Post Wrench Test

A flight test after maintenance needs to have more than just a careful preflight


Its somewhat astonishing to see how many accidents, or at least scary moments, occur on the first flight after maintenance. Perhaps the most striking one is the notorious case of a Navajo, in which the aircraft came out of the shop, took off, rolled over,and crashed, killing the pilot. The investigators discovered that the ailerons were connected backwards – yoke left rolled the plane right.

Its pretty easy to sit back in the cold, hard light of day and think, What a dummy! about that pilot. If the pilot really did follow the preflight checklist and note controls free and proper, theres a lesson for another day about seeing what you expect to see rather than what you really see. For the moment, however, the general approach to post-maintenance flights has far more intricate dynamics to consider.

Although its easy to dismiss the subject by assuring yourself youll do the best preflight ever, theres really more to taking an aircraft out of maintenance than running through a checklist slowly.

The military, as one might expect, has a very organized approach to the first flight after maintenance. The military has a procedure for everything. Each maintenance action has, in the maintenance manual, a specific notation as to whether any particular post-maintenance check is required. There might be nothing other than a ground check for removing and replacing a comm radio, but an engine removal/replacement will require a full Post-Maintenance Check Flight, or PMCF to those acronym-happy bureaucrats.

PMCF checklists usually have several sections. Each particular maintenance action requiring a PMCF will note which sections must be accomplished. Only specially trained flight crews are permitted to fly PMCFs. Prior to the flight, the flight crews sit down with the maintenance types, including the quality control people, to be briefed on each maintenance action performed.

The crew reviews the maintenance records and confirms each item worked on along with the specific checks required for each. For every check, there are fill-in-the-blanks sections to record any specific values to be noted, such as engine temperatures/pressures, time in seconds to retract or extend the landing gear, etc. After the flight, the crew again sits down with the maintenance folks to debrief everything they did and what happened.

For general aviation pilots, the question becomes how to apply this sort of thinking to light planes. The first step is the one most owners neglect: Make a thorough inventory of the airplanes condition before its worked on. If you are having the airplane checked out because of an engine problem, make sure you have a written record of the oil pressure and temperature, CHT/EGT data, static run-up RPM and other details on the last flight before the work is done.

If you dont have this baseline data, its hard to tell what effect the work had on anything other than your checkbook. Some things may be pretty subjective, like control feel. But its important to make some written notes to which you can refer later in order to confirm that the problem is solved, or identify significant changes in the aircrafts operation.

The next step is to become familiar with everything that was done. When you go to pick up the plane, sit down with the mechanic and go over all the work in detail. Theres more to this than simply hearing the mechanic say, Well, we did the AD on the ailerons.

You want to know exactly what they did, including all the pieces that were removed and replaced to obtain access to the area they worked on. This will give you an idea of all the things you want to check especially carefully on preflight, as well as in flight. Try to determine ahead of time exactly what differences you should see – engine temperatures or pressures, brake response, or any other components that were worked on.

For example, if the engine baffling was repaired, youll want to be sure to record CHTs and oil temperatures at various points in the flight, such as run-up, takeoff, climbout, cruise and descent.

The Paper Chase
Although more essential for the FAA than the safety of the flight, the paperwork on the repair job should be an integral part of this discussion. Your paperwork review should start with the bill for the work. Some mechanics, like Bill Broach of Chesapeake Aircraft Maintenance in Forest Hill, Md., attach a complete list of notes to the bill detailing everything he did in a manner far beyond the cryptic words used in the log book. Others put it in the itemization of the bill, with varying levels of detail. In any event, since the mechanic wants to be paid for everything he did, everything should be on the bill.

That means you should match up each item on the bill with the writeups in the airframe and/or engine logs. Remember – everything done should be mentioned in the logbooks. If your mechanic keeps the logs in the shop, stop by and make that verification rather than just mailing in a check.

During your discussion with the mechanic, make written notes of all this, including a list of each specific item to check during preflight inspection and during the flight. You can use these notes to establish a profile for the flight. This is called a test card by the experimental test pilot community, so youll be doing it just like Scott Crossfield.

Include in the list the speeds, altitudes, and times you want to fly, including spaces to record each piece of data at the point on the list where it will occur. This will help you organize each element you need to check in proper sequence, ensuring that you will test out each item requiring evaluation. Identify each flight point, such as:

• Startup
• Pretakeoff/runup
• Takeoff
• Vy climb to 1,500 feet
• Cruise climb to 5,000 feet
• Level off and checks in cruise at 2,400 RPM
• Cruise climb to 10,000 feet
• Level off and checks in cruise at 2,500 RPM
• Cruise descent to 3,000 feet
• Best glide speed descent to 2,000 feet
• Entry to pattern at 90 knots
• Normal landing
• Post landing
• Shutdown

The next question that may arise regarding paperwork is whether it is correctly documented. Many owners are familiar with the part of Appendix A to Part 43 of the FARs – the part that lists the preventive maintenance actions pilots can do themselves.

But as an aircraft owner, it is absolutely essential that you be familiar with that entire part. It tells you who is authorized to do what sort of work on your airplane and how that work must be documented. For example, if you are tired of the tangle of cables powering your handheld GPS from the cigarette lighter and want a wire installed directly into the electrical system, what documentation is needed?

To answer that question, you will discover in FAR 43.9 that you must determine whether this is a minor or major alteration. Go down to our old friend Appendix A, and it will tell you that this is a major alteration if it is a change to the basic design of the electrical system.

How do you know if you changed the basic design? You can ask an airworthiness inspector at the local FSDO. In fact, in this particular case, the answer obtained has varied from district to district. If your FSDO says its minor, a log entry signed by an A&P will do. If they say its major, you need a 337 signed by an A&P and approved by the local FSDO based on approved data (e.g., an STC or another approved 337 for the same work on another airplane) or field approval by an airworthiness inspector from that FSDO.

As a side lesson, this is why its best to ask the FSDO before you start the work. In some cases, you can go FSDO-shopping to find one that has approved the modification in the past, and will approve your execution of the same work again. Believe it or not, theres nothing that says the approval has to come from your local FSDO.

The point of all this discussion is that the pilot in command is responsible for everything in the maintenance logs on the plane you fly. Even though FAR Part 61 and virtually every Private Pilot training course have little or nothing about how maintenance should be performed or documented, FAR 91.7(a) (No person may operate a civil aircraft unless it is in an airworthy condition.) still makes the pilot in command completely responsible for the airworthiness of any plane he or she flies, including the maintenance performed and the writeups in the logs.

Remember, legal airworthiness encompasses much more than whether the airplane is safe to fly. Simply turning the airworthiness certificate around so its face isnt visible makes the airplane legally unairworthy.

Jumping through the legal loopholes does have a practical effect, however. Reviewing the paperwork gives you an opportunity to identify everything the mechanic did. This leads to the next step, which is examining the work done.

Hardware Check
Once you have sorted out and cross-referenced all the items on the bill, work notes, and logs, its time to look at the work. You should, to the maximum extent practical, examine it all personally. Clearly, you are not going to be able to inspect the bearings of an overhauled engine unless you live down the street from the overhaul shop and stop by periodically during the job.

However, you should open every panel you can do conveniently, lift up seats, crawl headfirst under the panel, open the access cover to the tail cone area, and do whatever is reasonable to take a direct, personal look at whatever the mechanic did. A good mechanic will not take any offense at this.

The best mechanics will be pleased and proud that you do, as you will see the quality of their work and are demonstrating an appreciation for that quality. Have the mechanic present for this inspection and get him to point out each item of work he performed. Oohs and aahs are good ego-strokers for your mechanic at this point in the operation.

Dont be afraid to ask about anything, either. Good mechanics are always happy to have more knowledgeable customers, as the more you know, the better you will in the future describe any problems in words the mechanic can understand. Ive personally heard pilots explaining a malfunction in terms that make the pilot sound like a bad blonde joke.

Being able to clearly tell your mechanic what the problem is can save you time and money, as well as saving the mechanic some aggravation, and you dont want an aggravated mechanic working on your plane.

You may also want to ask the mechanic to come along for the flight. This accomplishes several things. First, it provides a secretary to record data and notes on anything that crops up. (Even if he says, no, try to find someone else to ride along as flight test engineer.)

