Non-Destructive Testing (NDT)

>> Friday, May 7, 2010

Nondestructive testing (NDT) are noninvasive techniques to determine the integrity of a material, component or structure or quantitatively measure some characteristic of an object. In contrast to destructive testing, NDT is an assessment without doing harm, stress or destroying the test object. The destruction of the test object usually makes destructive testing more costly and it is also inappropriate in many circumstances.

NDT is used in a wide range of industrial areas and is used at almost any stage in the production or life cycle of many components.
The mainstream applications are used in below:
1) aerospace,
2) power generation,
3) automotive, railway,
4) petrochemical and pipeline markets

NDT of welds is one of the most used applications. It is very difficult to weld or mold a solid object that has no risk of breaking in service, so testing at manufacture and during use is often essential.

Personnel Qualification
Personnel Qualification is an important aspect of non-destructive evaluation. NDT techniques rely heavily on human skill and knowledge for the correct assessment and interpretation of test results. Proper and adequate training and certification of NDT personnel is therefore a must to ensure that the capabilities of the techniques are fully exploited. There are a number of published international and regional standards covering the certification of competence of personnel. The EN 473 (Qualification and certification of NDT personnel - General Principles) was developed specifically for the European Union for which the SNT-TC-1A is the American equivalent.

NDT Methods

The main NDT methods are shown below:
1)Ultrasonic Testing (UT),
2)Radiographic Testing (RT),
3)Electromagnetic Testing (ET) in which Eddy Current Testing (ECT) is well know and
4)Acoustic Emission (AE or AET).

Besides the main NDT methods a lot of other NDT techniques are available, such as Shearography Holography, Microwave and many more and new methods are being constantly researched and developed.

NDT Applications in Commercial Aircraft Maintenance

During aircraft maintenance 'NONDESTRUCTWE TESTING' (NDT) is the most economical way of performing inspection and this is the only way of discovering defects. In simply we can say, NDT can detect cracks or any other irregularities in the airframe structure and engine components which are obviously not visible to the naked eye.

Structures & different assemblies of aircraft are made from various materials, such as aluminium alloy, steel, titanium and composite materials. To dismantle the aircraft in pieces and then examine each component would take a long time, so the NDT method and equipment selection must be fast and effective.
In the present trend of NDT application on aircraft 70-80% of NDT is performed on the airframe, structure, landing gears and the rest carried out on engine & related components.

In order to maintain the aircraft defects free and ensure a high degree of quality & reliability and as a part of inspection programme, usually following NDT methods are applied;

1) Liquid Penetrant
Liquid penetrant testing is one of the oldest of modern nondestructive testing methods & widely used in aircraft maintenance. Liquid penetrant testing can be defined as a physical & chemical nondestructive procedure designed to detect & expose surface connected discontinuities in 'nonporous' engineering materials.

Detection of surface detects or structural damage in all materials of aircraft. Fluorescent penetrants are used in critical areas for more sensitive evaluation.

2) Magnetic particle,
Magnetic particle testing is a sensetive method of nondestructive testing for surface breaking and some sub-surface discontinuation in 'ferro-magnetic' materials.
The testing method is based on the principle that magnetic flux in a magnetised object is locally distorted by the presence of discontinuity. This distortion causes some of the magnetic field to exit & re-enter the test object at the discontinuity. This phenomenon is called magnetic flux leakage. Flux leakage is capable of attracting finely divided particles of magnetic materials that in turn form an 'indication' of the discontinuity. Therefore, the test basically consists of three operations : a) Establish a suitable magnetic flux in the test object by circular or longitudinal magnetisation. b) Apply magnetic particles in dry powder of a liquid suspension; and c) Examine the test object under suitable lighting conditions for interpreting & evaluating the indications.

Fluorescent or black oxide particles in the aerosol cans are used during critical areas of aircraft structure/components inspection when using either permanent or electromagnets. Fluorescent particle inspection method is evaluated by black light (Black light consists of a 100 watt mercury vapour projection spot lamp equipped with a filter to transmit wave length between 3200 to 3800 Angstrom unit and absorb substantially all visible white light).

