Maintenance Check Flight Best Practices 

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Background. From time to time, you may be called upon to conduct a post-maintenance check flight. This could range from an acceptance delivery flight of a new aircraft from an OEM’s a production line, an FAA-approved maintenance ferry flight, a test flight following a major overhaul, or post maintenance verification of a minor squawk. The purpose of this discussion is to provide some best practices for planning and execution of a maintenance check flight. Our goal is to conduct a thorough check with the lowest practicable risk. Flights post maintenance have the potential for elevated risk, and the procedures listed below are offered to reduce the risk of accidents and incidents, including safety of non-participants. 

References. The requirement to conduct an operation check flight is called out in Title 14, Chapter 1, Subchapter F, Part 91, Subpart E, § 91.407(b), ‘Operation after maintenance, preventive maintenance, rebuilding, or alteration’. For check flight procedures specific to your aircraft refer to the Manufacturers Maintenance Manual. 

Aircraft History. The first step in preparation is to determine the history of the aircraft. Once you understand why the aircraft has been subject to maintenance, and the scope of the work that has been performed, you will be much better prepared to inspect and subsequently conduct the check flight.  Reviewing the aircraft’s recent history might be easy if you are the owner or main operator but will require more effort if the aircraft and/or maintenance shop is not familiar to you.  

Aircraft logbook and maintenance paperwork. Before you look at the aircraft, conduct a review of the aircraft documents. If possible, talk with the technicians that conducted the work to better understand the following: 

  1. Have all the squawks been addressed and signed off?
  2. Have all the inspection steps been carried out?
  3. What systems have been maintained? You want to give these areas extra attention during your pre-flight inspection. You also want to scrutinize areas/systems in proximity that might have been inadvertently affected.
  4. What are the appropriate checks to determine all the maintained systems are serviceable? Review the maintenance manual and copy any relevant sections so you can review before/during flight to ensure you conduct the check correctly.
  5. Is there a “Maintenance Check Flight Schedule” from the OEM? If not, what are the applicable limits for each system to be checked. What criteria determine if a system is functioning properly?
  6. Confirm compliance with local tool control procedures.
  7. Confirm you have valid Airworthiness, Registration, Operating Limits and Weight & Balance.
  8. Don’t start pre-flighting the aircraft until maintenance has finished. Beginning your inspection while work is still underway puts perceived pressure on the maintainers which could make them rush and increases the probability of errors.

Pre-flight Inspection. The best place to find issues with the aircraft is on the ground. Give yourself plenty of time to give this task the necessary attention. Some tips: 

  1. Mindset. During any preflight you should be attentive. When conducting a post maintenance inspection, you need to be looking for all the normal items called out in the Rotorcraft Flight Manual (RFM) checklist, but you also need to be attentive for any potential errors that have been made during the maintenance. You need to approach the preflight with an attitude of attention to detail. This is your best chance to find and remedy issues that could become more serious if discovered airborne. 
  2. Location. If possible, conduct the preflight inspection in a hanger. Out of the elements you’ll be more comfortable and inclined to take the time a thorough inspection requires. You also have better access to maintainers, manuals, work stands, lighting etc.
  3. Use a flashlight. The additional illumination will allow you to see much better inside cowls. The bright light helps to draw your attention to a specific area. You’ll be surprised how much more you can see.
  4. Use a ladder. This will make it safer and more stable so that you can more easily spend more time checking items. The higher vantage point makes it easier to view the full extent of each space.
  5. Scrutinize not only the items that were maintained but also the items in proximity, that could have been inadvertently interfered with during the work. For an interesting historical view on this phenomenon follow the link (https://resources.savvyaviation.com/wp-content/uploads/articles_eaa/EAA_2011-03_the-waddington-effect.pdf) to an article on the “Waddington Effect”.
  6. Pay particular attention to fasteners. Check for correct use of split pins, lockwire (presence & orientation) and torque stripe. I make a point of physically checking flight controls and fasteners by hand. This won’t tell you if they are properly torqued, but if they move by hand, they surely aren’t tight enough. It is easy for a technician to ‘temporarily’ assemble components hand tight but then later forget to go back and properly torque them.
  7. Check fuel and hydraulic lines for leaks, especially connections that have been disconnected during maintenance. Check for leaks at connections, on the engine/transmission deck, and at the outlet of drain lines. Clean up any fluid residue before you fly and make a point to check post flight.
  8. If you are interrupted during your inspection, back up a few items or start your inspection of that area from the beginning.

Flight Planning. When planning the flight, aim to make it as safe as possible for yourself and any non-participants on the ground. Some considerations when planning the post-maintenance test flight. 

  1. Area. Choose an area that has the lowest possible population density. This gives you more forced landing options and lowers the probability you will injure anyone on the ground or cause damage to property. 
  2. Plan to fly in low traffic density airspace. The test flight will require you to pay more than normal attention to instruments and displays which will degrade your lookout. Ensure you use any built in or supplementary traffic displays (TCAS/ADS-B) to assist in deconflicting with other aircraft. If you are operating from a busy airport delay testing until you are clear of the traffic pattern or airspace boundary.
  3. Use ATC flight following if possible, to assist with traffic deconfliction and to have contact with an ATC in the event you need to declare an emergency.
  4. Weather. In the same way that observing and recording instruments makes traffic lookout harder it also makes avoiding clouds and the ground more difficult. Day VMC with a clearly defined horizon is what you are striving for. Don’t do maintenance check flights at night unless you are testing a specific system that requires night flight.
  5. Minimize the consequences of any incident by having the minimum crew required to safely conduct the mission. Having a technician or another pilot operating test equipment (track and balance for example) or recording data and helping with lookout is a good idea, but no passengers. Part 91.407(b) states only crew members can be carried on a check flight. Passengers are not permitted until the flight has been successfully conducted and logged in the aircraft logbook.
  6. Fuel. Start with enough fuel so that running out or getting caught in a low fuel situation is not an issue. This will allow you to take your time to properly set up and execute the various checks. Some checks might require a high fuel quantity (to achieve a target weight or test the fuel system) or a low fuel quantity (to achieve a low weight for autorotation RPM). Account for these specific fuel requirements by varying other sortie parameters to maintain an adequate reserve and level of safety.
  7. Ballast. If you need to increase the aircraft’s weight to achieve a check it is best to do so with fuel or inert objects securely tied down. Use properly rated tie down points and restraint straps when securing ballast. Do not use passengers to increase weight, you are unnecessarily increasing the consequence of an accident.
  8. Plan the sequence of the checks to efficiently use time and airspace while prioritizing the most critical systems early in the flight.

Flight

  1. Before strapping into the aircraft, do one last walk around to confirm all the panels and doors are closed and any ground handling gear has been removed.  
  2. Use an external power cart for the first start if available. If the aircraft has been in maintenance for a length of time the battery could be partially discharged. In a helicopter with turbine engines this could lead to a hot start.
  3. Test the most critical systems first. For example, test the Sprague clutch works and that you can safely autorotate before you test the ADF.
  4. Altitude. Maintain above 1000’ AGL, unless a specific check requires lower. The altitude increases your options for landing sites and provides more time to troubleshoot. Being higher alleviates the concern of CFIT if you are drawn inside the cockpit reading instruments or troubleshooting a problem. For hover work, minimize time spent in the H-V curve.
  5. Before each conducting each check review the sequence of events, parameters to be recorded, serviceability limits, risks and how to recover.
  6. Take regular pauses. It is important to take regular breaks from the maintenance check sequence to ensure the aircraft is still operating correctly. Every 10 minutes or so take the time to check all the aircraft indications, fuel state, airspace, and traffic.
  7. Notes. Take careful notes of your results so you can relay them to maintenance after the sortie. Have the copilot or technician take photos or video of displays so that you can accurately report what happened in flight.

Post Flight

  1. After shutting down, do a walk around the aircraft to check for leaks and any damage that might have occurred during the sortie. 
  2. Write your results in the aircraft logbook. If it is not recorded it is as if it did not happen and might not get addressed.
  3. Be detailed and specific in your report of the flight results to maintenance. Focus on the “what” the aircraft was doing, rather than how to fix it. In the same way that you the pilot are the expert at flying, maintainers are the experts at repairing the aircraft. Telling maintainers how to do their job seldom engenders a good working relationship. Jumping to a conclusion on how to fix an issue might circumvent the troubleshooting process and result in missing the real problem.
  4. Once the test has been successfully completed you will need to make an entry in the aircraft logbook to the effect that you conducted an operational check flight, and that the aircraft is in a condition for safe operation. Note that you need to hold at least a private pilot license to be qualified to conduct a check flight IAW FAR 91.407(b). An example logbook entry is shown below.

Nigel Speedy
Test Pilot
Leonardo Helicopters

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