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

TECHTALK

This 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 Carolina, 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 pilot observed an uncommanded drop in manifold pressure from 36 to 20 inches. He briefly considered returning 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 Unicom, 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 teardown inspection. TCM ultimately issued an analytical 

report (see Fig. 2) declaring the Bonanza’s catastrophic 

in-flight engine failure to be a “FAILURE FOR UNKNOWN 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 probable 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.

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

Volume 4, Number 2 CIRRUS PILOT • 21 
22 • CIRRUS PILOT March/April 2009

In flight heard loud noise followed by “humming.” 

Landed, inspected crankcase to find large hole 

above #6 cylinder. Cylinder loose. Oil flooded windscreen. 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 inspection 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 engine 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. Cylinder #4 was removed to facilitate removal of #6 (necessitated 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 cylinders are staggered so that each pair of through-bolts 

run 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 semicircular main bearing saddles that hold the main bearings 

to support the crankshaft. The main bearings are semicircular 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 journals. 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 crankcase saddles in an operation known as “crushing.” Once 

the bearings are crushed in place during engine assembly, 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 become 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 supply 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 phenomenon 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 remove 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


24 • CIRRUS PILOT March/April 2009

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 surgery, 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 selective 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 ownership 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/). He 

is 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.

AIRCRAFT ENGINE

HEATING MADE EASY!

F.B.O. & Dealer Inquires Welcome

Since 1984

AIRCRAFT ENGINE PRE-HEATER End your cold weather starting problems and

reduce pre-mature engine wear with PROVEN 

E-Z Heat Engine Pre-Heater. Heats ENTIRE engine

in about 4 hours, through conduction,

the safest means of heat transfer available.

•	 FAA accepted, No STC or 337 form required.

•	 Easiest to install of any pre-heat system.

•	 Reduces engine wear. Allows easy flow of

	 engine oil to vital components in extreme

	 sub-zero weather.

•	 Thermostatically controlled - No danger

	 of oil carbonization.

•	 Heats 12 quarts of oil from -40°F to +60°F in 1 hour.

•	 Uses less than 300 watts, compared to many

	 engine heaters that use up to 2500 watts.

•	 Permanent. Once applied, unit need not be removed.

•	 Weighs only 9 ozs. /3 year warranty.

•	 NEW • Double Insulated engine blanket & 

prop covers now available. IO-360 with 442

11 MODELS - BECAUSE EACH ENGINE IS DIFFERENT!

For more information, call or write: E-Z HEAT, INC.

779 Lakeview Dr . • Chetek, WI 54728

1-800-468-4459 • Fax: 715-924-4401

www.e-zheat.com

$

15995