Simple in principle, easily portable. Fast and effective for surface & subsurface defects in ferromagnetic materials of any shape, removed from engines, pumps, landing gear, gear boxes, shafts, shock struts etc. Widely used for bolts inspection.

3) Eddy current
Eddy current tests are important test & widely used method within the broad field of Nondestructive materials & evaluation. This method is particularly well suited for the detection of service induced cracks usually caused either by fatigue or by stress corrosion. Eddy current inspection can be performed with a minimum of part preparation and a high degree of sensitivity.

Eddy current test is used to detect surface & subsurface defects, corrosion in aircraft structures, fastener holes and bolt holes. Surface detects and conductivity testing by high frequency and sub-surface detects by low frequency methods.

Routine eddy current inspection is carried out on aircraft under carriage wheel hubs for cracks also used to detect cracks in different tubes, tublar components of aircraft & engine.

4) Ultrasonic
Sound with a frequency above the limit of audibility is called 'ultrasonic'. It ranges with a frequency of 0.2 MHz to 800 MHz.
Ultrasonic inspection provides a sensitive method of nondestructive testing in most materials, metallic, nonmetallic, magnetic or nonmagnetic. It permits the detection of small flaws with only single surface accessibility and is capable of estimating location & size of the defect Providing both surfaces are parallel, ultrasonics may be used for thickness measurement, where only one surface is accessible. The effective result of an ultrasonic test is heavily dependent on subject surface condition, grain size & direction and acoustic impedance. Ultrasonic techniques are very widely used for the detection of internal defects in materials.

Ultrasonic inspection operates on the principle of 'transmitted' & 'reflected' sound wave. Sound has a constant velocity in a given substance; therefore, a change in the acoustical impedance of the material causes a change in the sound velocity at that point producing an echo. The distance of the acoustical impedance (flaw) can be determined if the velocity of the sound in the test material, and the time taken for the sound to reach & return from the flaw is known.

Used for detection of surface & subsurface detects in welds, forging, casting main structural fittings of landing gear legs & engine attachments. Bolts in critical areas, aircraft structure joints & pylon. Also checks adhesive bond quality of lap joints & composite structure. Used for thickness measurement after damage or corrosion removal

5) Radiography (x-ray/gama ray)
Radiography is one of the oldest and widely used nondestructive testing methods. A radiograph is a photographic record produced by the passage of electromagnetic radiation such as x-rays or gamma rays through an object onto a film. When film is exposed to x-rays, gamma rays or light an invisible change called a 'latent image' is produced in film emulsion. The areas so exposed become darker when the film is immersed in a developing solution. After development the film is rinsed to stop development. The film is next put into a fixing bath and then washed to remove the fixer. Finally dried so that it may handled for interpretation and record.

Considering the penetration and absorption capability of x-radiation, radiography is used to inspect a variety of nonmetallic parts; for porosity, water entrapment, crushed core, cracks and resin rich/straved conditions; and metallic products; such as welds, castings and forging as well as locating discontinuities in fabricated structural assemblies such as cracks, corrosion, inclusions, debris, loose fittings, rivets, out of round holes & thickness variations. Gamma ray radiography is usually used for detection of internal flaws of aircraft structure (steel & titanium) and engine components which require higher energy levels or other assemblies where access is difficult.

6) Visual/Optical
Visual inspection is probably the most widely used of all the nondestructive tests. It is simple, easy to apply, quickly carried out and usually low in cost. The basic principle used in visual inspection is to illuminate the test specimen with light and examine the specimen with the eye. In many instances aids are used to assist in the examination.

This method is mainly used i) to magnify defects which can not be detected by the unaided eye, ii) to assist in the inspection of defects and iii) to permit visual checks of areas not accessible to unaided eye.

Detection of surface defects or structural damage in all materials. Optical instruments are used for visual checks of internal areas and for deep holes and bores of aircraft structure, landing gears etc. Widely used to monitor engine components, such as, turbine wheels and nozzles, compressor vanes and blades combustion cans without opening the engine. 'Borescopes', 'fibrescopes' and 'video imagescopes' are most important optical aids in remote - visual inspection, which area is normally inaccessible.

