA second-class medical certificate on April 5, 2017. He reported 27,600 hours total hours of flight experience of which 400 hours were flown during the 6 months before receiving the medical certificate. The DPE's logbooks were not recovered. AIRCRAFT INFORMATIONThe PA-28R-201 airplane, commonly referred to as the "Arrow," has been widely used by flight schools for complex airplane (meaning an airplane that has a retractable landing gear, flaps, and a controllable pitch propeller) flight training. According to FAA airworthiness and operator records, the accident airplane (serial number 2844137) was manufactured on September 17, 2007, and was issued a standard airworthiness certificate in the normal category. It was a single-engine, low-wing, four-place airplane equipped with a 200-horsepower, Lycoming IO-360-C1C6 four-cylinder engine that drove a McCauley two-blade, constant-speed propeller. ERAU purchased the airplane new (with 5.8 hours accrued) from Piper on September 25, 2007, and subsequently sold it to a leasing company, AVN Air, LLC, on September 28, 2007. ERAU leased the airplane from AVN until September 4, 2015, when it purchased the airplane back from the lessor. The airplane registration number was changed from N712ER to N106ER on July 10, 2014. ERAU was the only operator for the entire operational life of the airplane, which had only been used for flight instructor and commercial pilot training and was never used for initial flight training. The airframe had accumulated 7,690.6 hours of operation before the accident flight, and 28.3 hours since its most recent annual inspection, which was completed on March 21, 2018. Before the accident flight, the total landing cycles were 33,276 based on ERAU information documented in its education and training administration program. Maintenance Examination of the airframe maintenance logbook for the life of the accident airplane found that the airframe was inspected every 50 hours alternating between the ERAU 50-hour and ERAU annual inspection. The accident airplane's most recent inspection was an annual inspection completed on March 22, 2018, at an airframe time of 7,662.3 hours. Between March 28 and April 3, 2018, 12 flight instructors operated the accident airplane, and none reported any anomalies or flight operational safety issues with the airplane. One instructor noted "some side load on one landing" in the discrepancy log. There were no reported hard landings during this period, and all maneuvers were reported as "normal." The ERAU annual inspection contained all the provisions in the Piper recommended annual inspection with some items added by ERAU. According to chapter 5 of the Piper Airplane Maintenance Manual (AMM), the 100-hour inspection was a complete inspection of the airplane and was identical in scope to an annual inspection. The 100-hour inspection was divided into several major groups, including a wing group. Item 13 in the wing group stated, "inspect wing spar to fuselage attachment bolts and brackets" and indicated a recommended interval of 100 hours. According to ERAU, the accomplishment of this item involved removing the small plastic covers over the main spar attach bolts on the lower wing surface to ensure the torque striping on the nuts was not broken. The ERAU inspection would not normally look at the head of the bolts inside the center section even though the airplane interior would be removed from the airplane. All inspection items for the annual inspection were endorsed by a mechanic and/or inspector, including the wing spar-to-fuselage attachment bolts and brackets. There were no discrepancies pertaining to the wing spars. Examination of an airworthiness directive (AD) compliance list for the airplane revealed that all applicable ADs were in compliance. The most recent 50-hour inspection occurred on February 27, 2018, at an airframe time of 7,613.4 hours. All the items in the inspection list were endorsed by a mechanic. There were no discrepancies pertaining to the wing spars. Procedures for a detailed inspection of the wing spars were included in section 57-10-00 of the AMM. The procedures defined airplane usage classes of normal, severe, extreme, and unknown. The normal usage class encompassed most aircraft and included airplanes operated in normal flight training operations, such as the accident airplane. The recommended inspection involved removing the wings from the airplane, visually inspecting the main spar lower caps with a 10-power magnifying glass and performing a dye-penetrant inspection of the spar caps. The special inspections section of the AMM (5-30-00) recommended for airplanes in the normal usage class that the wing spar inspection in 57-10-00 be performed beginning at 30,600 hours, and every 3,000 hours thereafter. The accident airplane's airframe maintenance logbook indicated four entries for hard landing inspections, dated February 17, August 24, November 11, and November 14, 2011. These four dates corresponded with hard landing entries in the airplane's discrepancy log. The discrepancy log also indicated 14 additional hard landing events including five hard landing events in the 24 months preceding the accident. In each case, maintenance personnel conducted an inspection in accordance with chapter 5-50-00 of the AMM, section 3, which provides instructions for unscheduled maintenance checks. For reported hard landings, these procedures included inspecting the landing gear, attach points, wheel wells, and fuel tank for damage, as well as internal and external inspections of the wings. These procedures also called for inspecting for popped, cracked, and loose rivets, the wing attach bolts for slippage, damage, and overstress, and the wing skin for wrinkles. Access plates should be removed to inspect for internal damage to ribs, stringers and sparwebs. For each of the hard landing reports, resulting maintenance action determined that no defects were noted with any of the inspection items. A logbook entry