B727 and B737 Tire Slippage on Wheels

>> Friday, April 16, 2010

Two 737 operators and one 727 operator reported observing that their main gear tires were slipping or moving on the wheels. After initially suspecting that slippage was occurring, one of the operators began painting witness marks on wheels and tires during wheel and tire assembly. After the airplanes were in service for a number of landings, the witness marks were misaligned, indicating that the tires "slipped" or rotated on the wheels. In severe cases, slippage of up to 20 inches on the wheel periphery (about 100 degrees) was observed. In two of these cases, it was determined that the tires exhibiting the slippage were NOT approved by Boeing.

Boeing has determined that a SMALL amount of slippage is acceptable and is occasionally observed on in-service aircraft. A large amount of slippage (such as that reported above) is unacceptable and clearly indicates a problem. A large amount of slippage is likely to damage the wheels and tires, and might adversely affect stopping capability of the airplane.

In previous revisions to this article, Boeing recommended that slippage less than 20 degrees on the wheel periphery be considered acceptable. Twenty degrees on the periphery of a 727 main gear wheel is about 4 inches (100 mm). After this recommendation was made by Boeing, the referenced ARP was released (August 2002) which also recommends a 20 degree in-service slippage limit.
This FTD article provides recommended in-service limits for tire-to-wheel slippage.

Operator Action
If a tire is observed to have slipped more than about 20 degrees, we recommend that the wheel/tire assembly be removed from service and that the tire supplier be informed.

Also, we are aware that some operators use tire bead seat lubricants to facilitate wheel and tire assembly. It is reasonable to presume that bead seat lubricants may exacerbate tire-to-wheel slippage. As such, operators may wish to review their use of lubricants if they are experiencing excessive slippage.

SIL Aerospace Recommended Practice (ARP) 5507, dated 01-AUG-2002


Inertial Reference Unit (IRU) P/N: HG1150BD02 - Unscheduled Removal Report

>> Wednesday, April 14, 2010

Reason for Report
1.      Our fleet of 4 MD11 showed IRU P/N: HG1150BD02 poor reliability. We had experienced 12 IRUs unscheduled removals from May 05 till Dec 06.
2.      This study reviews shop data and gather information from OEM/Boeing to see how IRU reliability may be improved.

1.      From 2005 to 2006, we had experienced 12 unscheduled removals of IRU (refer Table)
12 Removals
2 under repair
Common Defect
s/n's with repeated removals
IRU failed (8 cases, 5 DC, 2 DNC and 1 under repair)
513 (2 DNC)

Shop Findings
Probability, %


Ring Laser Gryos failed

Accelerometer failed

a)      22-Oct-05, aux IRU S/N: 415 was rplcd due to IRU failed during flt.
b)      12-Dec-05, aux IRU S/N: 465 was rplcd due to IRU didn’t align.
c)      24-Dec-05, #1 IRU S/N: 426 was rplcd due to IRU failed after t/off.
d)     12-Feb-06, #1 IRU S/N: 513 was rplcd due to IRU #1 ind 10kts higher than #2 and #3 IRU.
e)      11-Jun-06, aux IRU S/N: 492 was rplcd due to IRU failed during cruise.
f)       13-Jun-06, aux IRU S/N: 94090712 was rplcd due to intmt and IRU drift after landing.
g)      08-Oct-06, aux IRU S/N: 94030672 was rplcd due to IRU failed.
h)     16-Oct-06, aux IRU S/N: 251 was rplcd due to IRU failed.
i)        23-Nov-06, #1 IRU S/N: 321 was rplcd due to intmt ops.
j)        23-Nov-06, aux IRU S/N: 513 was rplcd due to intmt ops.

