Fly 'Em Better
Understanding POH & AFMS Limitations
by Jim Irwin, Vice President, Products
Autopilot installations are required to have FAA certified and approved operating instructions. These instructions can be provided in the form of Placards, Supplemental Flight Manuals (SFM), Airplane Flight Manual Supplements (AFMS), or Pilot Operating Handbook (POH) Supplements, depending upon the method of information presentation used by the manufacturer when the aircraft was certified. By regulation (23.1581), aircraft over 6000 lbs. maximum weight must have an AFM; For 6000 lbs. or less, the required information can be provided in the forms listed.
A decade or two ago, it was not unusual to provide autopilot operating and limitation information on a Placard that was affixed to the instrument panel or on adjacent side panels. As the electronic equipment grew and became more complex, placards fell out of favor because there were too many and they cluttered up the panel, and operation information was too voluminous to be covered on a placard. For those aircraft certified as "placard" airplanes without Airplane Flight Manuals (AFM), equipment suppliers provided Supplemental Flight Manuals. This document is very similar to an Airplane Flight Manual (AFM) Supplement in format and content but it does not reference the AFM.
AFM supplements and Supplemental Flight Manuals are provided in a standardized format matching the format of the Airplane Flight Manual. The format was developed by GAMA and the FAA in the seventies. The manuals provide mandatory FAA approved information, as well as unapproved general information desired by the manufacturer. In the case of autopilots, the limitations section provides limitations on aircraft operations when the autopilot is in operation, and often revised aircraft limitations regardless of autopilot operation. These supplemental limitations become the limitations for specific operations for the airplane.
The AFM supplements provide general system information, limitations, emergency procedures, normal procedures, required operating equipment, and weight and balance sections.
This discussion will cover some autopilot limitations and why they are often imposed.
1. Autopilot operation prohibited above XXX KIAS (or KCAS or MPH CAS).
This is the maximum speed for operation of the autopilot, usually in a descent. This speed provides a margin to VNE in the event of an autopilot failure that results in a nose down maneuver entered from the limit speed. This speed is always either Indicated Airspeed (IAS) or Calibrated Airspeed (CAS), which is IAS corrected for instrument and Pitot/Static position errors. Notice that both of these speeds are basically Indicated. Some pilots confuse CAS with TAS (True Airspeed) and complain that the autopilot limit speed is too low because "My airplane cruises that fast. Autopilots are typically certified with a significant margin between the IAS/CAS at cruise and the limit speed achieved in a descent.
2. Flap Extension/Retraction limited to XX° during autopilot operation.
This limitation is imposed because in one or more flight conditions (including MAX-FWD and/or AFT-CG) lowering or raising the flap results in either an altitude gain or loss over 100 ft. or an excessive attitude change or rate of change. Often the problem occurs at a CG extreme, usually AFT, while at normal CG's the use of flaps is well controlled by the autopilot and almost no change occurs. Pilots then question "Why the limitation?" and proceed to use the flaps beyond the specified limit. It is good to remember that malfunction tests and other tests are not evaluated beyond the limitations provided, so no one knows what the attitude and altitude excursions might be in these areas outside the envelope. There is a reason for every limitation; don't ignore them.
3. Autopilot coupled missed approach or Go Around maneuver not authorized.
Pilots have a high work load during a missed approach maneuver - it doesn't last long, but attention must be divided between a number of activities. The most important is aircraft control and establishing the climb. If an autopilot is used, it is easy to minimize the aircraft control checks due to the confidence that the autopilot is taking care of it, while the pilot spends more time on the lesser important elements. If the autopilot has not been set correctly, then this confidence is misplaced, and an incorrect flight attitude may go unnoticed until a dangerous situation exists. For this reason, most manufacturers recommend that the autopilot be disconnected until the transition to the climb is complete, then re-engage the system. This procedure keeps the pilot in the loop during the critical part of the procedure.
4. Autopilot operation prohibited during single engine (S.E.) approach operations.
All autopilots certified in multi-engine airplanes must be able to withstand an engine failure in the most critical condition of flight without allowing excessive turn/yaw rates to develop in the period before the pilot can react to the failure. The limit is 3° per second after the initial yaw surge.