7) Sonic/Resonance
Sonic and resonance testing methods are used primarily for the detection of separations between layers of laminated structures.
Sonic and Resonance testing is effective for detection of crushed core or debonds in adhesive bonded honeycomb, impact damage and delimitations in composite structures and exfoliation corrosion.

The tap test method has demonstrated the ability to detect cracks, corrosion, impact damage and debonding. The sonic testing instrument operate in the audio or near audio frequency range.

Resonance testing instruments may operate either or both the sonic or ultrasonic frequency range. Different methods of transmitting and receiving energy have been developed. Basically, each technique introduces a pressure wave into the specimen and then detects the resonant, transmitted or reflected wave.

To examine bonding exists between honeycomb, detect delaminations in composite laminates. Large structures such as, fairings, cowl and wing trailing edge, rudder, flaps, ailerons, elevators etc. are made from composites and honeycomb materials.
Tap testing is limited to detection of disbonds or voids between upperfacing sheet and adhesive. It will not detect disbond or voids at 2 nd or 3 rd layer bondlines, such as doubler areas. It is limited to the detection of delaminations, approximately 25 mm (1 inch) in dia or greater, located less than 1.3 mm (0.05 inch) below the surface being examined.

8) Infrared Thermography.
Infrared and thermal methods for nondestructive are based on the principle that heat flow in a material is altered by the presence of some types of anomalies. These changes in heat flow cause localized temperature differences in the material. The imaging or study of such thermal patterns is known as 'thermography'. The terms 'infrared' and 'thermal' are used interchangeably in some contexts. Thermal refers to the physical phenomenon of heat, involving the movement of molecules. Infrared (below the colour red) denotes radiation between the visible and microwave regions of the electromagnetic spectrum.

The intensity and frequency/wavelength of the radiation can be correlated closely with the heat of the radiator. it follows that radiation sensors can be used to tell us about the physical condition of the test object. This is the basis of the technology of 'thermography'.

Used to detect certain voids, inclusions, debonds, liquid ingress or contamination, foreign objects and damaged or broken structural assemblies. Infrared thermography also been chosen for quick operational use and the reliability of defection 'liquid contamination' in the composite sandwich in compared to x-ray method. Detection of
thermal overheating in electrical & hydraulic system.

Specially thermographic inspection on aircraft structures are carried out to detect following defects :
(i) Composite laminate parts - for delamination debonding or foreign objects
(ii) Composite sandwich parts - for debonding and liquid contamination.
(iii) Metallic bonded parts - for debonding of corrosion on.
(iv) Metallic sandwich parts - for liquid contamination, debonding of corrosion.


B727 & B737 Window Heat Controller

>> Sunday, May 2, 2010

A) Reason for the study

1)       B727/737 Window heat controller unit (WHCU) p/n 231-2 (alt p/n 65-52803-8, 83000-05602, 10-61833-2) was among the top 5 unscheduled component removal for 4 consequences months from Nov 2007 till Feb 2008.  These studies analyze common reasons of such failures from year 2005 till Nov 2007 to enhance its reliability.
2)       There are various P/Ns (various OEMs) of Window Heat Controller Unit (WHCU) installed on B727/B737. All P/Ns are fully interchangeable.
Boeing P/N
83000-05601 / -05602
BAE Systems
3)       There were 4 units of WHCU installed on 727 (and B737) airplane which for pilot and co-pilot No. 1 and No. 2 windows. The WHCU located at E5-1 electrical rack. It consists of temp controller which a solid state device that performs overheat control and temp control, overheat relay which direct 115V AC power from window heat CB to temp controller when energized and transformer which provide high voltage for heating window.



B) Data

1)       Repair and Findings Data from 1st Jan 2005 till 31st May 2008 were reviewed and has been classified into various types of common defect and shop findings; s/n and aircraft with repeated removals. Total 43 units were removed unscheduled with 30 DC and 13 DNC. None were scrapped or overhauled.

Defect Confirm
Defect Not Confirm
30 (69.8 %)
13 (30.2%)
2)       Unscheduled removals unit in 2005, 2006, 2007 and 2008 (till May) are shown below:
2008 (till May)
Removal Units
3)       MTBUR for B737 is 2935 hrs and for B727 is 4112 hrs. The average MTBUR is 3838 hrs. Design MTBUR for WHCU p/n 83000-05602 is 14046 hrs while MTBF is 29200 hrs.

