3.1.2b Solution of the maintenance failure
The failure of the 737 with an “overstressed torsion link” was caused by a loose apex nut. This nut is probably not checked during scheduled maintenance of this aircraft. Although it must be checked according to the maintenance manual. The scheduled maintenance was done 21 hours before the failure.
After the failure the aircraft could not taxi, so the aircraft was towed into the hangar, where the damage was examined. The gear was completely removed and a new gear installed. This is a non-scheduled maintenance because the aircraft is taken out of service. To replace a main gear, the air-plane was grounded for two days. This has an impact on the flight schedule of the airline.
3.2 Controlling maintenance
To make sure the aircraft's maintenance is done correctly the maintenance has to be checked. This happens during regular checks.
The nose gear shimmy damper will be checked during the nose wheel steering system test. This test is done on the steering system of the nose wheel and is used to find if the steering rate, steering wheel centering and the angle of travel are in the necessary limits. The test also checks if the system is without leaks.
To do this test it is necessary that the hydraulic system is serviced and the steering system of the nose wheel is adjusted correctly.
To make sure the shimmy damper works correctly, the hydraulic system of the nose gear must be bled before it can be opened. When opened, malfunctions can be checked.
Another aspect of the nose wheel steering system test is the removal of the apex bolt of the nose wheel torsion link. After the test the bolt will be placed into normal position. The problem of the loose bolt, which caused the overstressed torsion link, may have been occurred by not properly re-turning the bolt on the gear.
However the torsion link did not collapse on the nose gear, but on the main gear. During the torsion link maintenance check the maintenance crew must check if the bolt is tightened and secured. By this check the torsion links will be removed and installed back on the landing gear.
To remove the torsion links, the main tires have to be removed and the shock strut has to be ex-tended. During the reinstallation of the torsion links all bolts
have to be tightened. This includes the torsion link apex bolt, which was loose when the accident happened. After tighten-ing, the bolt will be checked for being safe tied with lock wire (appendix X).
This lock wire (figure 3.3) is used to make sure the bolt re-mains tightened. Any losing of the bolt will result in the wire getting tightened, so the bolt will not move. This wire also makes it more easy to check if the bolt is still in place.
3.3 Cost
The maintenance of the landing gear results in some cost for the airline. There are different checks to replace or just check all the components of the landing gear. During these checks the aircraft has to stay on the ground (3.3.1). To execute the maintenance different ground staff and engineers are necessary (3.3.2). To reduce the maintenance cost the airliner can lease the landing gear (3.3.3)
Figure 3.3 Example of the use of lock wire
34 3.3.1 Aircraft on ground
When the landing gear is maintained the aircraft cannot fly, and therefore will generate costs. When the aircraft is on ground the airline cannot use the aircraft for transporting passengers or cargo. The A-check is done once a month and takes seven hours to complete. The A-check is mostly to check the most critical parts of the aircraft. The B-check is done four times in five year. Finally there is a C-check and a D-check to strip the landing gear and maintain it. This check take place once in five years of operation. These landing gear checks take place when the whole aircraft is taken apart.
For an AOG a standard of 2500 Euros per hour will be charged. The different checks takes different hours to complete. The total costs of all the checks during aircraft on ground are €1.875.000,-.
Check Numbers of
checks (per five years)
Hours Hours on ground Total costs (in five years)
The aircraft must be checked by well educated staff. The engineers salary is taken into account to the total costs of the maintenance. The different types of checks using different amount of engineers.
The hour costs per man are €50,-. The A-check can be done in five hours by four engineers. The B-check take place 3 times a year and can also been done by four engineers. The C and D B-checks for the landing gear are more intensive checks and can be done by 6 engineers. The C-check takes 15 hours to complete and the D-check take 2500 man hours to complete. The total cost of the landing gear maintenance per 5 years is almost 300.000 Euros.
