Airworthiness: Potential for Propeller Failures
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A Piper PA-28-200 experienced the liberation of about 6 inches of the tip of one propeller blade during landing. An investigation found significant fatigue damage to the cracked blade, likely caused by operating the propeller in a known resonant mode between 2200-2250 RPM. The airplane's tachometer was reading 80 RPM higher than the actual propeller speed, meaning the pilot may have unintentionally operated in the restricted range below 2350 RPM. Mechanical tachometers can lose accuracy over time, allowing a pilot to inadvertently fly in a restricted RPM range that subjects the propeller to damaging vibrations. Pilots should verify tachometer accuracy, be aware of any restricted RPM ranges, and check propellers
Airworthiness: Potential for Propeller Failures
- 1. Presented to: By: Date: Federal Aviation AdministrationAirworthiness and/or Operations Potential for Propeller failure 1
- 2. Federal Aviation Administration 2 Description of Airworthiness Concern On June 18, 2012, a Piper PA-28-200, had a liberation of about 6” of the tip of one propeller blade. The event occurred at Peoria Airport in Peoria, IL The propeller is a 2-bladed aluminum propeller.
- 3. Federal Aviation Administration Event Aircraft Just After Landing 3
- 4. Federal Aviation Administration 4 Leading Edge Damage – The propeller leading edge shows a significant amount of dents and scrapes from impact with small rocks (probably flown out of a gravel strip).
- 5. Federal Aviation Administration 5 Fatigue of Cracked Propeller Blade • There was significant fatigue evident (about 75% of blade showed fatigue damage), and with only a few hundred hours on the propeller, this leads us to believe the propeller was being operated in a resonant mode.
- 6. Federal Aviation Administration 6 Fatigue of Cracked Propeller Blade Dark grey is failure in overload Shiny silver is cyclic fatigue Dull grey is crack initiation site
- 7. Federal Aviation Administration 7 Cause of Fatigue • There is a known resonant mode with this propeller/engine combination that has it’s highest level between 2200 and 2250 RPM. – An AD had been issued, creating an operating restriction range between 2100 and 2350 RPM. – The propeller manufacturer had provided information to operators to avoid operation below 2500 RPM • The president of the flying club said their pilots have been told to operate at 2500 RPM. However, it is possible they have been flying at the more traditional 2400 RPM to save fuel.
- 8. Federal Aviation Administration 8 Probable Event Scenario • When tested after the event, the airplane’s tachometer was reading 80 rpm higher than actual rotational speed of the propeller. – Tachometer error discovered by NTSB. – If the pilot set the RPM to 2400, then he was probably operating in the restricted range below 2350 RPM.
- 9. Federal Aviation Administration Resonant Mode Evaluation 9
- 10. Federal Aviation Administration Why does this happen? • Mechanical tachometers can loose their accuracy over time. • Reports of tachometer errors of 50 rpm are common and they have been seen up to 250 rpm. • When the tachometer is out of calibration, you can inadvertently operate in a restricted range, subjecting the propeller to vibration that will cause damage. 10
- 11. Federal Aviation Administration What can you do? • Check with the propeller manufacturer regarding restricted ranges for your propeller/engine combination • Verify the proper RPM restrictions are accurately marked • Verify the accuracy of the tachometer readings – and repeat at regular intervals • Be vigilant in checking the propeller for damage that can initiate a crack 11
- 12. Federal Aviation Administration For more information…… • FAA Special Airworthiness Information Bulletin NE-08-21, issued May 14, 2008 • FAA Advisory Circular 20-37E, Aircraft Propeller Maintenance, dated September 9, 2005. 12
- 13. Federal Aviation Administration Taking these actions will help to ensure that this does not happen to you! 13
- 14. Federal Aviation Administration Updated FAA Contact Information • For General Propeller issues – Jay Turnberg, Propeller Specialist, Engine and Propeller Directorate, jay.turnberg@faa.gov • For Hamilton Sunstrand – Michael Schwetz, Boston ACO, michael.schwetz@faa.gov • For Hartzell Propellers – Chris Richards, Chicago ACO, christopher.j.richards@faa.gov • For Sensenich Propellers (wood) – Gary Wechsler, Atlanta ACO, gary.wechsler@faa.gov 14
- 15. Federal Aviation Administration Updated FAA Contact Information • For Sensenich Propellers (metal) – Ron Segall, New York ACO, ronald.segall@faa.gov • For McCauley Propellers – Steven Litke, Wichita ACO, steven.litke@faa.gov • For Aeroproducts/Pacific – Margaret Langsted, Seattle ACO, margaret.langsted@faa.gov • For Tarver – Jonathan Oxman, Los Angeles ACO, jonathan.oxman@faa.gov 15
Editor's Notes
- #2: 2013/01/14-028 (I) PP PPT Presenter Info: Author: Chicago ACO; FAASTeam POC’s: Kevin Clover, Operations Lead, Office Phone 562-888-2020 ; or Phil Randall, Airworthiness Lead, Office Phone 336-369-3948. Presentation may be used by FPM or Representative at any Operations or Airworthiness safety seminar. May also be used separately or in conjunction with other presentations to satisfy appropriate national business plan performance targets.
