1. Navy Landing Gear Perspective Tribaloy-Treated F/A-18C/D
Main Gear Shock Absorber
By Salvatore Piu
August 2001 HCAT mtg

2. F/A-18A-D Main Landing Gear Tribaloy T-400 - Treated Shock Absorber
Issue
Proposed modifications
Tests and flight evaluation
Status

3. Issue Severe wear/scoring found on cylinder and piston at overhauls:
Currently replace 100% Piston Rings (P/N 74A /-7001) and Snubber Rings (P/N 74A /-7001
Currently 90 % Piston Heads (P/N 74A /-7001)

4. Shock Absorber Configuration P/N 74A410850-1009
Two stages
Low pressure chamber, 150 psig, 8.87 inch stroke (shown in blue)
High pressure chamber, 1265 psig, 4.25 inch stroke (shown in green)
Total Stroke: inches
F/A-18C/D- shock absorber is not chrome-finished: Electrolysis-Nickel plating is used in stead of chrome.

5. Proposed Modifications
Purpose:
Reduce wear and internal damage of parts that typically get scrapped when worn -> increase time between overhauls
Changes:
Piston ring & snubber ring material change: from hard anodized aluminum to beryllium copper
Piston head treated with Tribaloy SWA UHP-180 T-400 HVOF-deposited coating
Tribaloy 400 Treated Area:
Not the main piston area typically treated with Hard Chrome
On an area where no previous coating applied

6. Piston head Piston Head Material: PH-13Mo
Triballoy surface rides on bare 300M material (piston head is constantly in an “oil bath”)

7. Potential Benefits Increased MTBF Improved fleet readiness
Longer component life
Reduced overhaul costs
Increased safety

8. Tests and Fight Evaluation
Two shocks modified, S/N EMC 0632 & EMC 0635
Lab Tests
Shock absorber ATP (service pressure)
Load-stroke tests (verify consistency of static, load-stroke performance)

9. Tests and Flight Evaluation
Flights
Both shocks installed on product support plane (at Boeing, St. Louis, October, 1997): 23 flights (31 touchdowns)
Performed post-flight load/stroke test (replaced leaking schroeder valve)
Both shocks installed on one in-service aircraft (May 5, 1998)
30 CV arrested landings to provide meaningful data
Flight clearance limited to 40 CV arrested landings
Extended flight clearance to avoid interfering with fleet operations
Right main landing gear experienced planing mechanism failure with EMC 0635 ( 60 field landings, 13 FCLP, & 80 CV arrested landings
Removed both shocks immediately after planing mechanism failure

10. Tests and Flight Evaluation
F/A-18A-D Planing Mechanism
Function: plane the main wheel axle and compress the gear (shock absorber) to fit in the wheel well
System has historical failures
8 primary components in system
Improperly functioning shock is a suspect contributor to failures
8 Primary Components of F/A-18A-D Planing Mechanism are:
Planing Link
Knee Joint Bell Crank Assembly
Connecting Link
Shrink Link
Upper Lock Link
Lower Lock Link
Crank (to axle)
Over-Center Spring (to lock links)

11. Tests and Flight Evaluation
Teardown inspection: shocks in excellent condition
Goal reset by Navy: 200 CV landings
Inspected, serviced, and placed shocks back on a fleet aircraft
Shocks on various aircraft from May 99 to June 00
Modified shocks both participated in planing mechanism failures
For each planing mechanism occurrence, shock removed from the failed gear, inspected, re-serviced, and placed on a fleet aircraft
5 on MLG’s with EMC 0635 (3 different MLG’s)
1 on a MLG with EMC 0632 (one of the MLG’s previously failed with EMC 0635)

12. Status Flight Evaluation Achieved 200 CV arrestments with one shock
Personnel associate modified shocks with planing mechanism failures due to repeated participation
In-service fleet evaluation halted by US Navy
Teardown inspection:
Wear observed was considerably less than would be expected (based on usage & compare to previous overhauled shocks)

13. Participated Planing System Failure
Status
Shock
S/N
Normal Field Landing
Touch & Goes
FCLP CV
Bolter
Field Arrestment
Participated Planing System Failure
EMC 0635
133 8 13
146 1 5
*1, *2
EMC 0632
137 12 54
200
*2, *3
Notes:
EMC 0635 participated in 2 planing system failures, not included in qty (5), prior to Triballoy-associated modifications.
Aircraft with modified shocks squawked for a pause in the stroking of the landing gear during landing (EMC 0635 lab tested for cause: nothing found)
EMC 0632 exhibited slow to extend characteristics prior to modifications

14. S/N EMD 0635: Piston Head & Ring

15. S/N EMD 0635: Cylinder

16. Current Plan One Additional Flight Evaluation
One aircraft with one new shock and and new-modified shock
200+ CV & FCLP landing
Tear-down and compare both shocks
If no issues found, implement change to spares and normal overhaul of existing shocks