Aircraft Elevators are designed primarily to transport aircraft between the hangar deck and the flight deck. They are, however, also used to transport cargo and equipment. Aircraft Elevators are found on aircraft carriers and on some amphibious assault ships such as LHA’s, LPH’s and LHD’s. All aircraft elevators use hydraulic engines, sheaves, and wire ropes to lift and lower the Platform.
There are two major types of aircraft elevators, the deck edge and the inboard types. These names refer to the location of the elevator platform on the ship. The machinery for inboard and deck edge elevators operates under the same principles and is similar in arrangement. Deck edge elevators are cantilevered off the hull of the ship. Inboard elevators use the platform as a hatch at the flight deck.
Guide rails and rollers constrain the platform so it can only move in the vertical position. Mechanical Locks hold the platform in place at the flight deck level taking the load off the wire rope.
Elevator Testing
New and modified aircraft elevators are tested prior to regular operation to ensure proper performance of all elevator components. There are no periodically required tests of aircraft elevators. However, because load testing and operational testing are required whenever hoisting wire ropes are replaced, every aircraft elevator will be tested at least once every five (5) years.
Load testing proves the ability of an elevator to perform properly through the complete range of operating limits with the platform loaded. The aircraft elevator require load testing with the rated load only. The stress applied by ship motion on the load bearing components is less than the stress due to normal operation.
The operational characteristics of the aircraft elevator require that additional loading effects of ship motion on the platform and its rated load at the flight deck level will not stretch the wire ropes and cause the platform to drop below the deck level. The flight deck is held tight to the flight deck because the hydraulic engine applies a load to the wire ropes beyond that which would be added by ship motion. The loading is applied every time the platform reaches the flight deck, regardless of weight on the platform. Hence, A static and dynamic load test (typically 150-200%) over the rated load capacity would not provide any additional assurance of proper elevator operation.
According to the Naval Ships’ Technical Manual (Chapter 588) an aircraft elevator must be fully load tested after repair or replacement of any of the following components:
1.Wire ropes
2.Sheaves and cross head
3.Hydraulic Engine (ram or cylinder)
4.Guide rails
5.Platform structure
6.Guide or face rollers
Traditional Method.
Labor and support equipment using current methods with steel or concrete weights with an average test time of 5 days for all elevators. Assumption is that the testing is done as part of a shipyard contract.
1.Support crane on pier (for testing inboard elevators)
2.YTD crane (for testing outboard waterside elevators) or Support crane lifted onboard the flight deck in order to reach waterside elevators.
3.Tug for positioning YTD crane or rental of barge mounted crane
4.Crane Operator
5.Minimum of 5 Riggers for positioning weights
6.Test Director
7.Electrician
Water Weights Method
Using the Water Weights method, all testing would be completed in two days. No dockside crane support is required. Two technicians complete all labor requirements. All equipment is transported in a pick-up truck.
Equipment used was as follows: -
8 x 8 tonne (17632 lbs) WATERWEIGHTS test bags
4 x Certified Flow Meters
1 x Manifold Filling Assembly
3 x 50ft Fire Hoses
2 x Diaphragm Pumps (used for emptying the bags)
Fill time (assuming 240 gallons per minute) to achieve full load is 40 minutes. Emptying (using diaphragm pumps) is approximately 30 minutes.
Given the figures above the obvious cost savings is huge compared to traditional methods. Additionally, testing takes 48 hours as opposed to 5 days. This might allow a shortening of the time the ship spends in dock, and therefore increased availability. Finally, two other important issues may have significant effect.
A)The above estimates assume that all elevators will be tested one after the other on the same mobilization. This rarely happens. The additional costs of separate mobilizations of tugs, YTD and shore based cranes dramatically widens the cost differential between the WATER WEIGHTS method and traditional methods.
B)Inevitably there are failures and the need for re-test. The cost of mobilization for re-test using WATER WEIGHTS will be lower.
C)There is also the added benefit of scheduling flexibility to both the testing activity and the ship itself because of the elimination of trying to schedule a support crane, tug or crane barge on scheduled test days.
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