Operational vs. Functional

Maintenance Matters

Operational vs. Functional

A look at the different checks

By Jeffery Howard

August 2004

Often the terms operational check and functional check are being used interchangeably by mechanics and pilots. To help avoid confusion, I would like to offer the following.

Operational check
Operational implies the concept of a number of systems functioning together. This comes from a military operation. This military operation may be a group of individuals or individual units working together to accomplish a single objective. While each individual or each unit has a specific job or function, without the successful completion of their task, the operation would not be complete.

On an aircraft, an operational check would consist of operating the aircraft to ensure that all the systems function together. Even though each system may have an independent function that does not mean that its function does not affect the function of other systems and therefore the operation of the aircraft. An operational check may be conducted on the ground or in the air.

Functional check
Functional, on the other hand, implies that it applies to an individual system or element within a larger entity. As with the example above with a military operation, each unit has a specific job to complete regardless of how the other units perform. The final outcome of the operation is determined by each individual unit's success. While all units' functional success or failure may or may not have an overwhelming impact in a negative manner on the success of the operation, the completion of the operation is what is important.

An aircraft is no different. While it is required that all systems of the aircraft be operative for the aircraft to be airworthy, not all systems must function for the aircraft to operate. This is not to say that a system that is not functioning will not impact the operation of the aircraft. On the contrary, the failure or nonfunctioning of a single system may be enough to bring an aircraft down.

System vs. aircraft
An operational check of an aircraft may be conducted to test the function of a system. While the operational check is performed to evaluate the entire aircraft and the interrelationship of all the systems, it is usually the objective of the operational check to perform a functional check of the individual system and its impact on the entire aircraft. The functional check may require the aircraft to be flown for the individual system to be properly evaluated. This would require the aircraft to be operated in flight only after a complete evaluation on the ground. This would be done to ensure the aircraft's safe operation and to ensure that the function of the system does not negatively affect the operation of the entire aircraft.

Therefore, an operational check evaluates the entire aircraft to include all systems and their interaction with each other. A functional check evaluates only a single system within the entire aircraft. This is not to imply that all systems in the aircraft must have a functional check to complete an operational check on the aircraft. The pilot will still have the responsibility to ensure the aircraft is safe before and during the flight. An operational flight check involves the pilot following the checklist and performing the preflight testing as with any flight. With this flight, the pilot, along with his other duties of flying the aircraft, may be involved with a functional check of a specific system.

In the air or on the ground
An example of an operational check that would not require the aircraft to be flown would be a position light change. With the latest changes to the general maintenance manual (GMM) and general operations manual (GOM) this will be referred to as an operational ground check. After the assembly is installed there is a functional check of the light to ensure it illuminates with the battery powering the aircraft and the switch that controls the light is in the on position. Each one of the functions is independent while each one affects the entire aircraft in its operation. Each functional check is completed on the ground with conclusive results. The operational check of the aircraft is complete based on the fact that there is no functional check that can be accomplished in flight that would change the results of the functional checks performed on the ground. This is in conformance with 14 CFR Part 91.407(c).

In this example, the aircraft would have to be flown to complete an operational check. This check is referred to as an operational flight check in the latest revision of the GMM and GOM. A tail rotor has been changed on a BO-105. After the assembly is installed there will be functional checks involving rigging, flapping, and balance. While the balance and flapping may be accomplished on the ground, only part of the rigging can be accomplished on the ground (operational ground check). The BO-105 maintenance manual requires the aircraft to be flown to adjust the balance weights (Chinese weights) on the tail rotor assembly (operational flight check). This adjustment and functional check requires the aircraft to be flown. Upon completion, the mechanic signs off the functional check and approves the aircraft for return to service. The pilot signs off the operational check. This is in conformance with 14 CFR Part 91.407(b).

Who may approve what?
In a 135 operation, the pilot has very limited authority to approve an aircraft for return to service. This authority is limited by the type of maintenance, 135 certificate holder, and by the training the certificate holder gives the pilot. If a pilot is trained within an approved training program, and it is limited to preventive maintenance as described in 14 CFR 43.3(g), the pilot can perform that maintenance.

An example of this limitation may allow the pilot to remove the chip plugs for an engine chip light. If no debris is found, the pilot may approve the aircraft for return to service. If there is debris on the plug, only a certified mechanic can evaluate and make a determination as to the airworthiness of the engine. If the mechanic makes the determination he may approve the aircraft for return to service. Again if there is no debris on the chip plug, an appropriately rated pilot may approve the aircraft for return to service.

In a Part 135 operation, a mechanic may approve an aircraft for return to service. Some pilots have limited authority on specific items to approve an aircraft for return to service. Only a pilot may return an aircraft to service after all the maintenance is completed. Maintenance is not complete until the aircraft is approved for return to service for the maintenance described in the entry.

So what does this mean?
If maintenance has been performed on an aircraft, and the system that had the maintenance performed requires a functional check, that check is part of a maintenance procedure and can only be approved for return to service by a certified mechanic. A pilot may conduct the functional check and report the results to the mechanic. The mechanic may then sign for the work completed (functional check) and approve the aircraft for return to service. The pilot may have conducted the functional check of the system during an operational check of the aircraft. After the mechanic approves the aircraft for return to service, the pilot may then make an entry returning the aircraft to service without further flight evaluation.

A sample logbook entry
For the mechanic, there are certain pieces of information that must be in the logbook entry. These requirements are found in 14 CFR Part 43.9, 14 CFR Part 91.417, and 14 CFR Part 135.439. The 14 CFR 43.9 explains the content of a logbook entry; 14 CFR Part 91.417 explains what information the owner/operator must retain; and 14 CFR Part 135.439 defines what information the certificate holder must retain and references 14 CFR Part 135.443. The 14 CFR Part 135.443 requires the certificate holder to ensure that logbook entries are made in accordance with the certificate holder's manual.

With this in mind, the mechanic and pilot have a format for a logbook. Since the mechanic will make an entry for approving the aircraft for return to service before the pilot, let's look at the maintenance entry first. The content of this entry is influenced by 14 CFR Part 43.9 and without a specific form or format, it would be the guiding and driving force for the content of the entry.

An operational flight check was conducted by (pilot's name) with the following results of the functional check of (system). (Results of the functional check.) These results are within the functional limits called for in (appropriate maintenance reference). (Date) (Total time) Henry Technician A&P 123456789

The example above is not the only way to sign off a functional check. Some functional checks may be observed by the mechanic in an official capacity as a crewmember. Changes in the wording could accommodate this variation in the functional check.

The pilot has a specific reference in the general operations manual. His entry would consist of the following.

Operational check flight performed, date 10-15-2003, aircraft T.T. 11984:45, and returned to service. Peter Pilot ATP 987654321

Revision: GMM Vol. 2, Chapter 2
There has been a change in the GMM and the GOM. These revisions have restated 14 CFR 91.407. It very clearly states what the pilot's responsibilities are with respect to ground checks and flight checks.
It does not change the requirement of the mechanic to ensure that any logbook entries that need a signature, have a signature. The change requires both pilot and mechanic to be involved with the operational ground check and operational flight check where a functional check is a result of maintenance. This is not a new concept, only stated differently than what we are used to seeing.

An example of a case where we are already in compliance with this requirement is blade tracking. The aircraft must be flown for tracking to be accomplished. The mechanic can not fly the aircraft and the pilot can not adjust the track of the aircraft. It now becomes a team effort.

When the aircraft requires tracking, the mechanic installs the tracking equipment. The mechanic briefs the pilot on the tracking procedure and the configuration the aircraft needs to be in as part of the tracking procedure. The aircraft is flown, adjustments are made, and the functional check is complete. The mechanic removes the tracking equipment and gets any additional inspections required for the removal of that equipment. The mechanic makes a logbook entry with the details of what was accomplished and that the functional check is complete. The mechanic's signature and certificate number approves the aircraft for return to service for the work accomplished.

Upon completion of the above functional check the operational flight check is also complete and may be signed off by the pilot as per the PHI GOM.

Example No. 2
After a day's flying, the pilot approaches the mechanic with a problem. The distance measuring equipment (DME) is giving erroneous readings. The mechanic troubleshoots the system and determines the antenna is defective. He changes the antenna. The mechanic does not have test equipment for the DME. The local avionics tech is on the other side of his area of responsibility. That makes him too far away to get back that night. The mechanic refers to the GMM Vol. 5. The mechanic discovers that there is a method to perform a functional check in flight using a local VORTAC station. The mechanic makes a logbook entry that an operational flight check for a functional check of the DME is required.

The pilot arrives the next morning to find the requirement for an operational flight check. He contacts maintenance to inquire about the functional check. After a briefing and explanation of the functional check required in the GMM Vol. 5, the pilot proceeds with the operational flight check. The mechanic may observe the functional check or rely on the results reported by the pilot. If the results of the functional check are not satisfactory further maintenance is required. If there is a provision to MEL the DME, further maintenance may be deferred. If the results of the functional checks are satisfactory, the mechanic will sign off the completion of the functional check in the logbook, approving the aircraft for return to service.

It is at this point, the pilot may make a logbook entry completing the operational check to include flight and returning the aircraft to service.

14 CFR Part 91.407
After reviewing 14 CFR Part 91.407, the sequence of events in the two previous examples follow this rule.

An aircraft has had maintenance performed and that work has been approved for return to service in accordance with '43.9 or '43.11. There is further testing on the system that can not be performed without flying the aircraft. An operational check is written up. The pilot may conduct the functional test, but the mechanic is the only one that can certify the system is operating as per maintenance instructions. After the operational flight check is complete and the mechanic has certified the aircraft is approved for return to service, a pilot with at least a private pilot certificate may return the aircraft to service. The pilot will document this by logging the flight in the aircraft records.


Jeff Howard is chief maintenance instructor at Petroleum Helicopters (PHI) in Lafayette, Louisiana. He can be reached at jhoward@phihelico.com.

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