Is AE300 TBO mandatory?

[Ed McDonald]

Further to my previous comment about unnecessary and indeed harmful “preventative maintenance” on aircraft by using irrational TBO values, this provides a bit more detail on my position on this.

Mike Busch’s “Maintenance Manifesto” is a short read and I highly recommend it. As it pertains to engine overhauls specifically, this book quotes research done from the NTSB database that shows that greatest risk of a piston engine failing is between 0-499 hours (or between year 1 to 4) after overhaul then continually declines thereafter, including well beyond TBO.

The airline industry has known this for decades but more recently with engines being monitored electronically on FADEC engines, like the Austro, the abundance of engine health data allows the engine to be monitored; the analysis of engine failures cited in the book predates sophisticated engine monitoring using digital engine monitors.

The application of TBO periods – hours or months and years – is completely arbitrary and lacking any good science or statistical analysis. Mike’s book explains this nicely and I won’t repeat it.

The aircraft I sold to buy the DA62 was a 1983 Piper Seneca III. Both engines were beyond their TBO, their oil analysis was showing no trends and the engine analysis (from Savvy) showed no trends either – the Savvy analysis, did, however point to spark plugs that were misbehaving and a timing issue but in terms of engine performance parameters the engine remained within specifications. My AME (A&P in American terms) told me that we could have easily taken the engines to 3,000 hours before overhaul and 4,000 hours if we were prepared to scrap the engines. But we did go beyond TBO based on data – engine oil analysis and engine data logging and analysis.

Disclaimer: the following material I present is based upon my experience as a professional pilot and Professional Engineer and aircraft owner for 15 years. I am not advocating that anyone else follow what I will be doing with my DA62 and its Austro engines but this a description of how I will be reducing the risks of an engine failure (job #1) and increase the overhaul period while reducing costs (job #2).
In the airline industry, maximum thrust take-offs are rarely used. Decades of data and research shows that by using reduced thrust while maintaining the balanced field and minimum climb performance requirements, yields reduced engine wear, reduced risk of an engine failure and extended overhaul periods.

The damage to a turbine engine occurs when the engine is being operated at peak temperatures, pressures and engine speed. The metal in the hot section can sustain high temperatures for a finite period of time and the longer the temperatures are at the maximum, the more damage is done to the engine. Same with internal engine pressure. As far as engine speed is concerned, the higher speed with higher centrifugal force combined with higher temperatures causes blade creep (stretching of the blades). Anything that can be done to reduce these stresses reduces the damage to the engine.

The highest demand for thrust is the take-off phase to accelerate the aircraft to take-off speed with a finite amount of runway to do so. Airliners have excess thrust for the vast majority of take-offs. In other words, a reduced thrust can be used to allow an aircraft to accelerate to a V1 (decision speed) then either reject the take-off and come to a stop in the remaining runway or continue the take-off, get airborne, and climb safely on one engine. Performance engineers calculate the reduced thrust settings for every runway an air carrier uses under varying outside air temperatures, weight and anti-icing configurations (altitude is fixed as it is specific runways are analyzed). Every airline I know of has a policy of using reduced power take-offs unless required by aircraft performance or icing conditions require otherwise.

In 32 years of airline flying and tens of thousands of take-offs, I can count on my hands and feet the number of full power take-offs I did. And the ones I did were on a B737-200 that was being flown beyond its design range and full power plus having the air-conditioning in on the APU was used.
Another way of looking at this is more pedestrian; when one acerates from a red light, do you normally floor the accelerator or is a more gentle, less aggressive amount of engine power applied? Imagine the stress in the engine if a maximum effort acceleration from every stop was used and extrapolate that across the life of the engine.

How does this apply to the Diamond aircraft and Austro engine? I have only been the owner of the DA62 for 4 months and have flown about 60 hours on the aircraft but what has struck me is how much power this aircraft has. This is presented not just in the acceleration at take-off but it is also manifested in the take-off distances required, even at maximum gross weight. This is due to both the power in the engines but also that massive wing.