Beyond that requirement, autopilots may be limited during S.E. operations. The most common limitation disallows S.E. approaches. This limitation is usually imposed if the autopilot does not have adequate authority to handle power changes during the approach or if it actually increases the work load rather than help to lower it. FAA certification tests usually do not evaluate stabilized, well conducted, approaches. Instead the certification pilot usually conducts transitions in IAS, aircraft configuration, etc. and overcontrols any power changes in order to assure that the autopilot is capable of handling its part of the load if the pilot gets behind.
This is another area where the pilot needs to be "in the loop", and an autopilot provides a false sense of security. Many times the manufacturer of the system will request of the approval authority that the system not be approved for S.E. approaches (or Go Arounds) for this reason.
5. Mandatory pre-flight limiter check must be conducted a minimum of once each flight day (or sometimes before each flight, in the case of electric trim).
Modern autopilot systems often employ designs that can suffer a single failure without an adverse operation occurring until a second independent failure also occurs. In these cases, it is important to identify before the flight whether the system is 100% or whether it has already suffered a failure that could otherwise stack-up with subsequent failures and allow an abnormal operation. Although these systems are tested for the effect of multiple failures, the test criteria differs from the standard tests, and in many cases, they allow shorter time delays to recovery or different maneuvering or stick force limits. For these reasons, it is mandatory that these pre-flight procedures be followed - they are important.
6. Autopilot operation prohibited below XXX Ft. AGL during coupled approach operations.
The altitude loss from an autopilot failure and the recovery from the failure on a nominal 3° Glide Path, cannot drop below a 1:29 slope underlying the Glide Path and projecting outbound from the approach end of the runway. When the measured altitude loss during an approach malfunction plus the height of the aircraft from the wheels to the G/S antenna, penetrates the 1:29 plane, the minimum approach altitude is increased to an amount that allows the maneuver without penetrating the underlying safety plane. If the resulting minimum altitude is less than 200 ft, normal Category I minima applies. If the minimum altitude is above 200ft, that higher altitude becomes the new minimum approach altitude for all approaches with lower published minimums when the autopilot is operating.
The primary document which provides acceptable criteria for meeting the general requirements of FAR 23.1329, covering autopilot approvals, is the Advisory Circular 23.1329-2. This revision was issued in 1991. Before that time, altitude loss information was provided, but the losses were not converted to explicit minimums, so earlier autopilot approvals typically do not include a limitation on approved altitudes.
7. Autopilot operation prohibited below XXX KIAS.
This limitation is usually provided because the performance of the autopilot system is not adequate below the reference speed during a specific maneuver and flight condition. Remember, autopilot performance is evaluated at full forward C.G. and with minimum clutch authority, as well as at maximum gross weight and maximum AFT C.G. These conditions often introduce flight characteristics and interaction between control axis that are not common in normal load conditions. Couple that with abrupt power changes and/or turns and autopilot dynamics or authority may not be sufficient, causing a limit IAS to be applied.
Even though, during normal operations, the autopilot may seem to fly the aircraft just fine below the limitation speed - remember, there is a condition that caused the limit, so don't be tempted to ignore it.
8. Autopilot off during take-off and landing.
Autopilots (Roll and Pitch) are not tested for operation, either malfunctions or normal, during take-off and landing. Therefore, the degree to which the system might interfere with normal vehicle control is not known and thus the limitation. If it ever became necessary to land with the autopilot engaged, pull the system circuit breaker (CB) and the electric trim circuit breaker to assure there will be no drive against your maneuvering of the aircraft. The servo override limits established in the FAR assure that a safe landing can be made, although the controls will be stiff.
Yaw Dampers are different. It is much more likely that a pilot will simply forget to turn the Yaw Damper off prior to landing. Because of this, they are routinely tested for the effect of a runaway during take-off and during landing and for the effects of interference caused by normal operation. As a result, Yaw Dampers often have only a limitation to operate at VNE or below. All Yaw Dampers are not approved for take-off and landing. Be sure to read the limitations and follow them for safe and comfortable flight.
Conclusion...
Not all limitations are covered here, but like those presented, they all have a reason for existence - always follow them. They are the Law. In some cases, like limitations on autopilot operations beyond certain C.G. limits, they become aircraft limitations as long as the autopilot is installed (it does not have to be operating). Always review them to make sure you understand them. If you do not understand or you want to know why, call the manufacturer's certification department. Remember, you cannot know too much about your aircraft or its systems.
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