- #3: This presentation is prompted by a concern that many mechanical tachometers in older aircraft have not been recently calibrated and may not be providing accurate readings. In the cases where this is combined with propellers with restricted RPM ranges, it may result in operation in the restricted range, causing high stresses in the propeller and potential damage. One recent example is an event that happened in June 2012. A Piper aircraft lost about 6 inches off the tip of one propeller blade in flight.
- #4: Here you can see the aircraft after it landed.
- #5: There are multiple components that lead up to the failure of the blade tip. Compounding the problem of operating in the restricted range, this aircraft also had leading edge damage. FAA data on propeller failures indicates that the majority of failures occur in the blade at the tip region, usually within several inches from the tip and often due to a crack initiator such as a pit, nick or gouge. This type of damage is commonly caused by impact with rocks and other small debris on or near the runway. The primary defense against a failure is a proper pre-flight visual inspection of the blade. Manufacturers maintenance documents should have specific instructions.
- #6: For this specific case, the blade was analyzed after the event. The analysis showed significant fatigue damage given the number of hours of operation. This is an indicator that the propeller is being operated in a restricted rpm range. A propeller’s restricted range is established to identify the range of rpm where the propeller and engine operate in resonance. When this occurs, the operational frequency of the propeller matches the operational frequency of the engine in such a way that they complement each other to magnify vibration and stress, eventually to the point of failure. The most famous example of this type of resonance is the use of a musical note to match the resonant frequency and shatter a glass.
- #7: The crack that led to this event was probably initiated by a rock chip or dent at the leading edge, as indicated by the thick red arrow. From there, as the propeller spun, the crack slowly grew through the shiny silver area. Once that amount of material had separated, the wind and centrifugal loads tore the rest of the blade off. All the material in dark grey separated at about the same time.
- #8: Many piston engine/propeller combinations have a range of restricted propeller RPMs. This is typically a maximum RPM. In addition, there can be a lower range where continuous operation should be avoided. These lower ranges need to be passed through in order to get the propeller up to operating speed, but this should be a transient and the speed should not be allowed to stabilize in this area. These restricted ranges are identified by the manufacturer and the limitations should be marked in the aircraft, typically on the tachometer. They are also typically located in the airplane flight manual and the propeller type certificate data sheet. Prolonged operation within these ranges exposes the propeller to high stresses and can increase the potential for a failure.
- #9: In the specific event we are reviewing, the tachometer was tested after the in-flight incident. The NTSB discovered that the tachometer was out of calibration and was reading 80 rpm higher than the actual rpm of the blade. The pilot was completely unaware of this condition.
- #10: This diagram shows the stress versus propeller RPM for a common propeller blade. It shows that there is an operational range where stresses are at an acceptable level, bounded on both sides by ranges where the stress levels increase significantly. If a pilot is operating an aircraft and is unaware of the tachometer error, the pilot could be operating in the ranges of higher stress rather than the range of acceptable stress.
- #11: Mechanical tachometers do not retain their accuracy over the life of the aircraft. In addition, the tachometer reading can be affected by the installation and may vary depending on the aircraft/engine/propeller combination. As indicated, data shows that errors of 50 rpm are common and errors of up to 250 rpm have been reported.
- #12: There are several actions that you can take to limit your risk of having a problem. Key items are listed on the slide. FAA AC 20-37E contains additional detailed information on tachometer and blade inspection.
- #13: More detail on the specific topic of rpm restrictions and tachometer calibration is located in FAA SAIB NE-08-21, dated May 14, 2008. We have included updated contact information at the end of this presentation. In addition, FAA AC 20-37E contains a wealth of additional information regarding the maintenance of propellers including some great pictures of types of damage and inspection techniques. These and other safety related publications can be found using www.faa.gov as well as the FAAASTeam website, www.faasafety.gov.
- #14: If you take the simple actions addressed in this presentation as well as read and adopt other actions in the two documents we referenced earlier in this presentation, you will reduce the risk of propeller failure seen here. If you’re a pilot, discuss this with your maintenance provider. If you’re a maintenance provider discuss this with the owner/operator of the aircraft your work on. After all, what do you have to lose!
- #15: This is the most current FAA contact information for propeller issues. I’ll pause before showing the remainder of the list on the next slide if you wish to copy these points of contacts.
- #16: Here is the remainder of the propeller points of contact at the FAA.