At 20,000 feet over Cam-Ranh Bay, Vietnam, the horizontal stabilizer on a C-141 pitched to full nose-down position when an aluminum bronze acme nut broke! A safety stop and good piloting saved the flight. The Air Force changed to leaded bronze in the C-5a Galaxy
On January 31, 2001, Alaska flight 261 at 31,000 feet lost all control of its horizontal stabilizer and crashed with the loss of all aboard when the aluminum bronze acme nut broke. There was no safety stop in the MD-80 design.
Loss of a Cessna 404 VH-ANV with all aboard when the aluminum bronze bushing in the fuel pump galled on the pump shaft causing the shaft to break.
The CFM-56 turbofan engines have an Airworthiness Directive (2002-NE-26-AD) calling for the removal from service of aluminum bronze fuel pump bearings at an estimated cost of $74,000 per pump (6,048 pumps in service)
Euclid Hitachi 180 ton dump trucks aluminum bronze bushings in the front suspension had to be replaced with nickel/tin based aluminum bronze because of (guess what), galling of the steel shaft.
King Air A100 (BE-10) C-GISH landed without its nose gear when the (guess what), aluminum bronze nut and steel screw suffered premature wear. A quick commentary on this one. The Canada Transportation Safety Board Report A0200131 made these stupid comments: "incorrectly filling the nut with grease at overhaul", and "inconsistent maintenance practices" In other words, make a bad design and when it fails, blame the mechanic.
But don't think that only the Canada Transportation Safety Board falls in this category. The US NTSB spent considerable amount of time investigating grease compatibility after Alaska Flight 261 crashed. No design changes have been made. After the C-141 incident, the Air Force made design changes and since then has not had an in-flight failure.
The MD-80 is "inspect to prevent failure", rather than "design to prevent failure". It is the aircraft mechanic who keeps this aircraft design in the air - not the pilots. Lets hope the mechanics do a good job - and increase their pay to pilot level.
Cross-section of an aluminum bronze bearing. Notice the red bronze color. Also notice the scratches. These are poor quality honing marks. Because aluminum bronze is hard - the surface needs to be precise and smooth.
A rough aluminum bronze surface produces aluminum oxide as the asperities are worn from the surface. Aluminum oxide is very hard and very abrasive. Both surfaces should be smooth.
Like all engineering materials, one needs to understand its properties before it can be successfully used. Aluminum bronze is a wonder material as it is very strong, temperature resistant, highly corrosion resistant, and easily manufactured. It does require the following:
In some cases, it may be better to substitute another bronze alloy like the Air Force did with the C-5a. Leaded bronze has spots of lead in the bronze that act as a natural lubricant. Tin/Nickel bronze also has good natural lubrication with high strength.
- Smooth surface finish
- Adequate lubrication (The Air Force has automatic greasers to the aluminum bronze acme nut on the C5-A. MD-80 does not)
- Good alignment. Because of its hardness, aluminum bronze will not conform to any misalignment.
Aluminum Bronze as a Valve Guide Material
Years ago, probably in the 1960's, most valve guides were aluminum bronze in Continental aircraft engines and Lycoming aircraft engines. They are still used as intake guides and work very well with little wear. In the exhaust position, aluminum bronze guides would wear out at about 800 hours.
Most engine models switched to other materials for the exhaust position. If you consider the exhaust guide in an aircraft engine, one end is exposed to 1,600 degree F oxidizing flame. This is above the annealing temperature for this alloy (1100-1300) and close to its melting temperature (1,800).
Aluminum Bronze Desirable Properties:
- High strength at high temperature
- Highest fatigue strength of any bronze alloy
- Hightly corrosion resistant
- Shock resistant
- Wear resistant
- Lower density than steel. Weights less
- High impact resistance at high and low temperature