dated July 15, 2014, stated that the airplane was stripped and repainted coincident with the registration change to N106ER. Logbook entries on March 31, 2015, and January 9, February 13, April 25, and July 11, 2017, detailed maintenance to replace rivet nuts, covers, or screws on the lower wing main spar bolt covers. No indications of airplane damage, major repairs, or other significant nonstandard maintenance were noted in the logbook. Wing Main Spar Each wing main spar on a Piper-28R-201 airplane is attached to the center wing box with 8 attachment bolts through the main spar upper cap and 10 through the main spar lower cap. As shown in figure 1, the locations on the lower spar cap forward of the spar web are identified as C-1 through C-5, and the locations on the main spar lower cap aft of the spar web are identified as D-1 through D-5 (later in this report, we use L [for the left wing] or R [for the right wing] in addition to the spar cap location identifiers where appropriate [for example, LC-1]). Doublers are riveted to the forward and aft sides of the spar at the attachment location and outboard beyond the bend in the spar that forms the wing dihedral. Flanges for the doublers extend over the main spar upper and lower caps at the forward side of the spar and over the lower cap at the aft side of the spar. Courtesy of Piper Aircraft, Inc. Arrow Wing Installation Drawings Figure 1. Diagram showing forward and aft side of wing main spars and associated attachment bolt designator map Weight and Balance The airplane was weighed after manufacture and had not been weighed since. The most recent calculated weight and balance report for the airplane listed an empty weight of 1,842.14 lb with a CG position at 86.28 inches. Airplane weight and balance computations and some performance data for the accident flight were recovered at the scene. The airplane's maximum gross takeoff weight (GTW) was indicated as 2,750 lb; the GTW, including occupants, baggage, and fuel, was indicated as 2,524.71 lb, which was 225.29 lb under maximum GTW and the CG was within limits at 86.94 inches. Previous In-flight Wing Separations on Piper PA-28 Series Airplanes On March 30, 1987, about 1257 central standard time, a Piper PA-28-181, N8191V, was destroyed when it collided with the ground following an in-flight left-wing separation while in low-level cruise flight near Marlin, Texas (NTSB case number FTW87FA088). The airplane was performing a pipeline patrol flight when the accident occurred. The investigation found fatigue cracking in the left-wing main spar lower cap near attachment bolt hole LC-1. The fatigue cracking initiated on the lower surface of the main spar lower cap and intersected the outboard edge of attachment bolt hole LC-1. The fatigue cracking had progressed from the hole forward through the forward flange and aft about halfway through the main spar lower cap. On April 10, 1987, the NTSB issued Urgent Safety Recommendations A-87-40 through -42 to the FAA as a result of our preliminary evaluations from the investigation. The recommendations asked for an AD to immediately inspect the main spar lower caps on certain PA-28-series airplanes (A-87-40), a recurrent inspection interval (A-87-41), and a study to determine other Piper airplanes with a similar spar design (A-87-42). The recommendations were respectively classified Closed—Acceptable Action, Closed—Unacceptable Action, and Closed—Acceptable Alternate Action between November 1988 and October 1989. See FAA Actions in Response to Wing Separations in the Additional Information section for more information. On August 24, 1993, at 2234 eastern daylight time, a Piper PA-28-181, N2093A, was destroyed when it collided with a vehicle and a tree following an in-flight right-wing separation while circling at low level near Provincetown, Massachusetts (NTSB case number NYC93FA140). The airplane was maneuvering in clouds after the non-instrument-rated pilot entered instrument meteorological conditions after takeoff. The investigation found fatigue cracking in the right-wing main spar lower cap near attachment bolt hole RC-1. The fatigue cracking initiated on the lower surface of the main spar lower cap slightly aft and outboard of the edge of hole RC-1 but did not intersect the hole. Several other cracks were noted in the lower surface of the spar cap parallel to the fatigue zone near both attachment bolt holes RC-1 and RD-1. The investigation also found that the nuts used on the wing attachment bolts were incorrect. METEOROLOGICAL INFORMATIONAbout 0953, reported weather at DAB indicated wind from 260° at 7 knots, 10 statute miles (sm) visibility, and few clouds at 25,000 ft. The temperature was 24°C, the dew point was 19°C, and the altimeter setting was 30.03 inches of mercury. There were no current turbulence or in-flight weather advisories for the area. AIRPORT INFORMATIONThe PA-28R-201 airplane, commonly referred to as the "Arrow," has been widely used by flight schools for complex airplane (meaning an airplane that has a retractable landing gear, flaps, and a controllable pitch propeller) flight training. According to FAA airworthiness and operator records, the accident airplane (serial number 2844137) was manufactured on September 17, 2007, and was issued a standard airworthiness certificate in the normal category. It was a single-engine, low-wing, four-place airplane equipped with a 200
Extensive fatigue cracking in the left-wing main spar lower cap and doublers, which resulted in the in-flight separation of the left wing. The fatigue cracks initiated and grew to a critical size due to flight and ground loads associated with flight-training involving flight-training maneuvers, significant operation at low altitudes and frequent landing cycles. Previously established inspection criteria were insufficient to detect the fatigue crack before it grew to a critical size.
Source: NTSB Aviation Accident Database
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