# ENGINE FAILURE FOR UNKNOWN REASONS?

**That’s what TCM’s analytical report said about the catastrophic failure of a TSIO-520 last year. Let’s see if we can figure out why the engine came apart.** 

by Mike Busch 

### TECH TALK



# T his column is about a catastrophic engine failure that occurred in a Beech Bonanza, but I think the lessons learned are vital for every Cirrus owner.

On November 7, 2007, a Rhode Island-based, 70-yearold, 4,000-hour instrument pilot climbed into his 2002
Beechcraft Bonanza B36TC and took off from Mountain Air Airport (2NC0) in mountainous western North Caro

lina, headed for FlightSafety International for refresher training in emergency procedures. He didn’t make it.

Mountain Air is an extremely challenging airport with a 2,900-foot runway carved out of the side of a mountain at a field elevation of 4,432 feet, and is surrounded by high terrain (see Fig. 1). I’ve flown there once – not as PIC, but in the right seat of a Cessna T210 owned and flown by a friend who has a condominium there, and has flown into the airport hundreds of times. Despite that, it frankly scared the hell out of me. It is one hairy airport.

Almost immediately after liftoff, the Bonanza’s TCM TSIO-520 engine started to lose power. An immediate left turn is required after takeoff from Runway 14 to avoid rapidly rising terrain. While in the turn, the Bonanza pi

lot observed an uncommanded drop in manifold pres

sure from 36 to 20 inches. He briefly considered return

ing to Mountain Air for a landing, and then quickly dismissed it as out of the question because the engine was losing power and an undershot landing at Mountain
Air would not be survivable. Keeping his cool, the pilot headed southeast towards Rutherford Country Airport (FQD) located in a valley 33 nm away at a field elevation of 1,078 feet with a 5,000-foot paved runway. The airplane was light – just the pilot aboard, plus relatively little fuel – and managed to climb to 6,500 feet, but engine power continued to deteriorate and soon the Bonanza was no longer able
to maintain altitude.
The pilot continued toward FQD and entered a high traffic pattern. He made a mayday call on 122.8 Uni

com, announcing that he had suffered an engine failure and was going to make an emergency landing at FQD. A flight instructor on the ground replied, “The airport is yours.” On base leg, the engine made a loud noise and quit altogether, and the windshield became covered with engine oil. The pilot proceeded to make a perfect landing and emerged without a scratch (on him or the airplane).
“The best thing about that flight,” quipped the 70-yearold pilot when I spoke with him recently, “was that my girlfriend wasn’t aboard.” (You just gotta love it!)

Upon exiting the aircraft, the pilot noted two dents on the forward left part of the cowling. Opening the left cowling door revealed that the #6 connecting rod had failed and punched a large hole in the case just above the #6 cylinder. Some of the #6 cylinder hold-down nuts were missing, while others were loose and in the process of coming off.

The Bonanza was towed to an FQD maintenance shop called Plane Werks. Teledyne Continental Motors dispatched a technical representative to inspect the engine – a TCM factory rebuilt with 687 hours total time – after which the engine was removed from the aircraft and shipped to TCM in Mobile, Ala. for a complete tear

down inspection. TCM ultimately issued an analytical report (see Fig. 2) declaring the Bonanza’s catastrophic in-flight engine failure to be a “FAILURE FOR UN

KNOWN REASONS.”
However, a closer reading of the TCM analytical report is quite revealing and instructive. After reading the report, it doesn’t take a lot of imagination to determine the prob

able cause of the engine failure. Let’s take a look at what the report says: 

### What happened?

FIGURE 1: Mountain Air (2NC0) is an extremely challenging airport surrounded by high terrain.

March/April 2009 20 • CIRRUS PILOTFIGURE 2: TCM’s analytical report on the catastrophic engine failure of B36TC N428LP on November 7, 2007.

