Special Airworthiness Information Bulletin| FAA SPECIAL AIRWORTHINESS
SUBJ: Instruments Date: January 18, 2011
This is information only. Recommendations aren’t mandatory.
This Special Airworthiness Information Bulletin informs you of an airworthiness concern that is relevant to all airplanes certificated under Title 14 of the Code of Federal Regulations (14 CFR) part 23, as well as those certificated under the previous Civil Air Regulations (CAR) part 3. This information is also relevant to any special light-sport category airplanes (S-LSA), experimental lightsport airplanes (E-LSA), and experimental amateur-built airplanes.
At this time, the Federal Aviation Administration (FAA) has determined that this airworthiness concern is not an unsafe condition that would warrant airworthiness directive (AD) action under 14 CFR part 39.
On November 12, 2001, American Airlines Flight 587, crashed shortly after takeoff from New York’s John F. Kennedy International Airport. The crash killed all 260 people aboard and 5 people on the ground. The National Transportation Safety Board (NTSB) determined “the probable cause of this accident was the in-flight separation of the vertical stabilizer as a result of the loads beyond ultimate design loads that were created by the first officer’s unnecessary and excessive rudder pedal inputs.” As a result of this accident and subsequent investigation, it was revealed that many pilots have a misunderstanding of what the design maneuvering velocity (speed), VA, represents. Many pilots believe that as long as the airplane is at or below this maneuvering speed, they can make any control inputs they desire without any risk of harm to the airplane. This is not true.
The design maneuvering speed (VA) is the speed below which you can move a single flight control, one time, to its full deflection, for one axis of airplane rotation only (pitch, roll or yaw), in smooth air, without risk of damage to the airplane. Even though the accident discussed above is a part 25 airplane, VA is applicable to part 23, CAR 3, and LSA airplanes. Also, even though experimental airplanes may not have a published VA , they will still have some maximum maneuvering speed associated with the maximum structural design loads. Therefore, the pilot should be aware of what speed this is, and adhere to the guidance herein. The regulations governing the design strength requirements for airplane structure require adequate strength for full control deflection (below VA). However, they do not require the manufacturer to make the airplane strong enough to withstand full control input followed by a full control input in the opposite direction, even below VA. Neither do they require the manufacturer to design the airplane for more than one simultaneous full control input such as full ailerons with full elevator and/or rudder.
VA, as published in the airplane flight manual (AFM) or pilot’s operating handbook (POH), is valid for operation at the gross weight stated, which is typically at max gross weight. It is especially important to note that VA decreases as the airplane weight decreases. At first, this may seem counter intuitive. All pilots understand that when the airplane is subjected to an external force, such as the 2 aerodynamic force from a control surface, the airplane responds by accelerating (rotational acceleration) about one of the airplane’s axes. This was stated many years ago in Newton’s Second Law of Motion. The law states that when an object of mass ‘m’ is acted upon by a force ‘F’, it will undergo acceleration ‘a’ in the same direction as the force. More simply stated in the widely known equation “F = ma”, which can be rewritten as “a = F/m”. Rewritten this way, it is clear for a given control force ‘F’, as the airplane weight ‘m’ decrezses then the acceleration ‘a’ will increase. This higher acceleration gives rise to higher loads on the airplane structure. Therefore, as the airplane weight decreases, the allowable maneuvering speed must also decrease, to ensure that the airframe is not damaged. Pilots may remember from their written exam that VA-NEW = VA √ (WNEW/WMAX-GROSS) as the way to calculate the corrected (new) maneuvering speed due to operating at a weight less than the maximum gross weight. NOTE: This formula is for calculating the VA change about the pitch axis; however, it can be used for all axes.
The FAA wants to clarify that operators should know what the maneuvering speed is and to caution pilots on what to avoid by adhering to the information described above and contained in the regulations. We recommend the following for maneuvering at, or even below, VA:
DO NOT apply a full deflection of a control, followed immediately by a full deflection in the
DO NOT apply full multiple control inputs simultaneously; i.e., pitch, roll and yaw
simultaneously, or in any combination thereof, even if you are below VA.
Reduce VA when operating below gross weight, using the following formula:
VA-NEW = VA √ (WNEW/WMAX-GROSS)
For Further Information Contact
Mark James, Aerospace Engineer, 901 Locust, Room 301, Kansas City, MO 64106;
phone: (816) 329-4137; fax: (816) 329-4090; email: email@example.com.
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