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Supervised Maintenance

by | December 12, 2024 | ,

Who is allowed to perform maintenance on a certificated aircraft? The FAA rule that answers that question is 14 CFR 43.3—Persons authorized to perform maintenance, preventive maintenance, rebuilding, and alterations. This rule authorizes repair stations, A&P mechanics, repairmen working for a repair station, air carriers, Part 135 operators, and aircraft manufacturers to perform maintenance. The rule also allows pilot-rated aircraft owners to perform preventive maintenance on an aircraft that they own. What if someone who is not an A&P—say, a maintenance-involved aircraft owner—wants to do something that goes beyond preventive maintenance—say a repair or alteration or component overhaul or replacement? Or what if an aspiring aircraft mechanic wants to go to work as an apprentice at a aircraft maintenance facility in order to gain the 30 months of practical experience required to qualify to take the knowledge and practical tests to become an A&P? Well, 14 CFR 43.3 addresses that, too. Specifically, paragraph (d) of 43.3 says: 43.3(d) – A person working under the supervision of a holder of a mechanic or repairman certificate may perform the maintenance, preventive maintenance, and alterations that his supervisor is authorized to perform, if the supervisor personally observes the work being done to the […]

Concierge Maintenance

by | November 14, 2024 | ,

Rethinking the business model for maintenance of GA aircraft. For more than a decade, my primary care physician was Dr. F. She was a wonderful doctor with a deep understanding of internal medicine and superb people skills who cared deeply for her patients. I felt fortunate to have her as my doc. But over the years, her reputation spread, and her practice grew larger. The lead time for scheduling an appointment to see her became uncomfortably long. Visits to her office became an ordeal involving lengthy delays in her always-packed waiting room, after which  I would get to see the doctor for 5-10 minutes if I was lucky. Then in October 2010, Dr. F died in the crash of a Beechcraft A36 Bonanza while on a humanitarian mission in Mexico. She was a member of The Flying Samaritans, a public benefit flying organization that operated free medical clinics at 17 locations in remote areas of Baja California. My doc regularly volunteered to participate in these clinics. (Decades earlier when I lived in Southern California, I was an active pilot for The Flying Sams.) That morning, the Bonanza took off from Ensenada headed for San Quintin. Aboard were Dr. F, another […]

Distrust and Verify

by | October 17, 2024 | ,

The Perils of Trying to Fix a Problem You Haven’t Seen For Yourself The phrase “trust but verify” was made famous by U.S. President Ronald Reagan in December 1987 after the signing of the Intermediate-Range Nuclear Forces (INF) Treaty with Mikhail Gorbachev. The phrase comes from a rhyming Russian proverb, “доверяй, но проверяй” (romanized as “doveryay, no proveryay”). More than three decades later, U.S. Secretary of State Mike Pompeo paraphrased the proverb when speaking about U.S.-China relations as “distrust and verify”—likely closer to the Russian proverb’s true meaning. This proverb—particularly the Pompeo version—is just as applicable to aircraft maintenance as it is to arms control. Far too often in my experience, A&P mechanics attack an aircraft with tools in an attempt to fix an issue based solely on a squawk by a pilot or aircraft owner without actually verifying for themselves that the description is accurate. The result is often a lot of wasted effort, unnecessary expense and downtime, and frustration for all involved parties. Let me illustrate what I mean with a couple of real-world examples… Cessna 310 Volt-Amp Meter Years ago when I served as the twin Cessna tech rep for the Cessna Pilots Association, I received a […]

Why Annual Inspections?

by | September 17, 2024 | ,

The big boys don’t do them—why must we? Given today’s acute shortage of GA mechanics and the difficulty owners are having getting their airplanes on shop schedules, I’ve been receiving an increasing number of inquiries about the need for annual inspections from frustrated aircraft owners. “Why is it necessary to have my airplane completely torn apart every 12 months?” one owner asked. “I’ve only flown about 40 hours since the last annual inspection.” “Wouldn’t it help solve the mechanic shortage if the FAA would let non-commercial operators like me do inspections every 18 or 24 months?” another owner asked. Those are great questions. I’ve long thought that the rule requiring small GA airplanes to receive a complete inspection every 12 calendar months didn’t make a lot of sense. Larger airplanes aren’t required to do it that way, so why are we? There are certainly parts of our small GA airplanes that need to be inspected frequently. For piston aircraft, that includes pretty much everything forward of the firewall. I’m talking about all that stuff that’s constantly trying to cook and vibrate itself to death.  Inspecting that stuff once a year might not be often enough. I would probably also include […]

Can’t Sign It Off?

by | August 12, 2024 | ,

Can a mechanic who works on your plane refuse to sign it off or put it back together? Boston-based Cirrus owner Mark (not his real name) was flying his turbonormalized SR22 home to Norwood Memorial Airport on what was probably going to be his last flight prior to putting the plane in the shop for its 2024 annual inspection. While cruising over northern New England, the engine started running rough and the engine page on the MFD showed that the CHT of cylinder #2 had risen above 400°F and was continuing to rise. Mark went full rich and throttled back to stop the thermal runaway, then declared an emergency with ATC and made a precautionary landing. After an uneventful landing, Mark taxied the Cirrus to the sole FBO on the field, which included a big Part 145 Repair Station that just happened to be a Cirrus Authorized Service Center. He told the director of maintenance (DOM) about the rough-running engine and the rising CHT #2, and asked that the shop troubleshoot the issue. Mark then rented a car and drove home to Boston. The Shop’s Diagnosis A few days later, Mark phoned the shop to learn what they had found. […]

Unaffordable/Unavailable

by | June 6, 2024 | ,

Recently, a client with an older Cessna 182 Skylane reported that his nose landing gear strut was leaking fluid and repeatedly going flat. Inspection revealed that the original chrome strut piston had become badly pitted and was tearing up the nose strut seals. The owner’s shop checked the Textron Aviation parts system and found that they had one replacement strut piston in stock, and clearly, they were extremely proud of it. The price: $9,200. That’s nearly 10% of the fair market value of the whole airplane. Yikes! Salvage yards like BAS Parts Sales, Dawson Aircraft Parts and Salvage, Texas Air Salvage, Air Salvage of Dallas and Preferred Air Parts sometimes offer these strut pistons in as-removed condition for $2,500 to $3,500, but we couldn’t find one in stock at any of them. We even checked on eBay, but no joy. So, we concluded the best bet might be to find a shop capable and approved to re-chrome the pitted strut. As I’m writing this, we’re checking with Delta Strut in Stockton, California who specializes in Beechcraft landing gear repair but sometimes will work on Cessna and Piper struts. Fingers crossed. Although the cost of a new replacement nose strut piston […]

The Tale of Two Prebuys

by | June 1, 2024 | ,

My company manages a lot of prebuys. At any given point in time, we typically have a dozen of them in progress. We’ve managed thousands of them over the years, and seen the good, the bad, and the ugly. Most of the time, the aircraft turn out to be in decent shape and the outcome is a successful purchase by our client, often after some last-minute price negotiations based on prebuy findings of airworthiness discrepancies that we think the seller should pay to correct. Once in a while, however, the prebuy uncovers problems serious enough that we advise our client to walk away and find another purchase candidate. We always try to arrange for the prebuy examination to be done by a shop or mechanic highly experienced with the particular make and model involved, and who is truly independent. By independent I mean that the selected shop or mechanic has no prior exposure to the aircraft being examined, and no prior relationship with the seller or (if applicable) the seller’s broker. This is the only way we can have confidence that the prebuy findings will be thorough, honest and unbiased. Let me tell you about two of these prebuys. One […]

Minimally Invasive

by | May 1, 2024 | ,

What we can learn from medicine about fixing things without taking them apart. A longtime friend who was suffering from extreme fatigue and shortness of breath. She was diagnosed with congestive heart failure caused by aortic valve stenosis, and she required an aortic valve replacement. This was a very big deal that required open-heart surgery. It involved cracking her chest, placing her on a heart-lung machine, stopping her heart, cutting into her aorta, surgically removing the defective valve, suturing a replacement valve (harvested from a pig) in its place, suturing her aorta, restarting her heart, and closing up her chest. Afterward, she spent a week in the hospital and more than two months recovering at home. That occurred about 15 years ago. Now, the replacement valve is starting to give out, and she needs another valve replacement. This time, remarkably, the valve will be replaced without opening her chest or stopping her heart.  Nowadays, most aortic valve replacements are done using a technique called transcatheter aortic valve replacement or TAVR. A small incision will be made in her groin, and a guide wire will be passed up through the femoral artery into the aorta, and through the aortic valve using […]

Savvy’s Borescope Initiative

by | April 1, 2024 | ,

Teaching owners (and mechanics) how to do borescope inspections right In my last column (“Ending the War on Jugs,” AOPA Pilot March 2024 issue), I talked at length about why we should use the borescope—not the compression tester—as the gold standard for assessing cylinder condition. Borescopes are now inexpensive (under $300) and capable of breathtaking image quality. These new borescopes let us diagnose cylinder issues accurately and often early enough that the problem can be remediated using modern minimally invasive techniques—lapping leaking valves in place, and freeing stuck rings with a simple solvent flush—thereby eliminating the need to incur the cost and risk of cylinder removal. But there’s a problem: Very, very few A&Ps know how to do a proper borescope inspection of cylinders. There’s little or no training on this subject in A&P school, and no relevant questions on the A&P knowledge tests. There’s nothing in any maintenance manual or service bulletin from aircraft or engine manufacturers that provides any guidance on this subject.  Interestingly, Continental has “required” that a borescope inspection be done any time a compression test is performed, but they offer not a syllable of guidance on how to do one! What’s wrong with this picture? […]

Monitor-Specific FAQs

by | March 7, 2024 |

Concerning E.I. UBG-16, MGL and GRT monitors How do I upload data from a UBG-16? Files that come from a UBG-16 do not contain “heading names” for each of the columns in the file (each column represents a data series, such as EGT1). As such, if you were to upload such a file to SavvyAviation, we would not be able to tell which column was EGT1, which was CHT1, and so on. To work around this, you have to configure your aircraft’s profile with these mappings. This will tell us, for example, that Column 1 is EGT1, column 2 is CHT2, etc. You should be able to find these mappings in your aircraft logs at the time your UBG-16 was installed. If you do not have such entries, you may need to open the file and do some detective work. If you open the file with an editor, be very careful not to accidentally save over the original. This will almost certainly make it unusable. It is best to make a copy and open the copy. Here are the steps to provide us with the UBG-16 configuration: Note: The mappings apply to file uploaded after the mappings have been set. In […]

Choosing and Upgrading Your Engine Monitor

by | March 7, 2024 |

Aircraft owners often ask us for advice about what kind of engine monitor equipment we recommend installing in their airplanes. Owners who already have engine monitors installed often seek advice about upgrading them with additional capabilities or replacing them with newer, more advanced equipment. Engine monitor technology has been a fast moving target. More and more avionics manufacturers are now offering engine monitor equipment, both as stand-alone monitors and as monitoring capability integrated into multi-function displays (MFDs). New makes, models and features are being introduced all the time. In addition to the traditional firms that manufacture avionics for certificated aircraft, there are now many companies specializing in uncertified avionics for the experimental amateur-build (E-AB) market. Consequently, we’re not going to recommend any specific makes and models here. (If we did, this document would probably be obsolete before it was posted to the Internet.) Instead, we will talk about what features and capabilities we believe are most important in digital engine data monitor equipment, and what you should be looking for when choosing an engine monitor for your airplane. There are two major dimensions to consider: what kinds of data does the equipment capture, and what features does it offer? What […]

In-Flight Diagnostics

by | March 6, 2024 |

The best way to diagnose an engine problem is usually in the air. Note to reader: The procedures discussed in this article for gathering flight test data are described more fully in the “Flight Test Profiles” document found under the “Learn More” or “Help” menu items. Pease refer to that document when performing the flight tests. “My engine started running rough about halfway home yesterday,” my client reported, “so I dropped it off at the service center. Could you please work with my mechanic to troubleshoot this problem?” Arggghhh!!! So many aircraft owners have a knee-jerk reaction to put their airplane in the shop whenever problems arise. Apparently they assume that diagnosing the problem is the job of a mechanic. That’s like having a bellyache and making an appointment to see a surgeon. Like surgeons, aircraft mechanics are primarily in the business of fixing things that aren’t working properly. But before you go to a surgeon or a mechanic, you need to figure out what’s wrong. You need a diagnosis. Every diagnosis starts with data. If you’re feeling unwell, the initial data would probably come from a Q and A session with your primary-care physician resulting in a detailed account […]

Controlling the Combustion Event

by | March 6, 2024 |

Each time you change mixture, RPM or MP, it affects combustion timing. Understanding how is your best defense against harming your engine by doing something dumb. My last few columns focused on the physics of the combustion event in Otto-cycle engines. I’m sure this was fascinating to the two or three of you who are interested in combustion theory, but of what possible relevance is it to a pilot whose main concerns are measured in knots and dollars rather than milliseconds and PSI? Actually, it’s extremely relevant. That’s because you modify the speed, intensity and timing of the combustion process every time you move the throttle, mixture and prop controls. If you don’t have a clear mental picture of how changing MP, RPM and mixture affects the combustion event, you’ll never truly understand optimum power settings or leaning, and you’ll simply have to operate your engine by rote – which is exactly what most pilots do. Little was known about this stuff back when Beech wrote the POH for your airplane. Much of what we now know comes from recent research done by my old friend George Braly at GAMI’s Carl Goulet Memorial Engine Test Facility in Ada, Oklahoma, arguably […]

Detonation and Pre-Ignition

by | March 6, 2024 |

Often confused and misunderstood, these two abnormal combustion phenomena are as different as night and day. Although we often hear people describe what goes on inside the cylinders of an Otto-cycle engine as being an explosion – i.e., a violent, nearly-instantaneous event – it’s not. The air-fuel charge does not explode when ignited by the spark plugs, but rather burns in an orderly fashion, starting at the spark plugs and progressing across the combustion chamber until it is quenched upon reaching the cylinder walls and piston crown when air-fuel charge has been completely consumed and there’s nothing more to burn. The combustion event takes a significant period of time – roughly 6 milliseconds or 90° of crankshaft rotation, give or take. It’s crucial that peak pressure occur well past TDC, because the geometry of the crankshaft and connecting rod near TDC does not permit combustion pressure to be converted into useful work (i.e., crankshaft rotation), but simply generates excessive stress on the cylinder, piston, connecting rod and crankshaft. Figure 2 attempts to dramatize this point. Detonation But if the combustion process moves too fast and the pressure peak occurs too early, the result can be excessive pressure, excessive temperatures, and […]

Understanding CHT and EGT

by | March 6, 2024 |

These two key measurements can tell us a lot about what’s going on inside our cylinders. My column last month talked about the fact that our piston aircraft engines convert only about one-third of the energy contained in avgas into useful energy to the propeller. About half the fuel’s energy goes out the exhaust pipe, while the remaining one-sixth is transferred to the cooling air that passes over the cylinder fins and through the oil cooler. The wasted energy is measured and displayed on a triad of cockpit gauges: oil tempera-ture, cylinder head temperature, and exhaust gas temperature. The one-third that actually makes it to the propeller is reflected on another cockpit instrument: the airspeed indicator. CHT vs. EGT From a powerplant management perspective, it’s crucial to understand that CHT and EGT tell us quite different things about what’s going on inside the engine. CHT mainly reflects what’s going on in the cylinder during the Otto cycle power stroke before the exhaust valve opens, while EGT mainly reflects what’s going on during the exhaust stroke after the exhaust valve opens: High CHTs generally indicate that the engine is under excessive stress for its own good. That’s why it’s so important […]

Interpreting Your Engine Monitor

by | March 6, 2024 |

The modern probe-per-cylinder digital engine monitor is a marvelous tool for keeping tabs on your engine’s health and troubleshooting its maladies. Here are some tips for figuring out what those bars and digits mean. This is embarrassing, but I might as well come clean: Up until 2003, I was still flying my Cessna T310R with only the primitive engine instrumentation installed by the factory in 1979. I’d long since upgraded my avionics stack with conspicuous quantities of glass, including a Garmin GNS-530 navigator and a Sandel SN3308 electronic HSI. I’d installed an XM Satellite Radio receiver to pipe stereo music to my ANR headsets. I’d added VGs to the wings and vertical tail. I’d even reupholstered my seats with the latest visco-elastic memory foam padding. But I was still relying on 30-year-old steam-gauge engine instrumentation. A modern digital engine monitor had been at or near the top of my wish list for years. Yet somehow the $5,000 I had set aside for this upgrade always seemed to get preempted by something else (usually nonaviation-related) every March when my annual comes due. In addition, I secretly dreaded what I expected to be a difficult, time-consuming and tedious task of installing such […]

Ending the War on Jugs

by | March 1, 2024 | ,

Weak compression doesn’t always mean that the cylinder has to come off. For most of my nearly six decades as an aircraft owner and three decades as an A&P, the rule about cylinders was simple: If the compression reading was less than 60/80, the cylinder had to come off for repair or replacement, period. The mechanics who worked on my airplanes, then later mentored me about maintenance, and then still later became my professional colleagues seemed quite happy with this straightforward rule. (Mechanics are always happiest when they have clear guidance to follow.) In 1984, Continental muddied the water when they issued Service Bulletin M84-15 that substituted a “master orifice tool” in place of the traditional 60/80 go/no-go criterion. This allowed Continental cylinders to remain in service with compression readings down to the mid-40s so long as the leakage was past the rings. This guidance was based on dynamometer test cell research by Continental engineering demonstrating that an engine with all cylinders at 40/80 made exactly the same horsepower as one with all cylinders at 75/80. (Most A&Ps weren’t comfortable with this, and would still yank jugs if they measured anywhere in the 50s.) However, M84-15 also said that no […]

Unleaded Avgas—Cure or Curse?

by | February 1, 2024 | ,

Does unleaded fuel really cause exhaust valve seat recession? The October 27 announcement by the University of North Dakota (UND) flight school that it was terminating its year-long test of Swift UL94 unleaded avgas and returning to 100LL came as a shock and disappointment to many in the industry, me included. The school’s Director of Maintenance, Dan Kasowski, cited evidence of “significant” exhaust valve/seat recession in some of the Lycoming-powered Piper Archers in its training fleet. Kasowski was not more specific about the amount of recession or the percentage of cylinders or aircraft affected, saying only that the data is still being collected and is being forwarded to Lycoming for analysis. The UND announcement was shocking because virtually all previous tests of FAA-approved unleaded avgas—both Swift UL94 and GAMI G100UL—seemed to indicate that the effects of operating piston aircraft engines on these fuels was all positive with no negatives. In addition, decades of experience using 91-octane premium unleaded automotive gasoline in a wide range of low-compression aircraft engines ranging from Lycoming O-320s to Continental O-470s uncovered no problems whatsoever. So the UND findings were quite unexpected, to say the least. Is this a major setback for the long-awaited transition to […]

Time & Materials

by | December 1, 2023 | ,

Why is GA maintenance done on a T&M basis, which places all the risk on the aircraft owner and none on the shop? A successful surgeon put his Beechcraft Baron 58 in an Arkansas shop for a makeover. He wanted both engines overhauled, new paint and interior, and the steam gauges replaced with a modern glass panel. The shop estimated they could get all this done in about six months at a cost of about $200,000. Much to the doctor’s dismay, the work actually took two years the final bill was about $500,000.. I learned about this when a mutual friend called me to ask if there was anything I could do for the Baron owner, who was extremely upset and understandably feeling seriously ripped off. I told my friend that while I certainly felt the owner’s pain, there was really nothing I or anyone else could do to help. Like the vast majority of GA maintenance, the work was done on a time-and-materials (T&M) basis. This meant that the owner agreed to pay for all the labor expended by the shop and its subcontractors (e.g., the paint shop) on an hourly basis, and for any required parts (typically at […]

Deadly Switches

by | November 23, 2023 | ,

That ubiquitous key-operated ignition switch is fraught with peril On July 26, 2018, private pilot Lanny Steven Kramer of Sarasota, Florida, and his wife Fran flew to the Cleveland Regional Jetport (RZR) in Cleveland, Tennessee to run some errands, after which they returned to the airport to depart. Shortly before 5 p.m., Lanny was preflighting the Cessna 182 Skylane while Fran had opened the righthand cabin door and was preparing to climb into the right seat. Then Fran heard a noise that sounded like the plane’s engine was trying to start. She found Lanny lying on the ramp in front of the airplane. He had sustained severe head injuries and was non-responsive. The Skylane’s ignition key was in his pocket.Fran called for help, which quickly arrived. Lanny was transported to the hospital by medevac helicopter, but succumbed to his head trauma. NTSB Findings Accident investigators found that the righthand magneto of the plane’s Continental O-470 engine was “hot”—the mag’s P-lead was not grounded out by the cockpit ignition switch. They found that the switch was not in the OFF position, but rather was in an intermediate position between the OFF and RIGHT detents. In that position, the switch did not […]

Miracle in Sioux Falls

by | October 1, 2023 | ,

My hapless blunder en route to Oshkosh triggered a series of incredibly fortuitous events. It was mid-July and my annual pilgrimage to AirVenture Oshkosh was rapidly approaching. I’d finished preparing PowerPoint for the 11 different presentations I would be making. Now I started thinking about the upcoming flight. Flying from California to Wisconsin solo can get lonely, so I called my colleague Colleen Sterling and asked if she was interested in flying with me. After conferring with her husband Dave, they both accepted my invitation. Colleen and Dave are two of my favorite people. Both are very accomplished pilots—Dave’s a recently retired 787 captain—and both are A&P/IAs. They own five airplanes between them, all based at San Diego’s Gillespie Field (KSEE), and race three of them at Reno. Serious aviators!. On Friday, I flew my Cessna 310 to KSEE and stayed overnight with Colleen and Dave. We planned to launch early Saturday morning and fly at least two-thirds of the way to Oshkosh, remain overnight (RON), then depart Sunday morning to arrive at Wittman Field by noon. Northern Route Colorado looked awful on NEXRAD, so a northerly route via Salt Lake City looked prudent. Using Foreflight, I found a promising […]

Fortunate Catch

by | September 1, 2023 | ,

A maintenance-aware owner is the last line of defense against maintenance errors Corey owns a 1978 Bonanza A36 and is quite involved in its maintenance. He does his own oil changes and other preventive maintenance. He even bought his own borescope and uses it to keep tabs on the health of his cylinders. He’s my kind of aircraft owner! His most recent borescope inspection revealed what looked to him like burned exhaust valves in the #3 and #4 cylinders. He reported this finding to his local shop, and after verifying his diagnosis a decision was made to send the better cylinder out for overhaul by an engine shop in Tulsa and to replace the worse one with a new Superior Millennium. (I’m not sure whether any consideration was given to lapping the exhaust valves in place—maybe they were too far gone for that.) The work seemed to be proceeding smoothly. Before long, the shop installed the new and overhauled cylinders and had the plane almost buttoned back up when Corey stopped by to check on their progress. Informed that the airplane was almost ready for the break-in flight, Corey went over to eyeball his newly installed jugs—and he saw red. […]

Legal interpretations

by | August 1, 2023 | ,

If you ask FAA lawyers what a regulation means, the answer might surprise you We’ve all studied the regs. If you’re a pilot, you’ve spent hours poring over Part 61 (pilot certification, ratings, currency) and Part 91 (operating rulese, owner responsibilities), both of which are voluminous. If you’re a mechanic, you’ve hopefully memorized Part 65 (mechanic eligibility, privileges and limitation) and Part 43 (maintenance) which are comparatively shorter but quite pithy. These regulations are written and maintained by a large team of FAA lawyers who work for the Regulations Division of the FAA Office of the Chief Counsel. If you have a question about the meaning of a rule, you can write to the Assistant Chief Counsel for Regulations (AGC-200) requesting a legal interpretation. (It’s sort of like asking the IRS for a revenue ruling.) Your request will be assigned to one of the rulemaking attorneys responsible for that particular section of the FARs, who hopefully will prepare a detailed written legal interpretation to be signed by the Assistant Chief Counsel and sent to you. Hopefully you’re not in a hurry, because the wheels of government turn slowly. Response time is generally at least several months, sometimes a lot longer. […]

Unbelievable Compression

by | July 1, 2023 | ,

How reliable and valid is the almighty compression test? Each annual inspection begins with a moment of terror when the IA removes the top spark plugs and takes a compression reading of each cylinder. We hold our breath awaiting the verdict. If the numbers are good, we exhale and relax. If not, we anticipate the sticker shock of cylinder removal and repair or replacement—and we pray that opening Pandora’s box won’t result in an even costlier verdict requiring an engine teardown or replacement. Sound familiar? Been there and done that? I certainly have.This agonizing compression testing ritual takes place hundreds of thousands of times each year. The requirement for performing a compression test at each annual and 100-hour inspection is written right into the FARs—specifically Part 43 Appendix D—so your IA has no choice about doing it. I’ve seen serious buyers walk away from an excellent airplane because they didn’t like the looks of the compression readings during the pre-buy. Yet this obsession with compression readings is so very wrong on multiple levels. Why wrong? I will explain… For one thing, the differential compression test is spectacularly unreliable. This means that you can test a cylinder multiple times and get  […]
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