Second, it can be a good way to build your relationship with your mechanic. Many of them really do like to fly and are quite pleased when you ask them to join you. Having them fly with you also gives them a better view of the problem and the effect of the solution – there is much that simply cannot be duplicated on the ground.

Try telling your mechanic, Well, in cruise configuration at 2,400 rpm it makes a noise like my sons bicycle with a baseball card in the spokes, only slower, but without the twanginess. The look in his eyes will say things his mouth dares not to a customer. And finally, it gives you a final test of his confidence in his work – but dont say that in his presence.

At this point, you reach a very important step in the process – paying the bill. Mechanics have this very strange reluctance to allow the customer to fly off in the airplane until the check is signed and handed over, even if you tell him youre coming right back for a leak check.

Although just about every work order that you as the owner will sign contains a mechanics lien on the airplane for the cost of the work done, its a real pain to collect on such a lien, and the mechanic would rather spend his time working on other airplanes generating revenue than chasing you down to collect cash he needs to pay his bills.

But this does bring up another point. When you pay the bill, check the work done against the work described in the work order you signed when you dropped off the plane. Use the work order to remind yourself of everything that was supposed to be done. This way, you can be sure the mechanic didnt forget to do something you wanted and also that he didnt accidentally do some work another owner had requested on a different airplane.

Lighting It Up
The next step in the process is to consider each item that had work done on it, how it worked before, how it should work now, and what might go wrong with it during the post-maintenance check flight. For example, after an oil change, you want to pay special attention to possible oil system malfunctions such as high oil temperature or low or high oil pressure at each stage of the flight – especially critical flight points such as takeoff and short final.

You will want to consider carefully what your best course of action at each point is, and especially to review the airport area for emergency landing sites if the engine gives up at 200 feet over the departure end of the runway. By working each of these possibilities in your mind before you fly and making some predetermined decisions about what you will do in particular instances, you will save yourself a lot of fumbling and confusion if something happens in one of those critical situations.

This is a standard practice for airline crews on each takeoff when they calculate a decision speed (V1) below which they will abort and above which they will continue if one engine fails on the takeoff roll. By making the conditional decision in advance, theres no time wasted trying to figure out what to do if something bad happens at a time when immediate action is required.

You have finally reached the point of strapping into the airplane. Be sure to give your flight test engineer a mission briefing on the flight profile, and all the items you will want to record. See that he knows where the instruments with those data are in your airplane because even two airplanes of the same model with consecutive serial numbers will diverge in their panel equipment layout as time goes by. This will also allow your assistant to become familiar with the test card so he can assist you by reminding you of each test point, and what comes next as the flight progresses.

Only now that youve properly planned the flight is it time to execute it. Start up, and fly it by the book, and by the numbers. The only additional point to make on that is to write down not only all the test data, but also anything at all that seems unusual or different, so you can discuss it upon landing. Which brings us to the last element of the PMCF – the debrief.

If you were considering the importance of closing the loop with the mechanic, you allowed yourself the time to fly the PMCF as a local flight, returning to the airport where the work was done, to allow you a chance to go over all the data with the mechanic. This also takes a lot of pressure off you if things arent going quite as expected – you dont have to make an inflight decision about continuing home or turning back, because you already decided ahead of time that youre going to land back at the mechanics airport.

Once you do, take the opportunity to go over both the plane and the data. Check to see that everything that was tightened is still tight, and that which was loosened is still loose. Sit down and look at each of the bits of information you recorded and confirm that it is what it should be after the work was done. If it isnt, you can examine the reasons why it isnt and map out a suitable course of action.

Doing this by long distance from your home airport is never easy. Mechanics are much better at evaluating what they can see, feel, hear and touch than trying to figure it out from someone elses description. And if there is a problem requiring additional work, you dont have to try to schedule it later. Often, a good shop will do what it can on the spot to rectify the situation, letting you get home the same day, if not quite as early as originally planned.

Getting an airplane out of the shop and taking it on its first flight can be a rewarding and educational experience when you do it carefully and prudently. While it requires some investment of time and effort on your part, its not that complicated a process, and will increase your confidence in the final result as you fly it home.

Also With This Article
Click here to view “A Measure of the Work.”

-by Ron Levy

Ron Levy is director of the Aviation Sciences Program at the University of Maryland Eastern Shore.


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