Robust Vibration Test (Categories R, U, U2)
- The robust vibration test demonstrates that equipment will operate satisfactorily while being subjected to vibration and that it will continue to operate satisfactorily after being subjected to endurance vibration levels.
- It combines a demonstration of the equipment functional performance and structural integrity. This test should be performed on all equipment where resistance to effects of long duration exposure to vibration must be demonstrated. The necessity for conducting this test in lieu of the standard vibration test shall be determined by the relevant equipment specification.
- Categories U and U2 are for equipment to be installed in helicopters with unknown rotor related frequencies.
- The robust test may or may not represent a service life test. A service life test depends on the vibration environment the equipment is subjected to relative to the test levels.
- If the vibration environment is known, and the relevant equipment specification requires a service life test, the robust test can be made to represent a service life test by adjusting the test levels and/or the test times using accepted fatigue scaling relationships.
High-Level, Short Duration Vibration Test (Categories H, Z)
- High-level short duration transient vibration levels are encountered during abnormal fixed wing aircraft vibration conditions that occur during an engine fan blade loss.
- This test should be applied to equipment in which a functional loss of performance can hazardously affect the aircraft's performance.
- The Category H test is a generalized test that encompasses all applications. The Category Z test covers restricted low fan frequency applications.
- These tests do not replace the standard or robust tests.
- Also see following CAUTION note.
CAUTION: A full analysis of vibration levels related to some specific engine imbalance conditions has not been evaluated against these limits. Therefore this test alone may not be sufficient for some applications without additional test or analysis.
Purpose of the Vibration Tests per DO160
- These tests demonstrate that the equipment complies with the applicable equipment performance standards (including durability requirements) when subjected to vibration levels specified for the appropriate installation.
- fixed-wing propeller aircraft
- fixed-wing turbojet, turbofan, and propfan aircraft
Standard Vibration Test (Category S)
The standard vibration test for fixed wing aircraft demonstrates that equipment will meet its functional performance requirements in the vibration environment experienced during normal operating conditions of the aircraft.
Effect of Vibration: Crawling of Cracks, Pressurization Cycles
Aircraft fuselages undergo repetitive cycles of differential pressure with each flight. The difference between the cabin and the external ambient pressure is about 6 or 7 psi at an altitude of 36,000 feet.
Note that cabin pressure at high altitudes is maintained at about 75% of sea level pressure, which corresponds to the air pressure at 8000 ft. This is done by pumping air into the cabin. Note that there is some variation in these numbers depending on the aircraft model.
Pressurization cycles along with vibration, corrosion, and thermal cycling can cause fatigue cracks to form and propagate.
1. Case Study: Southwest Airlines Flight 812
Southwest Airlines Flight 812 suffered rapid depressurization at 34,400 ft near Yuma, Arizona, leading to an emergency landing at Yuma International Airport, on April 1, 2011. Inspection of the 5 feet long tear revealed evidence of pre-existing fatigue along a lap joint.
The National Transportation Safety Board has concluded that “the probable cause of this accident was the improper installation of the fuselage crown skin panel at the S-4L lap joint during the manufacturing process, which resulted in multiple site damage fatigue cracking and eventual failure of the lower skin panel.”
2. Landing Gears and Vibrations:
Landing gears are designed to absorb the loads arising from taxiing, take-off, and landing. Hard landing shock is a particular concern. Vibration is another concern. Fatigue cracks can form in the struts and trunnion arms as a results of these loads. Again, corrosion can be a related factor.
2. Exhaust Cracks: