The ink was barely dry on the bill of sale when the baby blue and white Champ fell from the sky, clipping a tree, then taking out a parked minivan on a quiet urban street.
The pilot and passenger walked away, but I can’t imagine a worse way to begin the journey of airplane ownership.
The Flight
The proud new owner took off from Waunakee Airport (6P3) on the west side of Madison, Wisconsin, a little after one in the afternoon, bound northeast for Brennand Airport (79C), north of Oshkosh, a flight of about 80 miles. But he didn’t get far.
On initial climb out, right over the departure end of the runway, the RPMs rolled back. Uh-oh.
He gently turns toward the lowest of the surrounding trees. He tries to identify the problem, but is unable.
He later wrote, “Identified place to land and went for it.” He hits a tree and a car “before crashing into minivan.”
The Pilot
The new owner was a 51-year-old male flying with BasicMed. He had a private ticket with a total of 328.6 hours, 68.1 of them in make and model — enough to really know the type — and he’d logged 27.8 hours in the previous month, so plenty enough to be on his game. He was properly tailwheel trained and endorsed.
The little Champ was equipped with three point harnesses, no doubt an add-on, but one that very likely saved the pilot and his passenger from more serious injury when they practically parked the airplane inside the minivan during the accident sequence.
The NTSB
The pilot, even after reflecting on the crash, had no idea what caused it.
In his report to the NTSB on Form 6120.1, Pilot/Operator Aircraft Accident/Incident Report, he wrote, “the engine lost power and RPM and eventually quit, but I don’t know what caused it.”
A maintenance inspector from the FSDO, however, quickly found a smoking gun.
The engine fuel primer was unlocked and fully extended. And it was sufficiently worn-down that the inspector couldn’t get it to lock at all. He removed it and sent it to the NTSB’s Materials Laboratory for a more detailed look.
So now we need a quick primer on primers.
A primer is basically a manual piston-style pump installed in the fuel lines of carbureted-engine airplanes, which assists the pilot in starting the engine, especially for cold starts. Depending on the airplane’s design, it does this by delivering a volume of pre-vaporized fuel either into the induction system or into one or more of the cylinders.
To prime the engine for starting, the pilot rotates the primer handle to unlock it, pulls it back, which fills the primer tube with fuel, then smoothly pushes the plunger forward to deliver the cloud of fuel vapor. It works sort of like a syringe.
Most planes need two to three “shots” of prime to wake up. After delivering the shots, the pilot rotates the primer handle to lock it into place again.
The primer has a lock for good reason. If it is left open, as you apply power, excess fuel is pulled through the primer and into the intake system, over-richening the mixture to the point that your fuel-to-air mixture becomes a fuel-to-fuel mixture and won’t combust.
This is mentioned, briefly, in the Pilot’s Handbook of Aeronautical Knowledge’s chapter on aircraft systems. After a few sentences on what a primer is and how it works, the PHAK says, “It is important to lock the primer in place when it is not in use. If the knob is free to move, it may vibrate out of position during flight, which may cause an excessively rich fuel-air mixture.”
The primer locking mechanism itself is similar to what you see with coaxial cable: There are “nubs” that lock into a channel. The correct terminology for the nub is “locking pin,” while the correct terminology for the channel is the “retention lip.”
The locking pin is attached to the exterior of the primer handle (the shaft of the “syringe”), while the lip is inside the end cap, which is where the device passes through the instrument panel. Most of the primer itself is hidden behind the panel.
On inspection of the Champ’s removed primer, the NTSB lab found that the retention lip was “worn away around most of its circumference, leaving only a 0.143 inch-long section.” That’s in the ballpark of 9/64s of an inch. The pin itself is 0.093-inches in diameter, so the remaining lip was about one-and-a-half times as wide as the pin. Not much of a lip for the locking pin to hang on to.
The NTSB’s final report states, “It is likely that a slight movement or vibration caused the retaining pin to move around the small piece of retaining lip that remained, resulting in the fuel primer pump becoming unlocked and extending.”
Backing this theory up, investigators noted that the spark plugs were found “in a carbon-fouled condition, consistent with an overly rich mixture.”
The probable cause: “A total loss of engine power during the initial takeoff climb due to a worn manual fuel primer pump locking mechanism, which resulted in an unlocked primer and an overly rich fuel mixture.”
Analysis & Discussion
One concerning aspect of this accident, which I haven’t gotten into yet, is that the sale, an annual inspection, and the failed ferry flight to the airplane’s new home all took place on the exact same day. In fact, all in the morning of that day.
Is this one of those cases of “free annual with purchase” that we often see in airplane listings?
Which raises some interesting questions. Does that mean the airplane had been out of annual prior to the sale? Who did the annual? Was the seller an IA? And just how thorough was that annual?
In fact, the airplane does appear to have sat out of annual for some time. Copies of the logbooks on the NTSB docket show an annual signed off on the day of the sale, May 21, 2023, but the previous annual was in August of 2020, suggesting it was about a year and a half out of annual pending sale.
All of that said, this accident was not the result of a bad inspection. There’s no inspection requirement, nor guidance of any kind, for the ongoing airworthiness of an engine primer.
Still, let’s think more about this inspection-less (but legal) primer. Pretty much every time you start a carbureted airplane engine you need to use the primer. I think the only exception is a quick stop on a hot day, when priming isn’t necessary.
This airplane had 6,165 hours on it. If, just for the sake of analysis, we assume the average of each flight was one hour, and 80% of the flights needed priming, then the primer would have been unlocked — the nub rotating in the channel — 4,932 times over the course of its service life. Then, of course, it needs to be relocked. So now we’re talking about 9,864 cycles. (The piston is operated three strokes, per the POH, but those are typically completed on a single unlocking, with the primer being locked at the end.)
But the actual number is almost guaranteed to be higher. As pilots of carbureted airplanes know, such engines can be… ah… stubborn… to start. For many, a first attempt engine start is a rare thing. Add to that we are talking aluminum that’s at least 60 years old, and you can see why it might give up the ghost. I mean, how many times do you need to bend a paperclip before it breaks?
But what’s interesting to me is that, while high-ish for its age, we’re not talking about a crazy-high number of hours on the airframe. Of course, in many cases, primers get replaced, so not every vintage airplane is still sporting its original primer.
And while I “knew” that primer locks could wear out, without really engaging in any deep thought about it, I assumed the little locking nubs wear down. Those you can see to verify condition. But as this materials analysis shows us, the nub is stronger than the lip inside of the end cap. So it’s the inside guts of the primer that wears out. That was an eye-opener for me.
The Takeaway
We talk about the value of systems knowledge as the last defense in flight when it comes to reversing system failures. And for engine fuel issues, I think most pilots are pretty good about working the throttle and the mixture.
Accident statistics show us we aren’t so good at the next most obvious links: The fuel selector (if equipped) and the carb heat.
So, if, as a group, we aren’t too great about remembering the tank selector or the carb heat knob, I’m guessing that the poor neglected primer, all the way over on the far side of the flight deck, is even further from the mind of a pilot in an emergency. Had the primer been part of the new owner’s mindset, he might have been able to save his new ride — and the innocent minivan.
So, to me, that’s the first takeaway. When you are troubleshooting dropping RPM in a carbureted engine, don’t neglect checking that primer.
The second — and perhaps more important — takeaway is a new understanding of how time affects the primer locking lip, hidden down deep inside the end cap where we can’t see it without serious disassembly.
What are we to do about that?
I think the best test is to give your primer a firm tug now and again. I’d do it as part of post flight. No point in ruining your day right at engine start, right?
The Numbers
Want to read more? Download the NTSB’s final report here or view the items on docket here.
As an instructor and examiner, with technical background, for the last 30 years, i noticed many pilots have a little or no technical background at all. Noticeable during the many pre-flight questions, how things works and what to do if…. only big questions.
Then in the air with simulated engine failures (during the profchecks for renewals or revalidations), candidates stop doing checks after the basic drill: carb heat, fuel selector, and mixture rich, with or without the fuel pump switches, after these 3 checks, the focus is completly on finding an field to land and maintain a constant glide speed. As this last part is still a major struggle for many pilots, the second part of the trouble shoot will never come in the mind, missing out the chance of detemine the engine failure, caused by the primer unlocked, magneto failure, but also fuel tanks en engine instuments, like oil press or temp, can show the cause of failure.
I can only say to be happy, this accident only caused material damage and mayby some physical damage, but everybody walk away. The take away in this story says enough, but i would give a big advice to all instructors, practice these drills on the ground, show what happens when you turn that magneto key (or switches), show an unlocked primer, explain and practice over and over again, what to check, with a rough engine or dropping RPM, check that fuel indicator, check that oil temp and pressure and make the questions, NOW WHAT? The brain has to excersise these inputs, so automation will take over during a real engine failure. Unless a direct engine failure during or after take off, there is enough time to do it all….. Practice and knowledge can safe your life.
I am glad to know the cause was discovered. The pilot in this accident is a friend, and very competent pilot. I would fly with him any day. He told me the whole story and 99% of pilots would have had the same result or worse. I will be replacing the primer on my Cherokee. It’s not nearly as bad as the one in the photo, but it has wear in the locking ring and is original to the plane.
Looking at that accident, surely he must have known it would not fit? Even I can see that.
Really.? Why make such a non-constructive comment ?
Some years ago I was about 500 feet agl in my Aeronca 11AC when the engine power dropped. I went through the process, but there was no response. The primer was locked. I opened the primer and felt a slight increase in power. I stroked it and the engine power increased. Power dropped off at the end of the stroke. Being an A&P with IA, and having rebuilt this engine, I recognized the engine power loss was due to lack of fuel. I applied carburetor heat which helped richen the mixture. Success! The combination of carb heat and open primer restored cruise power. As I traveled the 10 miles back to the airport I tested closing the primer and carburetor heat (individually) but learned it took both to keep the power.
Upon disassembly of the carburetor, clay was found restricting the main jet. How that got there is another story.
THIS should be “THE PICTURE OF THE DAY!”
It should be noted that in this particular case, the engine lost power at the departure end of the runway over a populated area with trees and buildings. It’s one thing to have a sputter at a comfortable cruise altitude, but another altogether to lose power just crossing over the perimeter fence on climb out. Had the PIC invested his time and effort inside the cockpit trying to troubleshoot instead of pitching for airspeed and finding a hole in the trees, he and others might not still be with us. My number one takeaway: Pilot did a heck of a job with a bad situation. Context matters.
Up there in the cold country, I suspect primers get used alot more and wear out faster then in sunny Florida. The emergency procedure always says “mixture – full rich”. If time and altitude permit, be creative and try leaning it out. Sadly, he didn’t have that option.
Many years ago, while test flying my C-150 after maintenance and halfway down the runway at 100′, it went to “auto rough”. After performing a double handed ballet at the speed of sound, I found that leaning improved the performance enough to get back.
I had a primer disengage while on a night flight over Tampa Bay with a resulting sudden roughness. My instructor, a WWII pilot and instructor (this was in 1978) was quick to run through the possibilities and identify the issue. A quick fix – nothing sounds better than a smooth running engine especially at night!
From my experience, the lowly primer is not a forgotten item in the checklist during engine problems. The memorized emergency checklist for any aircraft with a primer is a flow that includes the primer. Even with aircraft where the primer functions correctly, it doesn’t mean the pilot will always properly engage it so it must be part of that flow if engine problems arise: “IN AND LOCKED.”
Yes – the popular C172P is a perfect example. The first three items are carb heat, fuel selector both, and mixture rich. All three of those items are near the throttle which may give the message that everything associated with the power has been checked. But there are two more items and the pilot must cross over the control wheel to check them – mags both and primer in and locked. I like to practice the flow and remember that there are five items to check. In many of the engine failure accident reports we see, the pilot reports what items were checked but it didn’t include all of the items on the checklist, and that made the difference.
I had a flight in a Cherokee 140 that was a different learning experience. The primer on those models don’t extend out very much. On one flight the engine would run rough at lower power settings but normally when power was increased. That was reported and the mechanic noticed we hadn’t locked the primer. Probably the small amount it traveled when pulled out had something to do with it, but bottom line, it apparently wasn’t pulled to verify it was in and locked after priming.
It should be mentioned that at the far end of the primer shaft/piston are 2 o-rings that shut off the outlet port when the knob is rotated in the lock channel. These wear too and should be inspected after some time. If the o-rings fail, the fuel can still flow to the primer orifices and cause a rich mixture on the affected cylinders.
When I push my primer all the way in, the last 1/2 inch is a bit stiffer, indicating that the o-rings are shutting off the fuel outlet.
The primer should be part of the annual inspection.