Earlier this year, I met with a student for an early morning training flight. As always, I conducted my own walk around the aircraft even though the student had already preflighted prior to my arrival. While inspecting the elevator for “free and correct,” I noticed a distinct “clunk” with an accompanying restriction in movement. Control surface binding is one of the most serious in-flight emergencies and can obviously be fatal.
Sensing a teachable moment, I said nothing and asked my student to show me how he checked the elevator. Observing him, I saw that he did not exercise the elevator throughout its full range of motion, instead checking the freedom of movement in a small range. When I showed him the issue I discovered, he was shocked. He told me that none of his prior instructors ever showed him how to properly inspect control surfaces for freedom of operation.
I have observed similar astonishment from pilots who, when ready to launch after conducting their own preflight, were pointed out major airworthiness items such as broken propeller spinner back plates, flat landing gear struts and landing gear pins that were left in place.
Mechanical failure is the second leading cause of aircraft incidents and accidents (human error is the top). The FAA, after analyzing over 20 years of Part 91 and Part 135 operations, found that 26 percent of all incidents and accidents involved a mechanical malfunction of an aircraft system. Granted, mechanical failures in aircraft are rare. However, nearly invariably, the aircraft presents signs that things are not normal well before a catastrophic failure takes place. Being attuned to those signs, especially when flying the same airplane regularly, can help uncover problems well before they develop into a catastrophic failures.
Preflight Deserves More Respect
So why do pilots routinely miss signs of impending mechanical failure? I find three core reasons:
First, many of us were never taught how to conduct a thorough aircraft preflight. While we all learned how to check fuel and oil levels and how to sump the fuel and check for contamination, most of us never learned what to look for in other areas of the aircraft. For example, how do we identify and spot areas subject to overstress, misassembled hardware or corrosion? What are general trouble areas for the specific aircraft model we are flying? The manufacturers’ preflight checklists use generic terms such as “CHECK” and generally do not help with the detailed nuances of a proper visual inspection.
Second, expectation bias presents a major challenge to conducting a thorough aircraft preflight inspection.
Admittedly, most of the time the plane is in an airworthy state. As a result, we expect the aircraft would be airworthy for today’s flight as well. Flying the same aircraft consistently exacerbates this bias, especially if we own the airplane. It can lead us to skip areas or glance over areas during preflight and miss critical items.
Third, many flight instructors are somewhat derelict in teaching the importance of preflight. The “standard practice” is for the instructor to demonstrate a preflight inspection and maybe observe the student conduct one. From that point on, the instructor will generally say something like, “Go ahead and preflight the airplane on your own.” The instructor arrives after the preflight is complete and off they go flying. In a way, the instructor is sending the message “Flying is important, so I will observe you when we fly together, but preflight is easy and simple, so I don’t need to observe you doing that.”
Fundamentals of a Thorough Preflight
While there is no need for pilots to have the same level of knowledge as an aviation inspector or an A&P IA, having some mechanical knowledge is very beneficial. It does not take a lot of effort to learn how to identify defects and abnormalities in a preflight. Investing time in acquiring this knowledge and then unfailingly putting that knowledge to use before every flight contributes immensely to flight safety. Here are some suggestions you can utilize as you follow the manufacturer’s recommended checklists for aircraft preflight.
A good preflight starts as you approach the aircraft. Looking at the “big picture,” take note of any
abnormalities. Is the airplane sitting evenly on the ramp? Is there any obvious damage, e.g., from a ground vehicle that may have contacted the airplane? Are there leaks or puddles of fluid under the aircraft? A flat tire?
When inspecting aero structures, a basic understanding of how loads are carried through the airframe is helpful. Attach points for vertical and horizontal stabilizers, spar lines and landing gear attach points are some of the areas of the airframe that experience the highest stress. Most metal aircraft employ a monocoque construction. In this type of construction, the skin is an integral component to the strength of the structure (just as an egg derives strength from its shell). Any sign of deformation in the skin or of popped or loose rivets in a metal aircraft structure is cause to suspect an overstressed member and to conduct a deep investigation. For example, look at Figure 1. Would you want to fly an aircraft missing rivets on its wing spar line? While in this case the rivets are completely sheared, observing cracked paint around a rivet or black oxide lines emanating from a rivet may indicate that the rivet is loose and strength is compromised.
Composite structures present a visual inspection challenge because they may be compromised without showing any external signs of damage. The only sign something is amiss in the empennage is a slight wrinkly in the paint in Figure 2. A “coin tap” test can indicate the health of a composite structure. By tapping the surface with a coin and listening for the resulting sound we can detect damage to the underlying composite structure. A localized change in stiffness, for example, due to delamination, will be indicated by a flattening tone from the coin tap.
Areas that carry high load should also be carefully inspected for cracks. Cracks usually emanate away from a stress point. They may initially present as small hairline blemishes and propagate and enlarge over time. The Pilatus PC-12 flap drive arm picture in Figure 3 caused the flaps to fail as the pilot deployed them on approach to landing.
Likewise, having a basic understanding of aircraft hardware – nuts, bolts, rod ends, etc. – is helpful. Generally speaking, aircraft employ hardware that utilizes some safety retention mechanism such as lock washers, lock nuts, cotter pins and safety wire. Sometimes, mechanics will use torque seal paint to mark the alignment of the nut with respect to the bolt after proper torquing. This makes it easier to detect any loosening of the hardware over time. Inspect safety wire for tension and proper installation. The safety wire should be installed in a direction that prevents the nut from loosening and is sometimes installed incorrectly. Never underestimate the importance of fasteners and treat even one missing fastener as requiring attention prior to flight. Just three missing cowl screws caused the engine cowl on a Citation to depart the aircraft in flight (Figure 4).
Corrosion, which is the oxidation of metal, may lead to significant loss of structural strength. Water from precipitation or from washing the aircraft, exposure to moisture in the atmosphere and exposure to deicing fluid can inflict insidious damage on the airframe. There are many kinds of corrosion, such as surface corrosion (observable as pitting of the surface), filiform corrosion (which appears as “worms” under paint), pitting corrosion (which shows as white or gray powder deposit), and others. Corrosion will normally manifest as visible imperfections on the surface. Figure 5 shows a trim tab rod end on a Hawker. A trace of red rust led the pilot to further investigate, preventing a possible control surface jam in flight. Pay specific attention to areas prone to corrosion: external skin areas, wheel wells, landing gear, wing flaps, spoiler recesses, battery compartments, and areas near lavatories and galleys. Related to corrosion is atmospheric degradation of polymers, such as those used in aircraft hoses and seals. Those will be evident by stiffness and cracking of the rubber and, left unchecked, as leaks.
Chafing is another cause of failure that, in many cases, can be detected early. Last year, an aircraft at my home field had to land on its nose due to a nose gear malfunction. A prop strike ensued. The culprit was an improperly long screw installed on a gear door, which jammed a component of the nose wheel. Long scratches on that component indicated that the interference existed over several flights. Chafing will show as scratches or wear on moving components such as landing gear and flaps. Chafing may also be a result of components that contact each other due to aircraft
vibrations. Figure 6 shows a broken aspirator high-pressure tube that a pilot found while inspecting the environmental control system in his Citation. Flying with the line broken would have resulted in pressurization issues.
If the engine cowl or inspection covers to the APU, ECS, etc., can be opened easily, it is a good idea to open those as part of a thorough preflight inspection. Look for leaks and loose components. In a turbine engine, inspect the bleed lines for looseness and cracks. In a reciprocating engine, pay particular attention to the exhaust system and the turbochargers (if equipped), as those are subject to high temperature and stress. Exhaust leaks will show as white streaks near joints or cracks in the system.
Preflight an Aircraft Post-Maintenance
The other week I was at the shop and observed a customer arriving to pick up their high-performance turbine aircraft after an annual maintenance event. The pilot loaded up his wife and dog into the aircraft, climbed aboard, and off they went. After all, the repair station mechanics already checked the aircraft when they released it back to the line, didn’t the
The recent fatal crash of a 2012 Piper Meridian on a post-maintenance flight in Olathe, Kansas, reminds us that post-maintenance flights are a serious matter. A thorough preflight inspection is in order anytime the aircraft has been out of your care, especially after any maintenance event. While the scope of such an inspection will be covered in another article, at the minimum, it should include the following:
- A review of the work performed, including a review of airworthiness paperwork. Going on a test flight in an aircraft lacking completed airworthiness paperwork may lack valid insurance coverage.
- Your visual inspection of the areas worked, preferably before the shop closes those areas. Look for rags and tools that may have been left behind. Examine fastener and components for security, to uncover cases where a mechanic hand tightened a component and forgot to torque it to spec.
- Conducting a thorough preflight inspection, with an emphasis on security of access panels and anything that may have been left behind, and on critical components such as control surfaces and powerplant.
- Conducting a ground run.
- Conducting a test flight. Naturally, the test flight should be performed in good VFR weather. In many aircraft, the maintenance manual will call for a post-maintenance test protocol.
Closing Words
A good way to become more familiar with what to look for during a visual inspection is to spend some time with the inspector or IA at your repair station as they inspect your aircraft during a maintenance event. In most cases, they will be happy to share their knowledge.
Conduct a thorough preflight every time. Expect to find something wrong! By approaching our aircraft with open eyes and a extreme vigilance prior to every flight, we can further reduce the risk of a mechanical failure that may lead to an incident or accident.
