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Emergency Inspections Ordered

Fri, Apr 8, 2011 — David Evans

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The FAA has ordered that all B737 Classics undergo intensive structural inspections over the next 20 days, but the number of aircraft with cracks need not be reported to the agency. Given the emergency nature of the directive, one would think the FAA would insist on knowing the extent of fracturing in the fleet.

Significant Regulatory & Related Activity

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5 April 2011                 Federal Aviation Administration (FAA)

AD # 2011-08-51

Emergency Airworthiness Directive to Owners & Operators of Boeing Model 737-300, -400 & -500 Airplanes

This action is based on the fuselage cracking 1 April 2011 which caused rapid decompression and a hasty landing in Yuma, AZ. (See Aviation Safety Journal, “Skin Rupture Explodes Myth That Safety Is Under Control”)

The AD calls for eddy-current inspections of so-called “Classic” model B737s and repairing any cracks that are found. An eddy-current inspection is one of a variety of non-destructive testing (NDT) techniques for assessing metallic structures. Using a hand-held electronic probe against the skin of an aircraft, very small cracks can be detected in or near the surface of the material.

Eddy current inspection probe.

In the case of this directive, technicians can use the eddy-current probe on the outside surface of the fuselage. Or, should operators desire, the probe can be applied to the inside surface of the skin. To do this, interior wall and ceiling panels and thermal acoustic insulation batts must be removed to expose the skin. Since the internal test involves considerable time and expense to remove and then replace the interior components, the vast majority of operators will doubtless perform the exterior test as the preferred survey method.

However, if any cracking is detected by the external method, eddy-current testing must also be performed from the interior of the fuselage.

Any cracks detected must be repaired before further flight.

The tests must be performed within 20 days of receipt of the AD, or before the airplane has accumulated 30,000 flight cycles. A flight cycle is a take off and landing, whereby the fuselage is fully pressurized and then depressurized as the airplane prepares for landing.

If the airplane has already accumulated 35,000 flights, the inspection must be completed within 5 days of receipt of the AD.

After the initial test, it must be repeated every 500 flights

There is no requirement to report results of the testing to the FAA. One would think the FAA would have an intense interest in the findings.

Boeing must be contacted for crack repair instructions. The AD refers to Boeing Alert Service Bulletin 737-53A1319 of 4 April 2010 for the area to be inspected.

Neither the number of aircraft nor the cost of the inspections is contained in the AD. However, eddy-current inspections are manpower intensive. The number of B737-300s affected is on the order of 762 airplanes worldwide. Of these, 196 are in U.S. registry, of which the vast majority – 169 – are operated by Southwest Airlines. Add in the -400 and -500 models, and the total increases.

Southwest has already inspected nearly 80 aircraft. Measurable cracks were found on four aircraft, according to the airline.

The B737-300 has a design service life of 60,000 flight cycles. The accident airplane had accumulated some 39,000 cycles. As such, it was not covered by the FAA’s Supplemental Structural Inspection Program (SSIP). The SSIP is intended to prevent a repeat of the Aloha Airlines accident of 1988, when the ceiling of one of its 22-year old B737-200’s peeled off in flight.

The SSIP takes effect when an airplane has accumulated 75% of its design service goal (DSG). For the B737-300, that point would be 45,000 flight cycles. Since the accident airplane had accumulated only 39,000 flights, or approximately 85% of the 45,000 flight threshold, it was not covered by the SSIP. Thus, even though the airplane had undergone an overhaul (D Check) are year ago, it is doubtful that the area now mandated for NDP had even been visually inspected.

Southwest is one of the better airlines in submitting service difficulty reports (SDRs). It has filed at least eight such reports with the FAA documenting that cracks have been found at less than the 45,000 cycle limit (at 75% of DSG). According to the SDR submissions, airplanes have been discovered at Southwest with cracks at 31,000 cycles, 43,000 cycles, and everywhere in between. Since NDT is not required for aircraft that have accumulated less than 45,000 cycles, it is assumed that cracks on these aircraft were detected visually.

Here is a typical SDR submitted by Southwest on a B737-300 with 35,000 cycles:

“DURING SCHEDULED 1/4D CHECK, FOUND FUSELAGE SKIN AT BS 727E + 13, STRINGER 23L, CRACKED. REPAIRED SKIN PER SRM.”

It is not known if this premature cracking situation was passed by the FAA to Boeing. A Boeing engineer in a 5 April conference call to reporters said the manufacturer had no requirement for inspection of the structure until the airplanes had accumulated 60,000 cycles (the 45,000 cycle threshold was established by the FAA for the SSIP). The SDRs indicated that cracking can occur at half that number of cycles.

Boeing is now hastily moving to modify its recommended inspection threshold.

When designed, the B737 Classics were subjected to only one lifetime’s worth of fatigue testing. A test airframe is subjected to the pressurization/depressurization of a flight in an accelerated ground test to determine if the structure will hold together for the duration of the DSG.

The so-called “Next Generation” B737s and all newer Boeing aircraft are subjected to two lifetimes worth of fatigue testing to assure their durability.

FAA head Randy Babbitt said in a 4 April statement, “The FAA has comprehensive programs in place to protect commercial aircraft from structural damage as they age.” What he didn’t say is that this 15-year old aircraft was not covered by the SSIP and, in the absence of the lap joint blowing out, would not have been subjected to any special structural inspections for at least four more years of service.

Four outcomes are possible:

1. Lap joints will be subject to intense visual inspections from 30,000 cycles onward.

2. The SSIP will be expanded to cover all aircraft at the 50% point of the design service goal (or 30,000 flights in the case of B737 Classics).

3. Internal NDT will be required from 30,000 flights onward.

4. FAA enforcement of SDR submission discipline will be tightened. Reporting among airlines remains highly variable. The eight SDRs referred to above may represent only partial reporting fleet-wide of premature cracking. Absent good, consistent SDR reporting from all airlines, fuselage blowouts as occurred at Southwest will continue to be a surprise.

What should also happen, but probably won’t, is an in-depth reconsideration of outsourced maintenance (of which Southwest is a notable practitioner). Unlicensed mechanics perform a substantial part of the outsourced work, with little or no supervision by licensed mechanics. An unlicensed mechanic may not recognize the significance of a missing rivet or a tiny hairline cracks at a lap joint.


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