Volume 4, Number 2 CIRRUS PILOT • 21 March/April 2009 22 • CIRRUS PILOT In flight heard loud noise followed by “humming.” Landed, inspected crankcase to find large hole above #6 cylinder. Cylinder loose. Oil flooded wind

screen. Landing was uneventful, no injury. Inspect engine for report. Remove for shipment to analytical in Mobile. Reported [cylinders] 2,4,6 were removed for repairs...

In fact, cylinders #2, #4 and #6 (on the left side of the engine) had been removed just 9.1 hours prior to the engine failure. Cylinders #2 and #6 were removed because of low compression readings; cylinder #4 was
removed to facilitate access to #6.

Made field visit to inspect the engine. Visual inspec

tion confirmed loss-of-lubrication signatures to #5 and #6 [connecting] rods with subsequent rod failure. Noted evidence of loss of cylinder torque on #6 cylinder. Two missing cylinder base nuts were found in cowling. During cylinder borescope [inspection], noted #1 piston not moving in the cylinder. Seven quarts of oil in the sump. Moderate amounts of aluminum in the [oil] filter. Forward crankshaft bearings appear to have been displaced. Visual inspection results showed indication of spun bearings due to loss of cylinder torque.

TCM informed the owner/pilot that there would be no warranty consideration, but that TCM would accept the destroyed engine for full core credit against purchase of a replacement factory rebuilt engine. The owner/pilot was also required to pay for the cost of shipping the en

gine to Mobile.

The maintenance shop in New England that performed the annual inspection on the Beech B36TC removed all three cylinders on the left side of the engine. Cylinders #2 and #6 were removed due to weak compression. Cyl

inder #4 was removed to facilitate removal of #6 (neces

sitated by the structural cowling of the Bonanza).

Removing each cylinder requires removal of the eight cylinder hold-down nuts that secure the cylinder base flange to the crankcase. Six of those hold-down nuts are
threaded onto short deck studs that are mounted in
threaded holes in the crankcase. The other two holddown nuts are threaded onto a pair of long through-bolts that pass all the way through the crankcase (See Fig. 3).

To understand the cause of this catastrophic engine failure, you need to know something about how the engine is put together. The left and right banks of cylin

ders are staggered so that each pair of through-boltsrun from the front hold-down nuts on a left-bank (evennumbered) cylinder to the rear hold-down nuts on a right-bank (odd-numbered) cylinder. The engine has four pairs of these through-bolts (see Fig. 4), and they are primarily responsible for holding the two crankcase halves firmly together. The crankcase contains precisely machined semicir

cular main bearing saddles that hold the main bearings to support the crankshaft. The main bearings are semicir

cular shells (see Fig. 5) consisting of steel back plates laminated to bearing surfaces made of a softer material called “babbit” to prevent scratching the crankshaft jour

nals. The main bearing shells have oil supply holes that must line up precisely with oil passages machined into
the crankcase halves.



### What really happened? FIGURE 3: Each cylinder
is held in place by six
deck studs
and two through-bolts.

FIGURE 4: Eight through-bolts hold the crankcase halves firmly together.

FIGURE 5: The main bearings are semicircular shells

held in place by through-bolt torque.Volume 4, Number 2 CIRRUS PILOT • 23 When the engine is assembled, the ends of the main bearing shells stick out slightly above the mating surfaces of the crankcase bearing saddles. As the case halves are mated during engine assembly by torquing the through bolts, the bearing shells are forced firmly into the crank

case saddles in an operation known as “crushing.” Once the bearings are crushed in place during engine assem

bly, it is absolutely essential that they not be disturbed.
If a mechanic removes all three cylinders from one side of the engine by removing all 24 cylinder hold-down nuts (8 per cylinder), this results in loss of torque on six of the eight through bolts that hold the two crankcase halves together. If the mechanic rotates the crankshaft while the engine is in this precarious state – in order to position each successive piston to facilitate removal of the associated cylinder – there is a serious risk that the main bearings will shift in their saddles.
If the bearings shift, the oil holes in the bearing be

come misaligned with the oil passages in the crankcase, partially cutting off the oil supply to the bearing. If the engine is then put back into service, the reduced oil sup

ply to the bearing causes increased heat and friction that can result in the bearing shifting more, further reducing its oil supply. Ultimately, the bearing can shift enough to cause the oil supply to be cut off completely, and the result is invariably a catastrophic failure of exactly the kind that the Bonanza’s engine suffered. This phenome

non is usually referred to as a “spun bearing.” After reading the TCM analytical report several times, talking to the owner/pilot, and looking at the photos he took of the engine after his forced landing, there’s no doubt in my mind that this is exactly what happened to the Bonanza’s TSIO-520 engine. In my opinion, this was
a classic maintenance-induced failure.

Could this failure have been prevented? Absolutely!

The correct way to remove multiple cylinders is to re

move one cylinder at a time, then retorque the associated pair of through-bolts (using a “torque plate” or a stack of washers in place of the missing cylinder flange) before moving on to the next cylinder. By doing it this way, six of the eight through bolts are always in a torqued state, firmly clamping the crankcase halves together and making it far less likely that any of the main bearings
will shift in their saddles.

In addition, the crankshaft should never be rotated while any through bolts are in an untorqued state . The mechanic should position a piston, then remove the eight hold-down nuts and the cylinder, then re-torque the two through-bolts (so all eight are torqued) before rotating the crankshaft to position the next piston. 

### The Right Way to Pull a Jug

March/April 2009 24 • CIRRUS PILOT Finally, TCM engine guru Bob Moseley – who knows more about these engines than anyone I know – suggests that the mechanic should measure the crankshaft end-play with a dial indicator before removing any cylinders, and then measure it again after the cylinder work is completed. If the crankshaft end-play changes as a result of the work performed, then the mechanic knows there’s a problem and won’t approve the engine for return to service until the problem has been resolved.

Here’s the bad news: I’d guess that only one mechanic in ten will use torque plates to re-torque the through bolts on one cylinder before removing another cylinder. Only one mechanic in 100 will check the crankshaft end-play before and after doing cylinder work.

The moral of this story? Owners need to understand that removing a cylinder is an invasive procedure that, like sur

gery, involves an element of risk, and that removing multiple cylinders is highly invasive and entails substantial risk.

Cylinder removal should never be undertaken lightly. Don’t let your mechanic do it unless you’re absolutely convinced that it’s really necessary. TCM service bulletin SB03-3 is the bible on this ( http://www.tcmlink.com/pdf2/SB03-3.pdf ), and strongly discourages cylinder removal regardless of compression reading unless a borescope inspection reveals a definite problem.

If you must remove one or more cylinders, be very selec

tive about whom you allow to do the work, because it’s easy to set the stage for a serious maintenance-induced failure if it isn’t done exactly right. If multiple cylinders need to come off, ask your mechanic whether he uses torque plates. Any answer other than “Of course I do,” should be grounds for considering another shop for the job. COPA

Copyright 2009, Michael D. Busch First publication rights granted Cirrus Owners and Pilots Association All other rights reserved by copyrightholder.

About the Author: Mike Busch – honored by

the FAA as its “National Aviation Maintenance

Technician of the Year” for 2008 – has been a pilot for more than 44 years and 7,000 hours,

and an aircraft owner and CFI for more than 40 years. He became increasingly interested in the maintenance aspects of aircraft owner ship about 20 years ago, and ultimately earned his A&P/IA. Mike is also a prolific aviation writer, with hundreds of technical articles published in American Bonanza Society Magazine, Aviation Safety, AVweb, Cessna Pilots Association Magazine, IFR, Light Plane Maintenance,

and The Aviation Consumer. He co-founded AVweb in 1995 and served as its editor-in-chief for more than seven years. Mike conducts weekend “Savvy Owner Seminars” at which aircraft

owners learn how to obtain better aircraft maintenance while spending a lot less money ( http://www.savvyaviator.com/ ). Heis founder and CEO of Savvy Aircraft Maintenance Management ( http://www.savvymx.com/ ) that professionally manages the maintenance of owner-flown aircraft including Cirrus SR20s and SR22s. Any questions for Mike may be emailed to mike.busch@savvyaviator.com. 

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