C) Current Maintenance Program




Every ‘C’ chk (Card: C-102A-2)

Every ‘C’ chk            (Card: 72C1-E-2-4-016)

Inspect cabin control window anti-icing syst for components such as window heat power relay, anti-ice control panel, heat conductive coating, thermal switches and heat sensors for security, wiring condition and evidence of overheat.

Every ‘C’ chk (Card: C-103A-1)


Inspect window heat control unit (4 places) installed on the E3 rack for security of installation, condition of wiring, cleanliness and evidence of overheat / moisture.


D) OEM comments.

Email from Boeing and OEM are described below:

1)        Boeing via email dated April, 30 and May, 9 provided comments per below:

a)        Boeing has not received any reports from other operator similar to WHCU defects.
b)        Boeing provides current MTBF reported from Koito is 29200 hrs.
c)        Boeing has limited knowledge of the Astronics p/n 231-5 due to this is an STC mod and therefore the p/n is not reflected in Boeing IPC.
d)        There is no BAE p/n that equivalent to Boeing improved p/n 10-61833-6.
e)        Boeing recommends to upgrades WHCU to the newest Koito p/n 83000-05604 (Boeing p/n 10-61833-6). Boeing drawing provides data allowing this p/n to be installed to B727 airplane. The IPC rev July 2008 will reflect the p/n 83000-05604 usage on 727-200 airplanes.

2)    Comments from Astronics Advanced Eletronics Systems (previously known as General Dynamics or Olin or Pacific Electro Dynamics)

a)        OEM does not track the MTBUR and MTBF of Window heat controller unit p/n 231-2.
b)        231-5 is the latest WHCU p/n produce by Astronics. It is fully interchangeable with p/n 231-2 with an addition of BITE circuits.
c)        Suspect the latest mods (Mod L or M depending on the age of the unit) have not been  incorporated for serial number that have failures of parts in the output transistor section (Q26, 27, 28)
d)        Astronics has produced 16 SBs related to WHCU p/n 231-2. Refer attachment 7B for modification history from p/n 231-1 to 231-2 mod ‘M’.
e)        Astronics only do repair and re-certify origin WHCU p/n 231-x from astronics or all predecessor company names for astronics. 
f)          Astronics provide recommendation per below:
1.     Due to cooling air system accumulates dust and dirt which creates thermal stress on the unit, operator is recommended to review the maintenance chk task to inspect the cooling system and cleaning the dust and dirt in E&E compartment to keep dust from clogging the units.
2.     Operator to record mod level for each WHCU installed on the fleet.
3.     Any units had RV1-4 or F1 changed should have Q1 changed as well or there is risk of recurrence of the problem.

3)    Comments from Koito Mfg

a)        Design MTBUR for WHCU p/n 83000-05602 is 14046 hrs.

b)        Since most of the operators has already incorporated new WHCU p/n: 83000-05604, there is no news about p/n 83000-05602 recent few years.
c)        P/n 83000-05604 has a BITE function while -05602 has not. The other differences are the weight of unit. -05604 weight is 4.3 kg while -05602 is 3.7 kg.
d)        Koito recommended below:
1.     To upgrade p/n 83000-05602 to -05604 (Boeing pn 10-61833-6). Info: p/n 83000-05604 is interchangeable with -05602. P/n 83000-05602 is no longer produce by Koito.
2.     Send repair or overhaul’s WHCU to Koito repair station or Aviation Technical Services, Inc (formerly Goodrich ATS).
3.     All new purchase of Koito WHCU must be obtained from AAxico.

4)    Comments from Aero Technology

a)        No expected MTBUR and no recommended improved part number from repairer.
b)        Other operators do not have low time failure (LTF) for window heat controller p/n 231-2.
c)        Two units (S/n 478 & 6205) LTF was warranty denied.
1)        S/n 478 was repaired at 1st time visit.  Nil faults found for 2nd shop visit.  
2)        S/n 6205 found internal damaged due to excessive heat on Oct 07. 1 month later the unit was sent for repair and found different circuit board had failed. It appears that the second failure was also caused by an excessive heat.
d)        Unable to offer exchange with upgraded p/n due to no stock available.

