NDT in Aircraft Maintenance

What can go wrong in service

Once an aircraft enters service, three kinds of damage dominate the inspection effort. Fatigue cracking grows from fastener holes, fittings and joints under repeated loads, especially pressurization cycles. Corrosion attacks aluminum structure in hidden places such as lap joints, floor beams and wheel wells. Accidental damage — hail, bird strikes, ground-equipment dings, lightning, and on composite structure, impact damage that may leave little visible trace — can happen on any flight or on the ramp.

The case that changed the industry is Aloha Airlines Flight 243. On April 28, 1988, a Boeing 737-200 flying from Hilo to Honolulu suffered an explosive decompression and structural failure at 24,000 feet, losing about 18 feet of cabin skin and structure above the floor line. The crew diverted and landed on Maui. A flight attendant was swept overboard and presumed killed, and eight other people were seriously injured. The National Transportation Safety Board determined that the probable cause was the failure of the airline's maintenance program to detect significant disbonding and fatigue damage, which led to the failure of a fuselage lap joint and the separation of the upper fuselage. Among the contributing factors it cited were the airline management's failure to supervise the maintenance program and to assess its inspection and quality control deficiencies (report NTSB/AAR-89/03). The accident became a defining case for the industry's attention to aging aircraft and to the reliability of inspection, including how human factors affect inspectors searching large areas of structure for small cracks.

Where NDT is used

• Eddy current testing (ET) is the workhorse of aircraft NDT. Surface probes find cracks around fasteners and in fittings, bolt-hole probes examine holes after fasteners are removed, and low-frequency techniques look for cracks and corrosion in lower layers of structure. Many wheel manufacturers also call for eddy current inspection of wheel halves, typically when the tire is changed.
• Ultrasonic testing (UT) measures remaining thickness where corrosion is suspected, finds cracks in thick sections and fittings, and is a primary method for disbonds and delaminations in composite and bonded structure.
• Visual inspection (VT) — general, detailed and special detailed, often with borescopes, mirrors and magnifiers — is the foundation of every inspection program. Engine borescope inspections look for damage to compressor and turbine blades without disassembly.
• Liquid penetrant (PT) and magnetic particle (MT) testing are used on removed parts in the shop: landing gear components, engine parts, actuators and fittings.
• Radiography (RT) finds water ingress in honeycomb panels and some hidden damage where access is difficult.
• Bond testing and thermography help assess composite and bonded structure after impact or repair.

How inspection fits into maintenance

Aircraft inspection is organized around a maintenance program, not around individual defects. For transport aircraft, the program is built from the manufacturer's maintenance planning data and the operator's own continuing airworthiness program, and it schedules inspections by flight hours, cycles and calendar time. Heavy checks open up structure that is otherwise inaccessible, and many NDT tasks are timed to them.

Specific NDT tasks come from several sources. The manufacturer's NDT manual gives the approved procedure for each task, down to probe type and calibration standard. Service bulletins recommend inspections as fleet experience accumulates. When the regulator finds an unsafe condition likely to exist in other aircraft, it issues an airworthiness directive (AD), which makes the inspection mandatory, often with repeat intervals and a required method. Damage-tolerance-based inspections for older aircraft and repairs close the loop on fatigue.

Findings are recorded, dispositioned against the structural repair manual or engineering, repaired and re-inspected. Good records let an operator see trends across a fleet, which is how many ADs and service bulletins start in the first place.

Regulations and standards

• FAA regulations in 14 CFR govern maintenance (Part 43), mechanics and inspection authorization (Part 65), repair stations (Part 145) and airworthiness directives (Part 39). FAA Advisory Circular 43.13-1B includes a chapter on nondestructive inspection and notes that the effectiveness of any NDI method depends on the skill, experience and training of the person performing it.
• EASA rules govern maintenance organizations in Europe, and EN 4179 covers NDT personnel qualification there.
• NAS410 (Level 1, Level 2 and Level 3) is widely used for NDT personnel certification in aerospace maintenance, and ATA Specification 105 provides airline-oriented guidelines for training and qualifying NDT personnel. The FAA's own guidance points to recognized standards of this kind.

Which personnel standard applies depends on the operator's or repair station's program and on the contracts and regulators it answers to. Our companion article on A&P mechanics, IAs and NDT goes further into who may do what.

What this means for operators and owners

For operators, NDT is only as good as the program around it. Are NDT tasks performed to the current revision of the manufacturer's procedure? Are reference standards and instruments calibrated and traceable? Are inspectors certified in the method and technique they are using, and are their certifications and eye examinations current on the day they sign? Are AD inspections tracked so repeat intervals are never missed?

For owners of business and general aviation aircraft, the questions are similar in miniature. When an AD or manufacturer's inspection calls for eddy current or ultrasonic inspection, make sure it is performed by someone qualified in that method, and keep the report with the aircraft records. A signed logbook entry is only as trustworthy as the inspection behind it.

How NDT LLC can help

NDT LLC helps airlines, repair stations and maintenance providers build NDT programs, write written practices and procedures, qualify and train personnel under NAS410 and related standards, and prepare for regulatory and customer audits. Get in touch to discuss your maintenance NDT program.