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.
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.
Here you can see the aircraft after it landed.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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!
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.
Here is the remainder of the propeller points of contact at the FAA.
Airworthiness: Potential for Propeller Failures
Presented to:By:Date:Federal AviationAdministrationAirworthinessand/or OperationsPotential for Propellerfailure1
Federal AviationAdministration2Description of Airworthiness ConcernOn June 18, 2012, a Piper PA-28-200, had aliberation of about 6” of the tip of onepropeller blade.The event occurred at Peoria Airport in Peoria, ILThe propeller is a 2-bladed aluminum propeller.
Federal AviationAdministrationEvent Aircraft Just After Landing3
Federal AviationAdministration4Leading Edge Damage– The propeller leading edge shows a significantamount of dents and scrapes from impact with smallrocks (probably flown out of a gravel strip).
Federal AviationAdministration5Fatigue of Cracked Propeller Blade• There was significant fatigue evident (about75% of blade showed fatigue damage), andwith only a few hundred hours on thepropeller, this leads us to believe thepropeller was being operated in a resonantmode.
Federal AviationAdministration6Fatigue of Cracked Propeller BladeDark grey is failurein overloadShiny silver iscyclic fatigueDull grey is crackinitiation site
Federal AviationAdministration7Cause of Fatigue• There is a known resonant mode with thispropeller/engine combination that has it’shighest level between 2200 and 2250 RPM.– An AD had been issued, creating an operatingrestriction range between 2100 and 2350 RPM.– The propeller manufacturer had provided informationto operators to avoid operation below 2500 RPM• The president of the flying club said their pilots have beentold to operate at 2500 RPM. However, it is possible theyhave been flying at the more traditional 2400 RPM to savefuel.
Federal AviationAdministration8Probable Event Scenario• When tested after the event, the airplane’stachometer was reading 80 rpm higher thanactual rotational speed of the propeller.– Tachometer error discovered by NTSB.– If the pilot set the RPM to 2400, then he wasprobably operating in the restricted range below2350 RPM.
Federal AviationAdministrationResonant Mode Evaluation9
Federal AviationAdministrationWhy does this happen?• Mechanical tachometers can loose their accuracyover time.• Reports of tachometer errors of 50 rpm arecommon and they have been seen up to 250 rpm.• When the tachometer is out of calibration, you caninadvertently operate in a restricted range,subjecting the propeller to vibration that will causedamage.10
Federal AviationAdministrationWhat can you do?• Check with the propeller manufacturer regardingrestricted ranges for your propeller/enginecombination• Verify the proper RPM restrictions are accuratelymarked• Verify the accuracy of the tachometer readings –and repeat at regular intervals• Be vigilant in checking the propeller for damagethat can initiate a crack11
Federal AviationAdministrationFor more information……• FAA Special Airworthiness Information BulletinNE-08-21, issued May 14, 2008• FAA Advisory Circular 20-37E, AircraftPropeller Maintenance, dated September 9,2005.12
Federal AviationAdministrationTaking theseactions will help toensure that thisdoes not happen toyou!13
Federal AviationAdministrationUpdated FAA Contact Information• For General Propeller issues – Jay Turnberg,Propeller Specialist, Engine and PropellerDirectorate, email@example.com• For Hamilton Sunstrand – Michael Schwetz, BostonACO, firstname.lastname@example.org• For Hartzell Propellers – Chris Richards, ChicagoACO, email@example.com• For Sensenich Propellers (wood) – Gary Wechsler,Atlanta ACO, firstname.lastname@example.org
Federal AviationAdministrationUpdated FAA Contact Information• For Sensenich Propellers (metal) – Ron Segall, NewYork ACO, email@example.com• For McCauley Propellers – Steven Litke, WichitaACO, firstname.lastname@example.org• For Aeroproducts/Pacific – Margaret Langsted,Seattle ACO, email@example.com• For Tarver – Jonathan Oxman, Los Angeles ACO,firstname.lastname@example.org