E) Findings and Discussion.

1)       16 out of 43 WHCU unscheduled removals were caused by overheat. From the study, overheat will affect the window and cause the window to crack. 5 unscheduled removals due to CBs tripped can also cause window to overheat.  
Reason for Removals
Overheat / fail overheat test
Nil heating / inop
Nil control / nil indication / not regulating / not function
CB tripped
Window cracked / arching
Green light intermittent

2)       29 (67.4%) units were sent to Aero Tech for repair, 6 (14%) to Aero Instrument, Avborne 3, High Tech Avionic 1 and Aero Control Avionic and ST Aero 2 each. Most of units were sent to Aero Tech due to low flat rate compare to other vendor.

Aero Tech
Aero Instrument
Aero Control Avionic
ST Aero
High Tech Avionics
29 (67.4%)
6 (14%)
2 (4.65%)
3 (7%)
2 (4.65%)
1 (2.3%)

3)       Currently there were 57 units installed on the fleet which 47.4% was manufactured by Astronics, 33.3% by BAE system and 17.5% by Koito.
4)       The S/Ns show the repeated removals are 478 (4 removals), M01372 (3 removals), 662, 1607, 1634, 3718, 5126, 6205, 7142 and 7145 (2 removals each s/n).
5)       Most of the WHCU (47.4%) belongs to Astronic. However, Boeing has limited knowledge of the unit and cannot determine the replacement of part 231-2 with 231-5.
6)       All of Astronics p/n 231-2 was not upgraded to latest mod L or M.
7)       Koito unit is the most recommended p/n due to 50% of the unit is in good condition and only one was removed unscheduled for the last 1 year. Furthermore, Boeing only recognizes Koito p/n compared to Astronics or BAE p/n.
8)       BAE p/n 65-52803-8 had shown only 5 units removed unscheduled last year (2007). Compared to Astronics and Koito, BAE WHCU reported fewer problems. However, the OEM (BAE) is NOT contactable. 

F) Recommendations

1)       To advice repairer;  unit found with varistor (RV1-4) or fuse (F1) defect/damaged, transistor Q1 must be replaced to prevent from tripping and overheat shutdown
2)       To evaluate upgrading Koito WHCU p/n 83000-05602 to the newest p/n 83000-05604 (Boeing p/n 10-61833-6)
3)       S/n 7097, to incorporate mod K at next shop visit
4)       S/ns 662, 2749, 3486 and 7142 to incorporate latest mod L or M at next shop visit.
5)       Close monitoring for WHCU s/n: 478, 6205 and M01372 that have low time failure (LTF) last year. Scrapped or exchange for those s/n that have more than 3 LTF within 2 years (until June 2009).
6)       Should spares need to order extra WHCU, always purchase latest version pn: 83000-05604 alt pn: 10-61833-6 from Aaxico (Koito prefer seller).
7)       Inform the maint crew to follow the troubleshooting chart in AMM 30-41-02 figure 101 before make a decision to replace the WHCU due to failure. Ensure that the window temperatures are below 75°F and No. 2 windows are closed and latched before using the troubleshooting chart.
8)       Review maintenance task (‘C’ chk and ‘B’ chk) to inspect the cooling system and cleaning the E&E from dust and dirt.  

G) Conclusions

1)     Operator Window Heat Controller Unit (WHCU) MTBUR is 3838 hrs which is lower than the design MTBUR (14046 hrs).
2)     Aging is the main reason of the unscheduled removals.

Email from Aero Tech dated 22, 29 April, 28 May 2008
Email from Astronics dated 6 and 7 May 2008
Email from Boeing dated 30 April, 9 and 31 May 2008
Email from Koito dated 23 May 2008, 14 and 17 June 2008
Email from Aviation Technical Services dated 24 May 2008
Email from Aaxico dated 26 May 2008
ISAR No. 93-10 dated 30 September 1993
ISAR No. 90-11 dated 12 December 1990


Click Below For More Infos on Acft Maintenance Support



  © Blogger templates Sunset by 2008

Back to TOP