Saturday, November 21, 2009

Turbos, oil temp, and overboosting

The owner of a Florida-based 1977 Bonanza V35B with a turbonormalized TCM IO-520-BA engine contacted me with concerns about fuel flow. He described a recent instructional flight involving a takeoff on a "coolish day" that showed 31" MP, 2700 RPM, and 31 GPH fuel flow. Later in the flight, a full-throttle go-around was executed, and the owner that fuel flow was 3 GPH below red line on the factory analog fuel flow gauge, and only 27 GPH on the Shadin digital fuel flow gauge. After landing, the owner described this situation to his mechanic and suggested that the fuel flow be adjusted up. The mechanic was concerned that if he adjusted the fuel flow, it might be excessive on takeoff.

I explained to the owner that his turbonormalized engine has a hydraulic wastegate actuator and controller that uses engine oil as its hydraulic fluid. A byproduct of this is that regulation of upper-deck air pressure (UDP) is not precise, and the system delivers higher UDP when the oil is cold than when it is hot.

I further explained that engine has an aneroid-compensated fuel pump whose output pressure is a fuction of two variables: engine RPM and UDP. Therefore, if RPM is held constant at 2700 RPM (red line), the fuel pump will put out more fuel flow when the oil is cold (and the UDP is high) than it will when the oil is hot (and the UDP is lower).

For safety's sake, I explained, the turbo controller should be adjusted so that it gives full red-line MP when the oil is hot -- so that, for example, if a full power go-around or missed-approach is executed, full rated power is actually available.

If the system is adjusted in this fashion, then when full power is applied with cool oil (e.g., first takeoff of the day), there will be some "overboost" (MP above red line) by an inch or two. This is okay. Momentary overboost will not harm the engine, and is a quickly self-correcting condition as the oil warms up during the takeoff roll. I suggested that the owner NOT try to manually limit the MP to red line under these conditions, and simply accept the brief overboost. (TCM has a SB that states that momentary overboost up to 3" is a non-event.)

Under such brief overboost conditions (MP over red line), if the system is adjusted properly, fuel flow will also be over red line momentarily until the oil warms up and the MP comes down, since the fuel pump is compensated to follow UDP. This is the way things are supposed to work. If the engine is momentarily producing a bit more than 100% power, then it needs a bit more than 100% fuel flow. The system knows what it's doing, so the pilot shouldn't try to help it.

The turbo controller should be adjusted so that it will give full red-line MP with hot oil. Making this adjustment will automatically increase fuel flow as well, because higher UDP commands higher fuel flow. Once the turbocontroller has been adjusted, full red-line fuel flow should be seen when full power is applied with hot oil. If fuel flow falls short of red-line, the high unmetered fuel flow (the adjustment screw on the fuel pump aneroid) should be adjusted to achieve full red-line fuel flow.

On takeoffs with cool oil, there will be 1" or 2" initial overboost above MP red line, and momentary FF above red line. All this is good.

It is essential that the turbocontroller and fuel pump adjustments be made when the engine and oil are hot.

High CHTs and cracked cylinder heads

A member of a German flying club wrote me about a Robin DR-300 that the club used in about a 50-50 mix of cross-country flying and towplane operations for the club's sailplanes.

When towing gliders, the airplane climbs at full power and very slow airspeeds, and CHTs regularly register in the high 400s Fahrenheit, close to Lycoming's 500F CHT red line. Once the glider releases, the engine is throttled back to idle and descends rapidly to land and hook up for the next tow.

The Robin's Lycoming O-360-A3A engine is now at 1,200 hours, just 60% of TBO. One cylinder started to show declining compression readins, so the mechanics reduced the compression-test interval from 100 hours to 50 hours. At the next check, the cylinder compression had fallen into the 40s, so the cylinder was removed and found to have a cracked cylinder head, with cracks running between the top spark plug hole and the exhaust valve seat.

    The club member's questions for me:

    1. Why did the cylinder head crack?

    2. Should the other three cylinders be removed?

    3. Should the mechanics have caught this earlier?

    I explained that any non-ferrous metal component (like an aluminum alloy cylinder head) will eventually crack if subject to repetitive stress for a sufficient period of time. The component will crack sooner if the repetitive stress is more intense and/or if the operating temperature is greater. Extended full-power operation during glider tows obviously increases the intensity of the stress, and the very high CHTs reduce the tensile strength of the cylinder head. (The head's tensile strength is reduced to 50% of its room-temperature value at a CHT of 400F, and to about 33% at 500F.)

    Although towplane operations are inherently very stressful on the engine, I suggested that cylinder longevity could probably be increased by making flatter, higher-speed climbs and less rapid descents. However, as a glider pilot myself, I understand that gliders generally have a fairly low Vne, and that glider pilots usually want to spend minimum time on tow, so engine longevity considerations may have to be subordinated to operational demands.

    I recommended to the German club member that the other three cylinders NOT be removed. Instead, they should be inspected for cracks internally using a good borescope and externally by direct visual inspection. Just because one cylinder cracked does not mean that the others will crack any time soon. In the 22 years I've owned and flown my 1979 Cessna T310R, I have had to retire 2 of the 12 cylinders because of head cracks. One crack occurred at about 2,000 hours time-in-service, and the second did not occur until about 4,500 hours. The remaining 10 cylinder heads have not cracked (yet), although they all will do so if kept in service long enough. It certainly would not have made any sense for me to pull all 12 cylinders when the first cylinder head crack occurred about 18 years ago!

    Finally, I suggested that it sounded like the club's mechanics had done a good job, but that had they performed regular borescope inspections of the cylinders they probably would have detected the cracks earlier. TCM requires at least annual borescope inspections (TCM SB03-3), but Lycoming does not. Nevertheless, I believe frequent and regular borescope inspections are an essential part of engine condition monitoring.

    Welcome to my new blog!

    Mike Busch
    For more than two decades, I've been helping aircraft owners deal with their thorniest maintenance problems, first as a technical representative for the 12,000-member Cessna Pilots Association, and more recently through the 10,000-member American Bonanza Society and the 2,000-member Cirrus Owners & Pilots Association.

    I also write monthly maintenance columns for the magazines of these three "type clubs," as well as for EAA's Sport Aviation and the online aviation magazine and news service AVweb which I co-founded in the mid-90s and served as editor-in-chief for more than seven years (at which point it was sold to Belvoir Publications).

    I find my technical support activities to be both rewarding and frustrating, rewarding because I love helping aircraft owners, but frustrating because those owners tend only to ask for my help after repeated trips to the maintenance shop, repeated expensive invoices, and repeated discoveries that their squawks remained unresolved. By the time they contact me, they are often desperate and angry, and I can't help but think (although I try not to say) "why didn't you contact me earlier?"

    Six years ago, in an attempt to do something more pro-active to prevent owners from enduring these kinds of frustrations, I started going around the country teaching in-depth 17-hour weekend seminars to help owners learn to manage their maintenance better. About 1,000 owners have graduated from this seminar. Some have become excellent maintenance managers, but many have been unable to take full advantage of the training, either because they are too busy to do a proper job of managing their maintenance, or because they're just too uncomfortable managing the work of their shops and mechanics, telling them what they want done (and not done) instead of the other way around. Many owners find the whole subject of maintenance quite intimidating and far outside their comfort zone, and so they put themselves at the mercy of their shops and mechanics to make their maintenance decisions, and then are often quite unhappy with the outcome and sometimes feel taken advantage of or even victimized.

    Consequently, about 18 months ago, I decided to start a new firm to provide professional maintenance management to the owners of owner-flown GA aircraft. Starting from scratch, we're now managing the maintenance of nearly 200 airplanes, mostly high-performance piston singles, light piston twins and cabin-class piston twins. To maintain these airplanes, we're working with hundreds of different maintenance shops throughout the U.S. Some of these shops are terrific, some are terrible, and some span the spectrum in between. One of our most important tasks is to try to persuade our aircraft-owner clients to patronize the good shops and avoid the bad ones. Once the shop has been chosen, we manage their work closely, making sure that they everything that is necessary and nothing that isn't. We scrutinize their estimates and invoices to make sure that our clients are getting maximum value for their maintenance dollars. Typically, we save each of our clients thousands of dollars a year in reduced parts and labor costs.

    In the course of my type-club support activities, my writing, my teaching, and my professional maintenance management work, Not a day passes that I don't encounter and interesting maintenance-related war story. Some of them are just too valuable not to share. I decided to start this blog to share these stories with my fellow aircraft owners and mechanics. I hope you find them educational and enjoyable.

    --Mike Busch A&P/IA CFI/A/ME
       2008 National Aviation Maintenance Technician of the Year
       President, Savvy Aviator, Inc.
       President, Savvy Aircraft Maintenance Management, Inc.