An abnormal vibration is felt while inflight, what may be the cause?

Vibrations are caused by out-of-balance forces.  The source of the out-of-balance condition may be unequal rotor mass distribution, inadequate blade tracking, and or unbalanced drive train components.

Low frequency vibrations are generally related to the main rotor and are often due to an out-of-track bade.  This results in a one-per-rev vertical vibration.  Medium frequency vibrations are generally related to the engine and transmission.  High-frequency vibrations are related to components that rotate at high RPMs.  For most helicopters, this relates to tail rotor and its components.  Vibrations originating from the tail rotor are usually felt through the pedals.  Any abnormal vibration should be investigated, as the vibrations can cause significant damage to the aircraft.


FAA-H-8083-21A – Helicopter Flying Handbook pg. 11-21
Principles of Helicopter Flight, 2nd Edition, pg. 181

Other Helicopter Flight Conditions

Who is responsible for maintaining an aircraft?

The owner or operator is responsible for maintaining the aircraft in an airworthy condition.

The primary responsibility for maintaining an aircraft in an airworthy condition falls on the owner or operator of the aircraft.  Certain inspections must be performed on the aircraft, and the owner must maintain the airworthiness of the aircraft during the time between required inspections by having any defects corrected.  Airworthy means the aircraft conforms to its type design and is in a condition for safe operation.


14 CFR 3.5 Statements about products, parts, appliances and materials.
14 CFR 91.403 General (Maintenance)
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 9-8

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What are the inspection requirements for an aircraft?

At a minimum, all aircraft need an annual inspection.  If the aircraft is used for hire, a 100-hour inspection is needed as well.  Depending on aircraft, the transponder and pitot-static system may need to be inspected as well.

The 100-hour inspection is similar to an annual inspection.  The primary difference that the annual inspection must be signed off by an Airframe and Power Plant (A&P) mechanic with and Inspection Authorization (IA) designation.  An annual inspection may substitute for a 100-hour inspection, but a 100-hour inspection may not substitute for an annual inspection.  When used for hire, such as flight instruction, the 100-hour inspection may only be exceeded by 10 hours, and this exception is only to allow the aircraft to be transported to a maintenance facility.  However, the timing of the next 100-hour inspection does not reset, it is based off the original time the 100-hour was to be completed.  If a transponder is to be used in controlled airspace, it must be inspected every 24 months.  If operating, instrument flight rules (IFR), the pitot-static system is inspected every 24 months as well.

Although more common in large operations, a progressive inspection program could be used.  In such a program, multiple inspections are made throughout the year in order to minimize the consecutive downtime to the aircraft, allowing for less disruption to operations.  For example, an inspection might be able to be completed at night or between flights, where a typical annual inspection could be a week or longer.


14 CFR 91.409 Inspections
14 CFR 91.411 Altimeter system inspection
14 CFR 91.413 Transponder test
14 CFR 43.15 Additional performance rules for inspections
14 CFR 43 Appendix D: Scope and Detail of Items To Be Included in Annual and 100-Hour Inspections
FAA-H-8083-25B Pilot’s Handbook of Aeronautical Knowledge pg. 9-8

Other Helicopter Maintenance Topics

How often does the transponder need to be tested?

The transponder needs to be tested every 24 months.

Unless it has been inspected and found to be in working order within the preceding 24 calendar months, no one may use ATC transponder.  The inspection requirements are outlined in 14 CFR Appendix F.


14 CFR Appendix F to Part 43, ATC Transponder Tests and Inspections
14 CFR 91.413 Transponder test

Other Helicopter Maintenance Topics

What is an airworthiness directive (AD)?

Airworthiness directives are legally enforceable rules that apply to aircraft, aircraft engines, propellers, and appliances.

The FAA issues an airworthiness directive when there is an unsafe condition that exists with a product, or the condition is likely to develop in other products of the same type design.

Compliance with the requirements of an airworthiness directive is mandatory.  Airworthiness directives may include specific inspections that must be completed, conditions or limitations to be adhered to, or any other actions that must be taken to resolve an unsafe condition.


14 CFR 39 Airworthiness Directives
14 CFR 91.403 General (Maintenance)

Other Helicopter Maintenance Topics

What equipment is required for VFR flight?

The required instruments for VFR flight during the day include airspeed indicator, altimeter, magnetic direction indicator, tachometer, manifold pressure gauge, oil pressure gauge, oil temperature gauge and fuel gauge.