Information from Boeing
1.      Boeing’s records indicate operators are seeing IRU MTBUR of approx 6500 FH
2.      IRUs may fail for a variety of reasons; bad laser gyros, accelerometers, electronics, etc.  When compared with VG/DG systems, the IRUs are extremely reliable.
3.      Our repair history showed the primary failures were laser gyros, and accelerometers. Boeing has seen instances where these components reach their useful life limits, and must be replaced.  Unfortunately, this may occur in a group since the IRUs were built at the same time.  So an operator may have a larger amount of certain component failures in one year, and significantly less for a couple of years afterwards. 
4.      Boeing has not had operator reports of significant reliability issues with the MD-11 IRUs; however, Boeing will continue to monitor in-service reliability for this unit.

Information from OEM
1.      Typical mean life of the ring laser gyros (RLG) is 75,000 operating hours (OP).
2.      When the RLG is approaching the end of life the failure rate will be higher. Accelerometers and RLG will fail towards the end of life of the sub assemblies.
3.      Currently Honeywell has no new P/Ns of sub-assemblies to improve IRU performance.
4.      OP hour:FH is about 1.5:1 and gyro life is based on OP hours (how long light bulb burns).
5.      Advised the operator to undertake a gyro refresh program which should improve IRU reliability since the four MD11 are high hours TSN.
6.      This advised should apply to the accelerometers if found faulty or the performance is suspect during the workshop visit. A replacement program would assist to improve the reliability.

Information from other MD-11 Operator
1.      Another operator (Martinair) haves similar IRU failure. About 60% to 70% of removals are caused by the gyros. They had a peak in gyro removals (9 gyro removals between August 2005 and December 2005). Since January 2006 they had only two gyro removals.
2.      Actual MTBUR for IRU is 10900 hrs.
3.      IRU’s main problem is Gyros. With increasing age the Laser Intensity decreases. If the value drops below a certain level a failure warning appears. If all units are in the same age there may be a peak in the removals of gyros.
4.      Gyros have a tendency for low reliability after about 10 year of average operation. As it is linked to OP hours and not equipment can be different for different airlines.
5.      Current worldwide fleet reliability for HG1150BD02  are 8200 MTBF and 5300 MTBUR

Findings and Discussion
1.      Majority of IRU defects are contributed by RLG and accelerometers.  Total 8 RLG were repld on 4 IRUs and 3 accelerometers were repld on 3 IRUs (1 each).
2.      IRU Design MTBUR is 10900hrs but our current MTBUR 4304 hrs is much lower.
3.      Currently there is no improved P/Ns and SB for IRU.
4.      As per Honeywell TN M23-3341-033 gyro should be replaced if the LIM voltage drops below 2.2V; or unit has set LIM fault. This is being followed by SRT.
5.      Operator does not have the replacement record of RLGs within the IRUs.
6.      Although Honeywell has given the expected RLG useful life, it is not practical to implement it as hard life limit due to mixed ages of the RLGs and the IRUs are in the SRT pool.
7.      Ignoring the OP hour:FH ratio and using only the TSN data divided by average utilization of 10 FH/day, the age of the RLGs and accelerometers are estimated and shown in Table.  Assumption: the IRUs TSN data are reasonable.  Estimated RLG useful life ranges between 9.4 – 14.1 years old.  Estimated accelerometer useful life ranges between 9.9 – 17 years.
1.      Recommend operator to replace ALL the gyros (x-, y- & z-gyro) within one IRU unit IF one of the gyros were found defective and required replacement IF the age of gryos are similar (±ONE year old vs the defective gryo) although it may pass the LIM Voltage test. 
2.      Recommend operator to carry out more extensive test on the other accelerometers should one of the 3 accelerometers in an IRU require replacement.
3.      S/N 513 should undergo additional testing to ensure unit is in good condition.
4.      Should one accelerometer be found faulty, subject the other accelerometers to additional testing and close visual inspn.
5.      Maintain more IRU spares due to the current reliability situation.
6.      MTBUR of IRU will be monitored and reported if any further deterioration.

MD11 fleet IRU reliability is below industry average.  Shop report shows ring laser gyros within are reaching their operating life limit. Recommendation is to replace all RLG within one unit if one of the RLG needs replacement per TN M23-3341-033 IF the other RLGs are of similar age.
With this implemented the MTBUR of IRU would be monitored for trends. 


Fuel Boost Pump Wire Chafing

>> Monday, April 12, 2010

Title: Fuel Boost Pump Wire Chafing

Aircraft Model: 737-100,-200,-300,-400,-500
Other Models: 707, 727

Issue Status: Open
Applicability: All 737-200/-300/-400/-500 airplanes

During the first repeat inspection after initial inspection per AD 99-21-15 and SB 737-28A1120, the #1 aft (left wing tank) fuel boost pump wiring of a 737-300 airplane was found with chafing through the Teflon sleeving into wiring insulation at three different locations. Approximately 21000 hours had accumulated since the wire bundle was initially replaced.

Boeing considers this an Airplane Level safety issue because chafing completely through the wire insulation may cause arcing within the conduit and possible burn through, leading to a potential ignition source.

In a 727 incident under investigation by the Indian government, wire bundle damage was discovered in the fuel tank conduit after an explosion that occurred on the ground in the left wing fuel tank. Approximately 10000 hours had accumulated since incorporation of AD 99-12-52. It is not known if chafing played a role in the 727 event.

Airworthiness Directives 99-21-15 (737), 99-12-52 (727) and 2001-17-20 (707) require removal of the fuel boost pump wiring from the in-tank conduit(s) for the boost pumps in main tanks number 1 and 2, and the center tank boost pumps, and a detailed visual inspection to detect damage of the wiring in accordance with Boeing alert service bulletins 737-28A1120, 727-28A0126 and 707A3500. These ADs also require repeat inspections, at intervals not to exceed 30,000 flight hours after accomplishment of the initial inspection. The 737 Classic aircraft have four conduits with wire bundles running to the fuel pumps; the 727 has eight and the 707 has ten.

Boeing is continuing to expedite root cause investigation including removal of the 737 aircraft conduit from the in-service aircraft for laboratory investigation.

The FAA has opened a planned AD worksheet. An FAA Immediately Adopted Rules (IAR) is anticipated to mandate the interim action that Boeing defines via service bulletin 737-28A1263(ECD TBD).

Compliance time is to be determined taking into consideration risk mitigation, operator impact and very large fleet size. To date, a compliance time of 90-120 days has been suggested by the FAA for the interim action, to be finalized based on parts availability, etc.

Boeing does not plan to release a service bulletin until parts are available.

Interim Action:
Boeing plans to address the 737 Classic first, immediately followed by 727. The 707 is also under review.

Accomplish within 90-120 days (to be confirmed) after new service bulletin and parts are available:

1- Remove and inspect all fuel boost pump wiring from the boost pump connectors to the splices on the front wing spar

2- Replace all Teflon sleeving with sleeving which has smaller OD than the conduit ID.

3- Replace all four conductor, unjacketed, or BMS13-51 wires with BMS13-60 jacketed wiring. Remove ground wire if found installed.

4- If any wire damage or arcing indications are found, inspect conduit (boroscope), leak check per AMM, and repair or replace the conduit.

5- Clean inside of conduit prior to installing the reworked/new wire bundle.

Do not use talcum powder as a wire pulling lubricant. S/B will specify the type of lubricant(s) to use

Report all inspection results to Boeing

Operator Action:
Incorporate service bulletin 737-28A1263 (when released), report all inspection results to Boeing, and comment on the FTEI bulletin board item EM-06-00063 with relevant information, if available.

Part Information:
Boeing is planning parts kits consisting of a roll of wire and a roll of sleeving in airplane ship set quantity.

A limited supply of miscellaneous parts will also be available at Boeing (separate from the kits) to support this inspection (conduits, connectors, splices, contacts, etc.).

Service Related Problem (SRP) 727-SRP-28-0113
Service Related Problem (SRP) 737-SRP-28-0113
Fleet Team Emerging Issue (FTEI) EM-06-00063, dated 06-JUL-2006
Airworthiness Directive (AD) 99-12-52 (727)
Airworthiness Directive (AD) 99-21-15 (737)
Service Bulletin (SB) 737-28A1120, dated 24-APR-1998


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