I have a personal policy of a minimum 3,000 ft runway length; my operating airport has a 4600 ft long runway. At 5071 pounds, 2000 ft elevation and +30 C, this thing only needs 2000 ft of runway and 3500 ft to clear a 50 ft obstacle! Even with a 3000 ft runway there is 50% more runway that what is required and at my home airport there is more than double the amount of runway.

Given my experience from the airline industry, the knowledge about jet engine wear and the performance of the DA62, I am going to use 95% load for all take-offs, unless other factors come into play – obstacle clearance, winds, etc. If load is a proxy for engine power, that would mean I would lose about 5% of the power I would normally have and will certainly extend the take-off run in theory by about 5% or 100 ft. I do not have any performance numbers to prove this.

I contacted Diamond (Trevor Mustard) to let him know about this as advisory only and asked whether Diamond had ever considered this. They have not and they are not prepared to endorse it – I don’t blame them as proper performance testing would be required.

What do I hope to achieve by this? First, by not operating the engine at it maximum load (RPM, temperatures and pressures) I expect the wear and tear on the engine to be less. This will be borne out over time as these engines get compared to other A330 engines operated normally with 100% load being used. Next, I believe I will reduce the risk of an engine failure during take-off as I am reducing the stresses on the engine – that only stands to reason. Finally, my plan is to operate these engines “on-condition” beyond the TBO values which is permitted in Canada, including commercially operated aircraft if engine monitoring and oil analysis is used.

This will be an interesting experiment but I am confident that it will deliver the objects noted above.

I am not advocating this for any other DA62 owners or Austro engined aircraft. But I do offer this as food for thought about the arbitrary TBO times that pre-date World War II and that science, statistics and experience are proving that this is not the right or rational way to maintain and operate aircraft or engines.

[dmloftus]

Makes complete sense. I usually hold back a bit on my DA40-XLS, often avoiding full power on the runway and, if I'm light, backing off to 24/24 at about 500 feet at airports where I have an out if I lose the engine (ie good highway adjacent, open farm land). But I will always use full power when taking off over populated areas to provide the increased altitude for a runway turnback in the case of power loss.

[Boatguy]

I am going to use 95% load for all take-offs, unless other factors come into play – obstacle clearance, winds, etc. If load is a proxy for engine power, that would mean I would lose about 5% of the power I would normally have and will certainly extend the take-off run in theory by about 5% or 100 ft. I do not have any performance numbers to prove this.

Derated thrust is certainly the norm. I was an IT "utility infielder" at a commercial airline in the early 70's that flew the DC-10-30. One of my tasks was to produce an abridged version of the Cruise Control Manual (aka AFM or POH) and that included derated thrust takeoffs. Here is an FAA note from the late 80's:

https://www.faa.gov/documentLibrary/med ... C25-13.pdf

I hope you will keep some records of the approximate runway length consumed by your DA62 at 95%.

What power setting are you using in cruise?

[dant]

I usually hold back a bit on my DA40-XLS

Conveniently with a naturally aspirated engine you don't stay at full rated power long!

[greg]

In the airline industry, maximum thrust take-offs are rarely used. Decades of data and research shows that by using reduced thrust while maintaining the balanced field and minimum climb performance requirements, yields reduced engine wear, reduced risk of an engine failure and extended overhaul periods.

I miss the old days before this started - travelling around the world as a kid in 707s and 747s during the 60's and 70's. After the plane turned onto the runway, the pilot would stand on the brakes as the engines spooled up to full power. The feeling as the brakes were released and you were pushed back into the seat was exhilarating. These days in a A380 the takeoff roll feels like taxiing. Boring!

(No doubt it's better economically, but I want excitement!)

Sorry for the thread creep.

[Ed McDonald]

What power setting are you using in cruise?

I am using 75% in cruise for now. One can do the math to determine the optimum power setting to produce a minimum cost trip by combining hourly, variable costs with hourly fuel cost, the famous cost index as you might remember. I haven't got that sophisticated with this aircraft yet.

I didn't mention that I own an L39 Albatross. With that aircraft I used Max Continous for take-off, max cruise for climb then a cruise power setting. In other words, every phase of flight had a derate attached to it. One becomes very sensitive to babying the engine when one pays all the bills and the aircraft is single engine.