Check Numbers of
checks (per five years)
Man hours per inspection
Total man hours Total costs
A 40 5 200 10.000
B 15 8 120 60.000
C 4 70 280 140.000
D 1 2500 2500 125.000
€290.000 3.3.3 Leasing landing gear
Maintenance of the landing gear are major costs for the airliner, therefore smaller airline companies are able to lease their landing gears. The total lease costs of a Boeing 737-300 landing gear is about 24,000 Euros a year. When an airliner leases a landing gear the only cost of maintenance are the brakes and tires. The costs of landing gear parts, that have to be replaced, are for the leasing compa-ny and not for the airliner.
3.4 Conclusion
This report is started with a main question, which meets the demand of the managing board of the ALA and the main purpose of the project team 2A2G;
35 ''In which way can the maintenance program be influenced by the impact of the B737's error, accord-ing to the costs of the maintenance and the airworthiness of the aircraft and which materials are certified for the forces on the landing gear? ''
The first two chapters of the report have been filled with technical and mechanical information of the B737's landing gear system. In the third chapter, the attention went out to two specific errors on the landing gear system of the B737. Answering this main question is done by using the information gained and written in all the previous chapters. The costs and airworthiness are distinguished to get a better view of both aspects.
According to the team’s forces calculation of the landing gear, the materials can be selected, such as steel, aluminum and titanium alloys. Steel has a high strength, toughness, fatigue strength and ductil-ity. Titanium and aluminum are both light weight and are having more corrosion resistant .
The maintenance program can be influenced by the errors in relation to continue the airworthiness, by giving more priority to these errors which only can be prevented by scheduled checks. When fol-lowing the maintenance programs which are being executed by certified personnel, the airworthi-ness of the aircraft can be kept at a standard.
36
Bibliography
Boeing 737 Manuals and Readers Aircraft illustrated parts B737-300
Aircraft maintenance manual Boeing 737-300
Boeing 737 Flight Crew operation Manual – Landing Gear Continental maintenance Manual Boeing 737
CS-25 amendment 7
Mel (minimum equipment list) Boeing 737 all types System schematic Manual
Books
Benjamin Chartier, Brandon Tuohy, Jefferson Retallack, Stephen Tennant Landing Gear Shock Absorber
Jelle Hieminga, Simon Ijspeert, Pieter van Langen Projectboek periode 5: Landing gear
Amsterdam, Augustes 2010
Hoge school van Amsterdam, Aviation studies Langedijk, C.J.A.
Landing Gear Design: Principles and Practices
American Institute of Aeronautics and Astronautics, Inc.
1984
Wenztel, Tilly
Het projectgroepsverslag Amsterdam, 2008
37 Hogeschool van Amsterdam, Domein en Techiek
Magazines
Aluminum and Aluminum Alloys :: KEY to METALS Article
http://www.keytometals.com/page.aspx?ID=CheckArticle&site=ktn&NM=2
Nederlandse vereniging voor van luchtvaart technici http://www.nvlt.org/archief/2006/thema-avond%20kl.pdf
38 2009 Smart cockpit
The Boeing Company
http://www.boeing.com/commercial/airports/acaps/737sec2.pdf 1995 - 2010 Boeing
Ungefallberichte November 2004
http://www.berlin-spotter.de/unfall/unfall2004/berichtenov.htm
39
List of appendices
I Maintenance Schedule 1
II MEL Repair Time Categories 10
III Centre of Gravity 11
IV Wing Design 12
V General Characteristics 13
VI Titanium Aspects 14
VII Aluminum Aspects 15
VIII Hydraulic pressure hose QD coupling leakage 16
IX Main Gear Failure Continental 18
X Use of Lockwire 23
XI Project assignment 25
XII Process report 27
XIII Group Agreements 28
XIV Distribution Of Tasks 30
XV Group members 31
40
I Maintenance Schedule
1
2
3
4
5
6
7
8
9
10
II MEL repair time categories
The time within a component needs to be repaired is defined in the MEL by a categorized system which runs from A to D.
