Boeing 737's Horizontal Stabilizer Must Be Scrutinized

FAA: This AD is effective May 20, 2013.

We are adopting a new airworthiness directive (AD) for all The Boeing Company Model 737-600, -700, -700C, -800, -900, and -900ER series airplanes. This AD was prompted by reports of an incorrect procedure used to apply the wear and corrosion protective surface coating to attach pins of the horizontal stabilizer rear spar. This AD requires inspecting to determine the part number of the attach pins of the horizontal stabilizer rear spar, and replacing certain attach pins with new, improved attach pins. We are issuing this AD to prevent premature failure of the attach pins, which could cause reduced structural integrity of the horizontal stabilizer to fuselage attachment, resulting in loss of control of the airplane.

 

We estimate that this AD affects 1,050 airplanes of U.S. registry.

We estimate the following costs to comply with this AD:

Estimated Costs
Action	Labor cost	Parts cost	Cost per product	Cost on U.S. operators
Inspection and attach pin replacement	39 work-hours × $85 per hour = $3,315	Up to $6,312	$9,627	Up to $10,108,350.

According to the manufacturer, some of the costs of this AD may be covered under warranty, thereby reducing the cost impact on affected individuals. We do not control warranty coverage for affected individuals. As a result, we have included all costs in our cost estimate.

 Applicability

(1) This AD applies to all The Boeing Company Model 737-600, -700, -700C, -800, -900, and -900ER series airplanes, certificated in any category.

(2) Installation of Supplemental Type Certificate (STC) ST00830SE (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgstc.nsf/0/408E012E008616A7862578880060456C?OpenDocument&Highlight=st00830se) does not affect the ability to accomplish the actions required by this AD. Therefore, for airplanes on which STC ST00830SE is installed, a “change in product” alternative method of compliance (AMOC) approval request is not necessary to comply with the requirements of 14 CFR 39.17. For all other AMOC requests, the operator must request approval for an AMOC in accordance with the procedures specified in paragraph (k) of this AD.

 

Unsafe Condition

This AD was prompted by reports of an incorrect procedure used to apply the wear and corrosion protective surface coating to attach pins of the horizontal stabilizer rear spar. We are issuing this AD to prevent premature failure of the attach pins, which could cause reduced structural integrity of the horizontal stabilizer to fuselage attachment, resulting in loss of control of the airplane.

Part Number (P/N) Inspection

For airplanes having line numbers 1 through 3534 inclusive: Before the accumulation of 56,000 total flight cycles, or within 3,000 flight cycles after the effective date of this AD, whichever occurs later, inspect to determine the part number of the attach pins of the horizontal stabilizer rear spar. A review of airplane maintenance records is acceptable in lieu of this inspection if the part number of the attach pin can be conclusively determined from that review.

 

Replacement

If, during the inspection required by paragraph (g) of this AD, any horizontal stabilizer rear spar attach pin has P/N 180A1612-3 or 180A1612-4, prior to the accumulation of 56,000 total flight cycles on the pin, or within 3,000 flight cycles after the effective date of this AD, whichever occurs later, replace with a new attach pin having P/N 180A1612-7 or 180A1612-8, respectively, in accordance with the Accomplishment Instructions of Boeing Special Attention Service Bulletin 737-55-1093, dated April 9, 2012.

 

 

 

Have You Been Confident of Corrosion Has Been Kept From Your Plane?

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"We want to make airplanes that fly like birds," said Fu-Kuo Chang, a scientist at Stanford University who developed the sensors and co-authored a recent article about the technology in the journal, Advanced Materials. "Aircraft that have all the sensing information about what is happening around them, just like birds do."

Aircraft could soon be covered in new technological cobwebs. Inspired by the gossamer strands of spider webs, scientists from Stanford University have created an ultra-fine mesh of strain and temperature sensors.

Wrapped around an aircraft, the sensors could help craft monitor their internal well-being. This added awareness could prevent microscopic cracks from developing into catastrophic failures. Beyond aircraft, the new technology could create a new breed of intelligent automobiles, packaging and medical devices.

Aircraft and birds both have various ways to sense their environment. Birds have eyes to see, ears to hear and mouths to speak (or sing). Aircraft have their own versions of these organs, such as radar, which gathers information about the physical environment, and radio, which allows them to communicate.

But aircraft lack nerves. Unlike birds, they don't have a way to sense tiny changes inside their bodies. For instance, a bird in a dive can sense, through its nerves and other tissues, whether the strain is too great and if they need to pull up before their bones break.

The new spider web-inspired mesh would give aircraft two new senses birds have had for millions of years: strain and temperature. The new mesh contains tiny structures that can, say, measure the temperature along the entire body of the aircraft, or map the air pressure flowing around a wing.

The new sensor is a plastic polymer that has the gold sensors laid down on top of it, which monitor the skin of the aircraft. The Stanford scientists are already developing technology that will allow pilots to image the interior of their aircraft similar to the way pregnant women can see their unborn children.

By adding ultrasonic wave-inducing piezoelectric devices, pilots could constantly scan the aircraft to discover, say, microscopic cracks in the supports long before they developed into life-threatening failures.

"This will help ensure the safety of air transportation," said Frank Chang, a scientist at the University of California, Los Angeles who is familiar with the research but is not involved in it.

To paper an entire aircraft with sensors would ordinarily add significant weight, and therefore require more fuel, something airlines are anxious to avoid. To get around this problem the California scientists stripped the sensors down to the bare minimum of material, eliminating 99.7 percent of it.

Spider web-like sensors that can detect touch and temperature in aircraft are just the beginning, say the scientists. The new sensors could eventually lead to smarter cars, wound dressings that tell doctors how quickly a patient is healing, shirts that allow pregnant women to see their unborn child whenever they want, or even synthetic skin for robots.

"This will have very extensive usage and importance," besides just aircraft, said UCLA's Chang.

CAUSES OF CORROSION

Corrosion is the destruction of metal by electrochemical reaction with its environment. Figure 1 illustrates some typical sources of the corrosion that affects airplanes. As shown in figure, three conditions must exist simultaneously for corrosion to take place:

The presence of an anode and a cathode. This occurs when two dissimilar metals or two regions of differential electrolyte concentration create a difference in electrical potential.

A metallic connector between the anode and cathode.

An electrolyte such as water.

Eliminating these three conditions in airplanes is restricted by practicality, functionality, and feasibility. Dissimilar metal contact cannot always be avoided because of weight, cost, and functional issues, but the potential for corrosion can be minimized by using surface treatments, plating, painting, and sealing. Water cannot be avoided, but it can be controlled with drain paths, drain holes, sealants, and corrosion-inhibiting compounds. Controlling the presence of water is usually the most effective means of preventing corrosion

Two of the most destructive forms of corrosion are stress corrosion cracking (SCC), also known as environmental assisted stress corrosion, and exfoliation corrosion. SCC occurs rapidly and follows the grain boundaries in aluminum alloys. Exfoliation corrosion also follows grain boundaries. It occurs in multiple planes, causing a leaf-like separation of the metal grain structure. Both forms of corrosion cause a loss of load-carrying capability. The most effective way to control this kind of corrosion is to use materials that are not susceptible to SCC at design stress levels or have a grain structure that is not susceptible to exfoliation.

An increasing number of operators are now providing ETOPS service to their passengers. For example, 76 percent of 767 operators and 42 percent of 757 operators are flying ETOPS routes. Several operators have discovered that the cost of ETOPS maintenance, compared to its benefits, also offers them a significant cost advantage when flying their non-ETOPS routes and when operating their non-ETOPS airplanes.

REFERENCE

DAVID BANIS
ENGINEER
MATERIALS TECHNOLOGY
BOEING COMMERCIAL AIRPLANES GROUP


J. ARTHUR MARCEAU
ENGINEER (RETIRED)
MATERIALS TECHNOLOGY
BOEING COMMERCIAL AIRPLANES GROUP


MICHAEL MOHAGHEGH
ENGINEER
STRUCTURES ENGINEERING
BOEING COMMERCIAL AIRPLANES GROUP


HARRY KINNISON, PH.D.
ETOPS MAINTENANCE PROGRAMS
MAINTENANCE AND GROUND OPERATIONS SYSTEMS
BOEING COMMERCIAL AIRPLANES GROUP