The last borescope of the engine (at 800 hours) prior to an unnamed maintenance organization screwing around with the fuel control unit showed perfectly clean combustion chamber and turbine section. This was due to not stressing the engine by keeping the temperatures and RPM's down.

As far as take-off distances are concerned, I will keep an eye on that and I might even do some flight testing using the EAA flight test cards. With the Garmin 1000 outputting so much data post processing might be done electronically rather than manually figuring out take-off distances.

I asked Savvy Analysis about trend analysis for the DA62 and got the following response:

Our analysis report focuses on EGT and CHT, and typically the G1000 / A330 doesn’t log those. I’ll attach a list of what we typically get.

We can help with RPMs, oil temp and pressure, FF, Volts — anything on this list. But we can’t run report cards and trend reports, and we can’t generate the standard Analysis Pro report.

So it’s not quite a full “no” but it’s a very qualified “yes”.

Surely EGT and CHT for each cylinder must be recorded somewhere. I am going to check with Diamond and see if that important information can be mined and added to the data file for analysis.

[TimS]

@greg

One point missed on the airlines. Spool time, most engines now spool in 2-3 seconds from flight idle to full power.
The older turbofans took much longer; I recall reading somewhere the first generation engines on the 747 took almost 15 seconds to make full rated power.

I went to St. Martin and was at the end of the runway (actually, I was in the ocean, I am not one the people getting knocked over on the beach). The plane with the biggest thrust over the beach of the I believe the MD-80; not even the Airbus 350 produced as much. The difference, the MD-80 came to the end of the runway, stood on the brakes and brought the engines to full power slowly over about ten seconds before brake release. Even the monster four engine A350 did not stop and you could hear the engines spooling up before it finished the turn.

Tim

[Boatguy]

As far as take-off distances are concerned, I will keep an eye on that and I might even do some flight testing using the EAA flight test cards. With the Garmin 1000 outputting so much data post processing might be done electronically rather than manually figuring out take-off distances.

Have you tried uploading the G1000 logs to Flysto.net? Poke around a bit on that site as it has a lot more facilities than are obvious at first glance. There are some excellent analytical tools for both piloting and engine performance.

[CFIDave]

Surely EGT and CHT for each cylinder must be recorded somewhere. I am going to check with Diamond and see if that important information can be mined and added to the data file for analysis.

Savvy is correct that the G1000 files they can capture don't track EGT or CHT temperatures for Austro engines, and neither does the engine itself (via Austro Engine Wizard file download and analysis from the ECUs.) This is a liquid-cooled engine without individual cylinders like an air-cooled Lycoming or Continental -- it's a single cast-iron block with coolant passages.

If you use the Austro Engine Wizard software (with special diagnostic cable) to download engine logs from each ECU, here are the available parameters that are recorded (and can be graphed):

Boost Pressure
Ambient Air Pressure
Propeller Speed
Engine Oil Pressure
Rail Pressure
Power Lever Position
Coolant Temperature
Intake Air Temperature
Battery Voltage
Fuel Pressure
Gearbox Oil Temperature
Engine Oil Temperature
Prop Actuator - Duty Cycle
Engine Status
Engine Oil Level
Engine Load

[CFIDave]

I believe in always using full power on takeoff; here's why:

For Lycoming DA40s, the fuel control unit is set up so that full power enriches the mixture to help cool the engine and prevent detonation. (Detonation is normal on compression ignition Austro diesel engines.) For this reason, "babying" a Lycoming engine by using less than full power on takeoff is not recommended by Lycoming.

For Lycoming or Austro engine Diamonds, full takeoff power is needed for improved energy management close to the ground. Unlike airliners (Transport Category aircraft) certified with considerable excess power and designed to easily climb on one engine, eking out a climb on a DA42 or DA62 twin (particularly near max gross weight, near the ground) with one engine takes considerable pilot skill. Full power permits climbing as high as possible as quickly as possible after takeoff, to maximize aircraft potential energy (airspeed + altitude) -- thus giving the pilot more options in the event of an engine failure.

If you intend to operate Austro engines past TBO, I agree that operating at only 75% is likely to reduce wear on the engine (not to mention provide better fuel economy). Unless fighting really strong headwinds, I generally operate my engines that way.