Category A:
Items in this category shall be rectified within the time interval specified in the remarks column of the MEL item.
Category B:
Items in this category shall be rectified within three consecutive calendar days following the day of discovery.
Category C:
Items in this category shall be rectified within ten consecutive calendar days following the day of discovery.
Category D:
Items in this category shall be rectified within one hundred and twenty consecutive calendar days following the day of discovery.
11
III Centre of Gravity
12
IV Wing Design
13
V General Characteristics
14
VI Titanium
15
VII Aluminum
16
VIII Hydraulic pressure hose QD coupling leakage
17
18
IX Main Gear Failure Continental
19
20
21
22
23
X Use of Lockwire
Lock wiring* is the securing together of two or more parts with a wire which shall be installed in such a manner that any tendency for a part to loosen will be counteracted by an additional tightening of the wire.
For general purpose lock wiring, use the preferred sizes in Table 1-1. Use smaller diameter wire where parts are too small to permit a hole diameter to accommodate the preferred sizes, or where space limitations preclude the use of the preferred sizes. The larger sizes are used where stronger wire is required. Wire diameter of .032 is the most commonly used.
The common method
lock wired by a twenty-four inch length of wire.
Wire shall be pulled taut while being twisted. The number of twists per inch as recorded in Table 1-1, represents general practice and is given as guidance information only.
Caution must be exercised during the twisting operation to keep the wire tight without overstressing.
Abrasions caused by commercially available wire twisting pliers shall be acceptable but nicks, kinks, and other damage to the wire are not.
Lock wire shall not be installed in such a manner as to cause the wire to be subjected to chafing, fati-gue through vibration, or additional tension other than the tension imposed on the wire to prevent loosening. In the event that no wire hole is provided, wiring should be to a convenient neighboring part in a manner so as not to interfere with the function of the parts. Hose and electrical coupling nuts shall be wired in the same manner as tube coupling nuts.
Various examples of lock wiring are shown in Figures 1-1 through 1-12. Figure 1-12 shows the single-stranded method, while the other figures show the two-single-stranded or double twist method.
Figure 3 The use of lock wire
24 Check the units to be lock wired to make sure that they have been correctly torqued. Under-torquing or over-torquing to obtain proper alignment of the holes is not advisable. If it is impossible to obtain a proper alignment within the specified torque limits, back off the unit and try it again or select another unit.
In adjacent units, it is desirable that the holes be in approximately the same relationship to each oth-er as shown in Figures 1-1 through 1-4 (for right-hand threads), thus the lock wire will have a tenden-cy to pull the unit clockwise. This should be reversed for left-handed threads.
Where lock wire is used to secure a castellated nut on a threaded item, selection of locking hole di-ameter for the item shall be based on cotter pin requirements.
Table 1-1
Lockwire and Lockwire Hold Data
Wire Diameter Twists per Inch Recommended Hole Diameter
0.020 9-12 0.037-0.057
0.025 9-12 0.060-0.080
0.032 7-10 0.060-0.080
25
XI Project assignment
26
27
XII Process Report
The start of the project was a bit cautious but good. In the first meeting we exchanged numbers and mail addresses and we planned a brainstorm session, where the project has been inventoried.
The meetings where useful. Through these meetings could the project group quickly see which parts is completed and need to be done. A few things can be done better. The project group needs to create discussions, this is to create more ideas to enrich the information provision of the report. In the start document a chairman and secretary list has been set up, where all the members of the group first get a turn of secretary and then as chairman. The chairman functioned in general good, this is due that every member of the group complied to the rules during an meeting. The agree-ments and task distributions where good noted by the secretary. The secretary placed it within 24 hours on BSCW and sent it to all the members per e-mail with a hyperlink of it. Another thing what can be done better is that all the minutes had been checked with feedback by the members , this is to keep the quality of the minutes high and it is a very handy tool to keep the member posted of the current situation. When a member was absent this always been reported beforehand the meeting to the chairman, eventually the absent was noted on the minute.
