An aviation’s early days, there were few if any regulatory criteria on designs and operations. Pilots and maintenance personnel were up to their own on deciding what limits they would adhere to. Accidents were plentiful and often unnecessary. The public demanded that safety be improved, and government, engineers and the military stepped in to establish limits as aviation became more advanced.
Manufacturers responded to the demand, a process greatly accelerated by World War Two. Engineering started with design analysis using the parameters requested by the customer to come up with an aircraft that fulfilled the request. Sometimes in-flight or structural testing revealed a bad design and accidents occurred.
Along came the Civil Aeronautics Agency, which later became the Federal Aviation Administration. These organizations established aircraft airworthiness standards, which were enhanced as years went by thanks to the reams of data collected from various activities including accidents, flight testing and wind tunnel tests. After meeting all the standards type and production certificates were issued manufacture. For small airplanes, those weighing no more than 12,500 lbs., FAR Part 23 is the regulatory standard type-certified small aircraft must now meet. Additional FAR Parts 33 and 35 cover certification of engines and propellers, respectively.
Why limits? Limits have been established for many different reasons. As pilots, we may think that operation at over maximum weight would only result in reduced performance. Not necessarily. Although performance will suffer as a result of being overweight, other important factors also are involved, such as controllability. In other words, you may not be able to recover from a gust of wind on takeoff or landing due to insufficient flight control response.
Operation above airspeed limits, especially in turbulence, may result in buffeting or even structural damage. In extreme cases, aerodynamic control-surface flutter and the stresses imposed will damage the aircraft structure, perhaps resulting in subsequent loss of control of the aircraft as well. Certainly, maneuvering speed is an important value to memorize. The reason for this is very much to keep the structure within the designed airframe load limit envelope and in one piece.
Landing gear operation above limit speeds places an additional load on the operating mechanisms. Multiple operations at or above the limit can certainly cause a failure of the landing gear, but when that might happen is anyone’s guess.
Flap overspeed operation is frequently dismissed as “no big deal.” Unfortunately, it is a big deal. The flap operating system is designed to operate at a maximum load and not only is the operation compromised at higher-than-limited speed, it is the wing structure that is impacted as it absorbs the stresses created by changing the center of lift. Again, not only is this compromising aircraft control but it’s damaging the structure as well.
Life limits exist for not only an airframe but major components and accessories as well. “Life limit” means that the specified parts or components must be removed from service and disposed of when reaching the life limit, whether expressed in hours, cycles or another metric. A life limit differs from time before overhaul, for example in important ways: A life limit can’t be extended except by a regulatory change and the component can’t be overhauled to return it to a zero-time item.
It is the aircraft owner/operator’s responsibility to maintain accurate records of each and every life-limited component and have those parts replaced at or before the appropriate time. The details of what parts and when to dispose of them can be found in the latest revision of the type certificate data sheet, or current maintenance manual. Airworthiness directives also play an important part of requiring replacements prior to failure.
Examples of GA aircraft having life limited parts or structures include the Beechcraft 58P Pressurized Baron, Piper’s PA-44 Seminole, PA-34 Seneca and PA-38 Tomahawk, Cessna’s P210 Pressurized Centurion and Cirrus Design’s SR series, to mention just a few.
With advances in technology, it is now possible to accurately determine when a part or structure will fail due to fatigue. Aging aircraft and components have a varied life expectancy due to the effects of corrosion and repeated excess loads (exceeding limits), if any, which weaken the structure. A “fudge” factor is incorporated into the life limits as it is a definite safety requirement that these parts must be replaced on schedule.
Maintenance plays a very important part in keeping aviation safe. It is very important that maintainers be notified if any limitation has been exceeded, and that there’s a record. When an aircraft is undergoing a routine inspection, maintenance technicians should concern themselves with not only carefully inspecting the entire aircraft but also checking to see that required placards, instrument markings and warnings are properly displayed according to current revisions to type certificate data sheets, and that Pilots Operating Handbook (POH) or Aircraft Flight Manual (AFM)information is current. In addition, within the aircraft is the required POH/AFM specific to the serial number of the aircraft.
Limitations may change during production or even years later due to airworthiness directives, service bulletins and flight manual revisions. Replacing components like propellers may result in changes that alter limits. Maintenance and inspections must always be performed according to current maintenance manuals.
In addition to maintenance-manual requirements, instructions for continued airworthiness may have been issued for modifications made according to Supplemental Type Certificates. The mods themselves may include limitations and must be followed. Special inspections may be necessary, such as when a flap overspeed, hard landing or engine/propeller overspeed occurs.
An exceedance—an event in which a limitation is not met—can have consequences: As pilots over the years, it is likely that we have bumped into or exceeded limits either by neglecting them or in the heat of the moment unintentionally been in the “red.” What happens, in most cases, is nothing, so no worries, right?
Well, it’s not that nothing happens. Most important is the risk undertaken in the event itself and the later possibility of structural failure at some point in the life of the aircraft due to a cumulative effect on it or its components. The truth is, due to the engineering in the design of the aircraft and inspections completed by maintenance personnel, the danger of complete failure is mitigated and the seriousness is not often immediately realized by the owner/operator until the maintenance bill is received.
An example of this is an aircraft undergoing its first annual inspection after a recent purchase by the new owner, it was revealed that both flap spars were cracked and the repair cost was estimated at $5000. How did this happen? Further investigation revealed that the flap limit speed was incorrectly marked on the airspeed indicator. A little more research indicated that this early model of the aircraft had a lower flap limit speed and later models identified by serial number had a significantly increased limit speed due to structural improvements during production. A service kit was available for early model aircraft but was not incorporated in this aircraft.
Another extreme example of exceeding limits is a popular early business jet with a nasty habit of losing control at high Mach speeds in the flight levels. Incorporated into the design of the aircraft was an audible and visual overspeed warning.
After a few fatal accidents, the truth came out: Flight crews were disabling the speed-warning device by tripping a circuit breaker and then purposely exceeding the speed limit. The aircraft flight manual explained the reason for the limitation—loss of control—and what could happen if the limit speed was exceeded, to little avail. Flight training and instructors emphasized this adverse condition and the consequences, and the problem basically went away.
While accidents involving exceeding limitations are rare, exceeding any limit potentially has penalties requiring expensive repairs or leading to a serious accident. The reality is that it’s not just a limit that has been exceeded or was in the past. Instead, it’s usually a combination of factors that cause an accident. Combine a slightly out-of-CG airplane that’s a little overweight, a little slack in the control cables and a gust of wind or turbulence, and an accident happens. Acknowledging that limits should not be exceeded is the mark of a conscientious pilot.
Mike Berry is a 17,000-hour airline transport pilot, is type-rated in the B727 and B757, and holds an A&P ticket with inspection authorization.