Basically, VFR flight during the day requires instruments to navigate, control the aircraft and monitor its performance.  The airspeed indicator, altimeter and magnetic direction indicator, are used for navigation.  These gauges indicate speed, height, and direction.  The tachometer, and manifold pressure gauge provide indicators for controlling the aircraft and the oil pressure gauge, oil temperature gauge and fuel gauge provides indicators to monitor the engine. *  In addition, seat belts, anti-collision light, and emergency locator transmitter (ELT) are generally required.

If operating over water, a flotation device is required. If flying at night, additional equipment is required.  This includes the equipment for day VFR along with approved position lights, and anti-collision light, a source of electrical power, spare fuses, if applicable, and if for hire, one electric landing light.

A specific helicopter may require additional items beyond what is required by regulation.  The pilot should review the helicopter’s rotorcraft flight manual to confirm.

*There are other requirements for liquid cooled aircraft and ones with landing gear or if operating over water.


14 CFR 91.205 Equipment Requirements

Other Helicopter Maintenance Topics

How does the pilot in command handle inoperative equipment?

If the equipment is not required, the item should be disabled and placarded as inoperative.

In order to determine if an item is required, there are a few steps to consider.

1. Is the item required by the aircraft type certificate?
2. Is the item required by a minimum equipment list?*
3. Is the item required by regulation for the type of flight, such as VFR day or VFR night?
4. Is the item required by company policy or operation specifications?
5. Is the item required by the pilot in command for safe flight?

* Minimum equipment lists are not common for part 91 operations.  A minimum equipment list is a formally approved document for a specific aircraft that identifies which items or equipment may be inoperative and under what conditions.  For example, an MEL may allow a pop-out float to be inoperative if the flight does not occur over water.


14 CFR 91.213 Inoperative Instruments and Equipment

Other Helicopter Maintenance Topics

What documents are required to be in the aircraft for flight?

The following documents are required to be in the aircraft during flight.  Airworthiness certificate, registration, radio operators permit (if outside the U.S.), operating limitations, and weight and balance.  The mnemonic ARROW is often used to remember these documents.

A-Airworthiness Certificate


R-Radio Operations Permit (not required in U.S.)

O-Operating Limitations (included within POH/AFM or placards)

W-Weight and Balance (generally included within the POW/AFM)

The airworthiness certificate does not expire as long as the aircraft is maintained in airworthy condition.  To maintain it in airworthy condition, an annual inspection must be completed and compliance with applicable airworthiness directives (ADs) is required.  The registration must be current and it expires every 3 years.  If operating internationally, a radio operator’s permit is required. A radio operator’s permit is not required if operating solely in the U.S.

The “W” in the ARROW is not clearly outlined in 14 CFR 91. Part 91.9 states to operate within limitations and the approved flight manual (or alternative) must be in the helicopter.  The regulations governing helicopter certification, 14 CFR 27, requires that the aircraft weight limitations (14 CFR 27.25) and the center of gravity (CG) envelope (14 CFR 27.27) are to be established and that these limits are to be included as operating limitations (14 CFR 27.1519).  As such, the helicopter’s weight and the W/B or CG limits must be in the helicopter as they are limitations.  However, the actual W/B for a specific flight does not need to be in the helicopter, but the W/B should be calculated for each flight to demonstrate the flight is operated within the limitations (14 CFR 91.9).


14 CFR 91.9 – Civil aircraft flight manual, marking, and placard requirements
14 CFR 91.203 – Civil aircraft: Certifications required
14 CFR 47.40 – Registration expiration and renewal
14 CFR Part 27 – Airworthiness Standards: Normal Category Rotorcraft
14 CFR 27.25 Weight limits (requires establishment of maximum weight)
14 CFR 27.27 Center of gravity limits (requires establishment of CG envelope)
14 CFR 27.1519 Weight and center of gravity (W/B are established as operating limitations)

Other Helicopter Maintenance Topics

If a monocoque tailboom had a small dent, is the helicopter still airworthy?

No. The helicopter is not airworthy with any dents in a monocoque tailboom.

Most training helicopters, such as the Robinson 22/44, and Sikorsky S300, use a monocoque designed tailboom.  With a monocoque design, there is no internal structure for strength.  The strength is derived from the external structure or skin.  Any dents in this structure would significantly weaken its overall strength.  A common example is an aluminum soda can. The can is difficult to crush when there are no dents.  You add a small dent in the can, and it can be collapsed with much less pressure.


Aircraft POH

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