The planning of the start document was very global. Every chapter was in general planned for a week with a margin of a week at the end. This couldn’t get reached because the problems we encountered during writing the report. This was not a big problem because the deviation was maximal a few days.
When a member couldn’t get his job done on time, then he must reported this on time to the group with the question to finish this later on.
To ensure that there are no spelling or content errors the group used a buddy system and at the end of every chapter a beamer session. The buddy checks the whole document of spelling and content , there are no documents rejected but there are many spelling mistakes made. We as group should work on that. During an beamer session with the whole group it is hard to keep the members fo-cused, that’s why it is better to divide the whole group in smaller groups to keep the attention level high. At the end of the control rounds the group sends the chapters to the project manager for last comments.
During this project, we have learned how the landing gear works and which systems are related to it.
The project group can apply mechanical calculations at various conditions or situations to the land-ing gear. Also the group has analyzed the failures of the landland-ing gear and adjusted the maintenance schedule with the corresponding costs to it.
28
XIII Group agreements
The project should be finished within a deadline. Therefore milestones are created which has its own deadline. Each group member should complete their tasks correctly and also needs to be up to date with changes and agreements within the group. For these reasons rules are made to even out every-thing regarding group work and to avoid misunderstandings;
Too late
Too late is later than the agreeing time of the meeting, or later than the time the college COM/Project-lesson is beginning with a margin of five minutes.
Important: when a group member is too late he call or sent the captain a text message.
In case this is not possible call or sent the minutes secretary a text message. When you can’t reach each of these people, call one other person of the group.
When a group member is more than one time too late in the whole project time, with-out a valid reason: note the protocol.
Absent
When a group member is absent it’s just as important that he call or sent the captain a text message. The group member makes sure that he catch up with the subject materi-al. The group member will do this as soon as possible, namely in consultation with the group. The group will decide with the person who is absent a date when the subject ma-terial should be finished. Eventually will the captain be the person who will tell the ab-sent person when the subject material should be finished. The group member also makes sure that he, before the next meeting, read the minutes of that
meet-ing/COM/project lesson. The group member makes sure that he often check his mail that day.
When a person is more than one time, without a valid reason, absent in the whole project time: note the protocol
Not completing assignments or minutes.
When a group member thinks that he can’t make it to finish the assignment (under as-signment we mean: asas-signments given by the project/COM-mentor and asas-signment giv-en to each other at a groups meeting. So also presgiv-entations.) or minutes: tell it to the captain at latest two days before the deadline. The group member also let the other group members know that he could not make it so they can help him. Of course the group member should have a valid reason for not finishing assignments. Eventually the group will decide when this person should hand in the assignment.
It is always possible that a group member don’t understand an assignment. The group is a team so it is very important to let the group know that a person doesn’t understand, so we can help each other. There is always a person who can help a group member to explain something.
When someone is not following the agreement more than one time in the whole project time, without a valid reason: note the protocol.
Protocol
First time: the group member get a warning from the captain on behalf of the whole group.
Second time: During the group meeting the group will have a talk about the person who is not following the agreement, without a valid reason. It could me possible that this person is holding up the group. So the group will also have a talk about this with the project mentor and with the person who is not following the agreements to find a
solu-29 tion. It is possible that the person who isn’t following the agreements, will be put out of the group.
Valid reasons
The next valid reasons will only be valid when these will be told on time to the captain or other group mates. With on time the group means: before the meeting or
COM/project lesson is starting. When something happens on short time the group member can tell the captain that something happened by text messages him or call him (not too late). Valid example reasons are:
COM/project lesson is starting. When something happens on short time the group member can tell the captain that something happened by text messages him or call him (not too late). Valid example reasons are: