Post-Maintenance Test Flying

Aviation lore veritably brims with tales of the fearless test pilot flying into harm's way to probe the limits of an experimental airplane's performance, the quintessential scenario being the flutter dive. You know the scene: Pull up, Buck, pull up!

Aviation lore veritably brims with tales of the fearless test pilot flying into harm’s way to probe the limits of an experimental airplane’s performance, the quintessential scenario being the flutter dive. You know the scene: “Pull up, Buck, pull up!”

Thank you, Hollywood. But in the real world, especially the one in which actual test pilots work, the flying is a mostly prosaic business, characterized more by methodical preparation and exceptionally careful and (usually) boringly repetitive execution. With a multibillion-dollar program at stake — not to mention the lives of those within the prototype being tested — there is no room for barnstormer antics in the cockpit. Every maneuver performed is authorized in accordance with a long slate of scheduled test points determined by the engineers who designed the aircraft, as well as the certification requirements of the relevant aviation authority.

Thus, the test flight is a carefully choreographed set of tasks intended to verify the aircraft’s performance envelope. (In the cybernetic era, it is also important to acknowledge the roles of wind-tunnel testing and computer modeling in aircraft design, allowing engineers to explore a design’s flight characteristics months before the actual aircraft leaves the ground. Accordingly, the test pilot’s job today is to confirm what has already been determined in the computer and ensure no unpredicted behavior emerges. If it does, then corrective action will be taken, sending the engineers back to their computer terminals.)

Which brings us to the moment of truth every business aviation captain will eventually face if she or he remains with an FAR Part 91 or 135 operator long enough: the post-maintenance test flight. This is generally necessary when serious work has been done on an airframe — for example, a corrosion inspection requiring a wing de-mating or empennage disassembly; maintenance of a major system like pressurization; scheduled overhauls (“C” or “D” checks); or completion of a modification affecting aircraft performance, such as the installation of winglets or new props. Other evaluation flights are scheduled over the life of the aircraft to test certain systems such as the ram air turbine (RAT) emergency electrical generator on the Bombardier Challenger product line. In all of these cases (and others), it is necessary to assure the safety of the aircraft and its systems before signoff and return to line service.


Such a post-maintenance flight is not an experimental test of new design. Rather, the industry-accepted nomenclature for such an evaluation is “functional check flight,” or FCF. (This would also apply to production testing of new aircraft as they roll off the assembly line.) Still, an FCF should never be approached casually, and a careful preflight planning process with an emphasis on risk management should be conducted before the aircraft leaves the ground. This should include consultations with maintenance staff and, in some cases, representatives of the aircraft’s manufacturer.

And post-maintenance FCFs also carry an imbedded message to the business aviation pilots tapped to fly them, according to one heavy jet operator who flew FCFs in military and business jets: “The sooner we line pilots stop talking about ‘test flights’ the better. It takes a special rating and a lot of oversight from the FAA to be a test pilot.” He notes that before an FCF can even occur, the aircraft must have been signed off as being airworthy.

“Our only task is to fly it using normal procedures to validate that fact,” he said. “We need to plan in great detail, follow the plan, and be spring-loaded to the ‘knock-it-off’ position [if a contingency occurs] so we can get the airplane back on the ground and talk things over, with the altimeter reading home field elevation and the airspeed indicator resting gently on its peg.”

Is This Flight Necessary?

The process begins with a thorough understanding of the work that was done on the aircraft. According to another heavy business jet operator, the first question to ask is whether a functional check flight is really necessary for the maintenance that was performed.

“Most maintenance procedures on modern aircraft do not require flight checks,” he observed. The vice president of operations for a large charter/management operation added that his maintenance department relies on the Aircraft Maintenance Manual (AMM) for guidelines on flight testing “because it gives you specific directions on checking the components that have to be flight-tested. The task cards for the event will cover the specific things the flight testing should include.”

For example, once a hot-section inspection has been completed, and assuming no deterioration was discovered, runs can be conducted on the ground. For major overhauls, rigorous operating cycles will be conducted in test cells by the engine manufacturer or repair station that did the work before returning powerplants to customers, necessitating only routine check flights after the engine was reinstalled and signed off.

“After major engine maintenance,” Lou Gray, captain and operations director at Duncan Aviation in Lincoln, Nebraska, said, “we would perform a static takeoff engine power check per the AFM [Airplane Flight Manual] and verify that power and temperature limits were within proper parameters.”

But there are some scheduled maintenance actions that require extensive, carefully planned evaluation flights. “The Gulfstream has an electrical load-shedding system that drops off nonessential stuff to reduce the electrical load on the airplane [in an emergency],” the vice president of operations from the charter/management company explained. Testing this feature “has to be done in the air because there are designated trip-level altitudes where certain functions will be performed by the systems, so you can’t do these tests on the ground. The electrical system does this only in the case of a contingency, and there are certain steps that the flight crew has to perform to make it happen.”

Another test requiring inflight evaluation is an APU start at altitude.

“If we have flap maintenance,” the vice president of operations said, “we do a flight test to load them up and check them out. You do whatever you can on the ground, but there are just some things that have to be checked out in the air, and flap rigging is a good example of one of them.”

In addition to the AMM and AFM, other references for planning an FCF include the Aircraft Owner’s Manual (AOM) and various customer support services offered by the original equipment manufacturer (OEM), including advice, publications and training from OEM flight operations departments. “Typically, we use the factory checklist,” Duncan’s Gray said. “In addition, we have a Flight Risk Assessment Tool [FRAT] sheet in our SMS [Safety Management System] for normal as well as maintenance test flights that will be completed prior to flying. A score above 30 on a maintenance FRAT requires mitigation or canceling of the test flight.”

So, Rule 1 of how to prepare for and fly a safe FCF is do your homework; study every source of information at your disposal, and, if necessary, use the aircraft’s OEM as well.

If there’s a choice among flight crews, obviously you want the best and most experienced pilots flying an FCF, especially those with high time in the aircraft type who are intimately familiar with the aircraft’s performance envelope and proclivities.

“In our company,” that same vice president of operations noted, “we go above and beyond the usual requirements in our training in terms of systems knowledge, and so we’re comfortable and confident with our pilots flying these flights. Nevertheless, you want very knowledgeable and experienced people flying these missions in both seats, usually training captains or check airmen.

“But I still brief them with the maintenance staff to see what they [the maintainers] want us to pay attention to,” he continued. “So, depending on the extent of the maintenance event, we will sit down with the maintenance department people to determine what was done on the airplane and what they expect of us. And we brief to the actual task card. A lot of times we take a maintenance tech with us on the flight, usually one of the ones who’ve performed the maintenance task.”

At Duncan, “We have a DOM [director of maintenance] who oversees all maintenance on company-operated aircraft,” Gray said. “He prepares a thorough briefing on what was done and what systems were affected. The DOM also prepares a check flight item summary with a list of what we are to look for and what checks he would like to have performed in addition to normal flight operational checks. For non-company operated aircraft, we meet with the lead mechanic and Duncan Aviation project manager to discuss what systems were affected and what needs to be accomplished on the test flight. These then follow the same procedures as we follow on our own company-operated aircraft.”

Fully understanding the work done on the aircraft can be summed up by the heavy business jet operator quoted earlier: “It is one thing to hear a stabilizer actuator was replaced, but quite another to hear the entire stab had to be removed to access the actuator. You need to really understand what components were affected.”

And this knowledge should then frame how the check flight will be planned and what will be carried out once aloft.

Rule 2, then, is to rely on the maintenance personnel — either in your flight operation or at the repair station that did the work — to brief the flight crew and explain everything that was done to the aircraft and how it will be verified on the FCF. If necessary, too, work with the DOM to plan the check flight, keeping everyone involved with the maintenance action in the loop.

Planning the Low-Risk Flight

After consulting with maintenance personnel, the FCF should be mapped out in detail while employing all the risk-management strategies necessary to ensure a safe operation. That task completed, pass it around knowledgeable sources to ensure you aren’t missing something critical.

Duncan Aviation’s Gray advised that special precautions to consider on the FCF include aircraft limitations, speed, altitude, VMC, weight and balance, crew qualifications and experience, stall flight, presence of a non-flight department crewmember, MEL items, fatigue and short-notice test flight — all addressed in some way in his company’s SMS.

He added: “Some mitigations for our test flight operations are: Was it minor maintenance such as avionics? Is a mechanic on board? Are both pilots type-rated in the aircraft? Is the test flight within 50 nm of [home base]? Was there a detailed briefing? And so forth.”

Should the flight crew reserve flight test airspace with ATC? There are mixed opinions on this subject, and readers should use their own discretion on whether to set up a block of protected airspace for the FCF. If the decision is to do it, Gray offered this advice: “This would involve typical flight plan routing to coincide with estimated time to complete all function tests. Always add ‘maintenance check flight’ in the remarks section [of the flight plan].

“Once we make contact with the ARTCC controller,” he continued, “we advise of our route or altitude specific for the checking needed for that flight. If there is an unusual requirement, for example, a 10-mi. arc to be flown at 2,000 ft. AGL through the active runway’s approach, then a call to ATC always saves fuel and everyone’s patience. Here at Duncan Aviation-Lincoln, we have a low traffic area north and west of KLNK that we try to do our check flights in so as to not disrupt normal air traffic.”

But the former military pilot noted that in the civilian world any such flight follows a determination by maintenance that the aircraft is airworthy. Thus, “If you think you need special airspace to do the check, you might be in over your head.”

The next consideration in planning the flight is whether to require a maintenance technician on the flight — preferably the individual who signed off on the work.

“Our DOM or an experienced company technician rides along on all of our post-maintenance check flights to assist the flight crew with the flight check items,” Gray said. “Also, the maintenance person on board records all discrepancies and/or items that need to be addressed after the flight.”

Duncan also doesn’t allow “nonessential passengers” to ride along if major maintenance of engines or flight controls or structural repairs were involved. “Also, if a stall or similar flight check is required,” Gray said, “it is performed during a separate flight with only the flight crew aboard. Afterward, a post-flight debrief is conducted followed by a final signoff.”

Not only is the tech aboard the test flight accountable for the work, but they can support the flight crew in accomplishing the test points and assist in the event of a failure.

While it’s impossible to anticipate every possible contingency, the FCF should be “built” around the maintenance performed on the aircraft with action plans addressing failures that could occur of the systems or components that were inspected, repaired, replaced or modified. To the best extent possible, this plan should reflect the essence of risk management.

Rule 3: Plan the functional check flight around the maintenance that was performed on the aircraft with an emphasis on risk management and an understanding that the flight should be a collaboration between the cockpit crew and maintenance staff.

Conducting the Check Flight

Now it’s time to fly. But beforehand, “Thoroughly brief before departure, and review and prepare for possible emergencies,” Gray advised. “Fly on an IFR flight plan under ATC control and in radio contact at all times. The flight crew and maintenance personnel should hold a preflight briefing and cover emergency situations that may occur during the intended flight that are related [or maybe not related] to the maintenance performed. If an incident or problem arises on the flight, the crew and maintenance tech will assess and determine if the flight can be continued or if return to base is required.”

Bob Howie, who captains a Gulfstream V out of Houston and in an earlier chapter of his career flew FCFs for a major Southwest U.S. repair station, offered a primer on how to preflight for an FCF of an aircraft that has just emerged from heavy maintenance.

First, make sure the aircraft documentation is up to date. “Even before you get in the airplane,” Howie told BCA, “spend some time reviewing the ‘can’ — the binder containing all the aircraft documents. Go through all the paperwork. Make sure there are no open write-ups, that everything has been signed off, that all the dates have been complied with.

Also, confirm that all the squawks have been addressed and signed off, that the weight and balance is correct — everything that makes the airplane legal to fly. You can’t make the FCF until the airplane has been signed back into service, that it is truly airworthy.”

Then move to the exterior preflight inspection. “This is straightforward, but take time to look at everything,” Howie advised. “I once heard of a crew doing a walk-around who noted that the nosewheel tires had been put on backward so the rain-deflectors, or chines, were on the inside. They were doing their job correctly and caught this. It’s the same walk-around you do under normal circumstances but more intense, ensuring that the mechanics put the airplane back together correctly.

‘Touch everything physically,” he continued. “Use your flashlight as a pointing device as much as you’d use it as an illumination device — it keeps your attention focused on what you’re looking at, that something is what it is and not what you’re expecting it to be.”

“After every maintenance event,” Duncan’s Gray said, “our flight crews do a thorough preflight inspection to make sure that all switches are in the proper position as well as that all panels and cowlings are secured properly. We use the same process on customer aircraft.”

Howie, too, advised that the flight deck preflight be equally thorough and hands-on.

“Make sure all switches, knobs and buttons are in the correct place, that the circuit breakers are all pushed in, and if after the plane’s been on jacks, the gear handle’s down, as sometimes the techs will lower the aircraft to the ground and leave the gear handle in the up position,” he said. “Make sure that everything is in the position where it’s supposed to be. As you go through this, touch everything with your finger — use your tactile sense to verify. Again, that way you will see it as it is and not what you’d like it to be. This process will typically take 2 hr., so don’t try to rush through it.

“Then when you turn the airplane on, that is, strike the battery switches,” he said, “make sure all the lights come on as they’re supposed to. Do the APU start and stop lights come on? Do the annunciators function? Do all the lights come on, including the one showing that the annunciators are armed? Then go through all the function checks. Before you move the airplane, make sure everything is in its proper form and function.”

Howie is also a proponent of taking a maintenance rep on the ensuing check flight since maintainers “take as much pride in what they do as we pilots” and in case things go awry. “Once on a check flight, we got a bunch of warning lights,” he recalled, “and the maintenance tech recognized immediately that it was a problem with the breaker, which didn’t pop but was going bad. We returned and they fixed it.”

But what if a serious problem arises on the check flight?

“This is critical,” the pilot of the heavy jet quoted earlier observed. “We often take off with a laundry list of things to do and are tempted to write off one missed item and move on to the next. You need to think these through on the ground. If your attempts at validating a yaw damper failed, for example, it doesn’t make sense to next check a rudder limiter. But we don’t often think of this in the heat of the moment. You need to have clear guidance on what needs to happen correctly before proceeding to the next item.

“If something doesn’t go well,” he continued, “any person in the cockpit should be able to say ‘knock it off.’ I once did a yaw damper check on a GIII that went awry. The airplane nearly inverted itself. My gut instinct was to try it again, but the looks on the faces of the other pilot and engineer told me I should rethink that plan.”

For his part, Howie recalled an FCF during which the nose gear locked properly in the down position but wouldn’t lock up when retracted. He returned for a safe landing and when the hydraulic fluid was drained from the nose gear strut, it was found to be the wrong hue and viscosity. On another FCF in an Astra, “we got ‘ground spoilers’ when we weren’t supposed to. Fortunately, the Astra didn’t mind, and kept on flying, but if that’d happened on the Gulfstream [he flies now], I wouldn’t be here to tell the story.”

Rule 4: Before launching on an FCF, work out a plan for handling contingencies that could happen as a result of the maintenance performed.

And Afterward . . .

If something unplanned occurs on the functional check flight, the response should be clear: Make an assessment, take corrective action if possible, decide whether to continue the flight, and if not — especially if the aircraft is at risk — land as quickly as possible. You’ve already made a risk-management plan and assigned duties to each crewmember — including the maintenance tech riding along. So function as a crew, and back each other up.

Whether there has been a glitch to be corrected or all goes well and the aircraft is deemed safe and ready for reentry to service, the last task will be to conduct a post-flight review of the operation. The object here is to learn from the experience, exchange information and points of view, and avoid problems in the future.

Rule 5: Debrief after the FCF and review the entire post-maintenance verification process. Encourage feedback from all involved, identify any mistakes that were made, and modify your operations manual and SMS accordingly.

As Lou Gray concluded, any functional check flight is an extraordinary event. “We make a point to remind everyone that this will not be a normal flight! They should assume nothing. When rolling down the runway, as soon as the flight controls become effective, the pilot needs to make sure he or she gets the proper response from his or her input for all three axes. And whenever someone ‘functions’ [i.e., carries out] an item, he or she always moves one switch at a time, waits, and observes the outcome. If all is OK, they can then try another function. It is also vitally important, as always, to work as a team.”

This article appears in the June 2016 issue of Business & Commercial Aviation. Subscribe here.

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