Date:	Saturday 10 July 2021
Time:	12:54
Type:	Beechcraft C90 King Air
Owner/operator:	Falcon Executive Aviation
Registration:	N3688P
MSN:	LJ-915
Year of manufacture:	1980
Total airframe hrs:	17126 hours
Engine model:	U/A CANADA PT6A SERIES
Fatalities:	Fatalities: 2 / Occupants: 2
Other fatalities:	0
Aircraft damage:	Destroyed
Category:	Accident
Location:	near Wikieup, AZ -   United States of America
Phase:	Manoeuvring (airshow, firefighting, ag.ops.)
Nature:	Fire fighting
Departure airport:	Marana Regional Airport, AZ (AVW/KAVQ)
Destination airport:	Marana Regional Airport, AZ (AVW/KAVQ)
Investigating agency:	NTSB
Confidence Rating:	Accident investigation report completed and information captured

Narrative:
On July 10, 2021, about 1254 mountain standard time, a Beech C-90, turbo prop airplane, N3688P, was destroyed when it was involved in an accident near Wikieup, Arizona. The pilot and Air Tactical Group supervisor were fatally injured. The airplane was operated as a public use firefighting aircraft in support of the Bureau of Land Management conducting aerial reconnaissance and supervision.

The pilot was conducting a firefighting support flight, with an Air Tactical Group Supervisor on board the airplane. Automatic Dependent Surveillance Broadcast (ADS-B) data indicated that the airplane was on station for about 45 minutes completing multiple orbits over the area of the fire. The last ADS-B data point showed the airplane in a descent, and its airspeed was about 151 knots at an altitude of about 2,300 ft agl.

According to a witness, the airplane descended in a steep dive and impacted the side of a ridgeline in mountainous desert terrain. No distress call from the airplane was overheard on the radio. Another witness observed the outboard left wing falling to the ground after the aircraft had impacted the terrain. The outboard left wing, which had separated outboard of the engine nacelle, was located about 0.79 miles northeast of the main wreckage and did not sustain thermal damage.

During a scheduled maintenance inspection several months before the accident, eddy current (EC) non-destructive testing (NDT) of a left wing’s lower forward spar cap detected a crack in a fastener hole. The hole was then oversized/reamed to a larger size, but the EC reinspection still produced a crack indication. The operator then submitted a structural damage report and service request detailing the crack indication to the aircraft manufacturer. The aircraft manufacturer responded to the operator that the crack indication necessitated the replacement of “the center section forward spar cap, center section forward lower fittings and both outboard main spar assemblies.”

However, the operator and their maintenance provider elected to repair the wing spar instead of replacing the spars, as indicated by the aircraft manufacturer. The maintenance facility owner contacted a Federal Aviation Administration (FAA) Designated Engineering Representative (DER) for the design of the repair. The repair involved oversizing the affected fastener hole and installing an external doubler around the hole location. The repair was installed and signed off several months before the accident. The DER claimed no knowledge of the communication between the aircraft manufacturer and the maintenance provider about the crack indication, though the maintenance provider claimed otherwise.

After the repair, an eddy current inspection conducted by a commercial NDT inspector showed the wing spar repair to be successful and did not reveal a crack indication. Further, there were no other crack indications on the airplane.

A postaccident examination of the spar fracture surface revealed that the left wing’s lower spar cap fractured from a fatigue crack that initiated at the aft inboard fastener hole. The fatigue crack measured 2.484 inches in length and exhibited striations consistent with crack propagation. A study comparing the crack length. striations, flight hours, and number of cycles suggests the crack was large enough to have been seen visually at the last inspection. Therefore, it is likely that the NDT inspector omitted the EC inspection of the fastener hole or missed the fatigue crack indication given its length.

The fatigue separation of the lower spar cap was not in the same area where the repair was accomplished. The repaired area was inboard of where the left wing separated. The DER approved repaired area was not identified in the recovered wreckage and therefore could not be examined. Nevertheless, the area of the wing spar fatigue crack would have been removed from the airplane if the airplane’s operator and maintenance provider had followed the procedure outlined by the aircraft manufacturer, which noted that the crack indication necessitated the replacement of the spars.

Probable Cause: The failure and separation of the left wing’s outboard section due to a fatigue crack in the lower spar cap. Contributing to the accident was the operator’s decision to repair the wing spar instead of replacing it as recommended by the aircraft manufacturer. Also contributing to the accident was the failure of the Non-Destructive Testing inspector to detect the fatigue crack during inspection.

https://www.seattletimes.com/nation-world/nation/2-firefighters-die-as-plane-goes-down-while-surveying-blaze/
https://www.azfamily.com/news/2-dead-in-plane-crash-near-wikieup/article_f4a7642a-e1f1-11eb-a16b-774558ec6aba.html
https://edition.cnn.com/2021/07/11/us/arizona-cedar-basin-fire-fatal-accident/index.html
https://fireaviation.com/2021/07/10/two-aerial-firefighters-killed-in-crash-of-air-attack-aircraft-in-arizona/
https://eu.azcentral.com/story/news/local/arizona-wildfires/2021/07/13/new-details-arizona-plane-crash-killing-2-arizona-wildfire-officials/7942189002/
https://aerossurance.com/air-accidents-incidents/in-flight-break-up-of-king-air-c90-after-wing-spar-repaired-against-manufacturers-advice/

https://registry.faa.gov/aircraftinquiry/Search/NNumberResult?nNumberTxt=3688P
https://flightaware.com/live/flight/N3688P/history/20210710/1850Z/KAVQ/L%2034.72902%20-113.32084/tracklog
https://globe.adsbexchange.com/?icao=a42d82&lat=33.435&lon=-112.840&zoom=8.4&showTrace=2021-07-10×tamp=1625944328

https://cdn.jetphotos.com/full/2/84795_1240545649.jpg (photo)

In-Flight Break-up of King Air C90: wing spar repaired against OEM advice then failed from fatigue cracking elsewhere that was missed during the same inspection. https://t.co/cqd6n7xuRp #airworthiness #aviationmaintenance #firefighting #flightsafety #aviationsafety pic.twitter.com/NObLzweXQc

— Aerossurance (@Aerossurance) November 26, 2023

Date/time	Contributor	Updates
11-Jul-2021 05:34	Geno	Added
11-Jul-2021 05:43	Captain Adam	Updated [Aircraft type, Embed code, ]
11-Jul-2021 06:23	Aerossurance	Updated [Location, Embed code, Damage, Narrative, ]
11-Jul-2021 06:27	Aerossurance	Updated [Source, ]
11-Jul-2021 09:30	Aerossurance	Updated [Embed code, ]
11-Jul-2021 09:32	Aerossurance	Updated [Location, Embed code, Narrative, ]
12-Jul-2021 08:34	aaronwk	Updated [Registration, Cn, Operator, Departure airport, Source, ]
12-Jul-2021 08:37	harro	Updated [Source, ]
12-Jul-2021 09:54	RobertMB	Updated [Nature, Source, Narrative, ]
12-Jul-2021 14:49	Anon.	Updated [Time, Nature, Source, Category, ]
12-Jul-2021 14:49	harro	Updated [Departure airport, Source, Narrative, ]
14-Jul-2021 10:42	Iceman 29	Updated [Source, Embed code, ]
14-Jul-2021 10:43	Iceman 29	Updated [Source, Embed code, ]
28-Jul-2021 06:27	aaronwk	Updated [Time, Source, Narrative, Category, ]
24-Aug-2023 12:06	Captain Adam	Updated
25-Nov-2023 17:44	Aerossurance	Updated [Source, ]
26-Nov-2023 10:26	Aerossurance	Updated [Source, ]
26-Nov-2023 10:42	Aerossurance	Updated [Embed code, ]