Which system to record TTIS?

[turboplanner]

TTIS has a relationship with airframe flexing and cracking and from that perspective doesn't need to include the unstressed time on the ground.

 

Engine time needs to be by hour meter because engine components wear and fail usually at quite predictable run times, but since there will be several engine rebuilds/replacements per airframe, engone time needs to be kept separate.

 

On that logic FAA is correct, CASA is wrong in their concept, and wrong not to define TTIS.

 

 

[old man emu]

It seems that CASA has decided to go with an ambiguous definition for "time" in relation to aircraft usage.

 

Having no definition for TTIS, this is CASA's definition of Flight Time:

 

In the case of heavier-than-air aircraft, the total time from when the aircraft first moves under its own power for the purpose of a flight until the moment at which it comes to rest on completion of the flight. This in no way identifies to what "Flight Time" applies. Is it airframe/engine time, or pilot I"n Command time"?

 

If you apply the definition of "driver" in the Australian Road rules, (a driver is the person who is driving a vehicle [Note  "Drive" includes "be in control of"]), then "Flight Time" commences when the engine is started to begin a flight and ends at engine shut-down. That's the time that is used in the pilot's logbook. It is not the accepted FAA definition of TTIS. Therefore, it is reasonable for a pilot to log In Command, or Dual time based on the time recorded on the tacho, and also to record TTIS based on the readings of a Hobbs meter which is activated when the speed of the airflow over the wings is in excess of the aircraft's lowest design stall speed in takeoff and landing configurations.

 

In relation to TTIS limits for engines and propellers, the stresses on their components during ground operations are much less than when they are being used for propulsion. These stresses are a result of the torque generated by the power strokes frequency which is measured as RPM. This is a depiction of a generic Torque -v- RPM relation:

 

 

 

 

At the RPM levels used for ground operations (from approx 750 RPM to 1000 RPM), the torque produced by an engine is mostly less than 1/3 of the maximum torque the engine is designed to produce. Therefore the stresses and wear on the moving parts is much less than when the engine is operating at maximum torque.

 

 

[aro]

I would have thought that whoever developed the maintenance schedule should define how the intervals are measured.

 

Rotax definitely specify their engine hours include any time the engine is running - not just flying time. It looks like Lycoming specify "engine operating hours" which to me also suggests any time the engine is running.

 

Are you suggesting that an engine that spends lengthy times idling on the ground should have less frequent oil changes? Most people would say the opposite. Rotax engines probably have most stress on the gearbox at idle. Time spent in cruise should be when the engine is at its optimum temperature, oil pressure etc. and experiencing least wear.

 

CASA in their infinite wisdom might have their own definition of time, but that is not necessarily a good idea.

 

 

[Thruster88]

At the shop I see VH aircraft using either tacho or vdo. It is specified on each maintenance release along with the current reading and TTIS, often different due to instrument  replacement.  

 

 

[old man emu]

Are you suggesting that an engine that spends lengthy times idling on the ground should have less frequent oil changes? 

 

Let's look at this realistically. Does anyone regularly sit in an aircraft on the ground and run the engine for hours on end? Apart from generating a wind towards the rear, the risk is lack of cooling (unless you've got a water-cooled engine, and even then there is reduced cooling on the ground) is enough to raise a concern of excess wear.

 

How many people will cancel a flight just because the magic 25, 50 or 100 hours tacho time would be passed during that flight. Is the engine going to become a block of molten metal for an hour or two? There's many an aircraft flying our skies with engines past TBO, but have been OK'd to operate "On Condition".

 

Can any of you accurately tell me the number of hours that the engine manufacturer logged running-in the engine before releasing it for service? If you can, do you include that time in the engine's TTIS? What about the amount of time you spent sitting in the cockpit of your partially-built project enjoying "flights of fancy", pulling and pushing at the controls? Do you count that as airframe TTIS?

 

 

[aro]

Let's look at this realistically. Does anyone regularly sit in an aircraft on the ground and run the engine for hours on end?

 

Rotax allow 100 hours between oil changes. If you do 1.0 VDO flights including 0.1 on the ground at both the start and end (which seems to be fairly common flight school pattern) your 100 hours airswitch is 125 hours engine running time. Is 25 hours on the ground "hours on end"?

 

How many people will cancel a flight just because the magic 25, 50 or 100 hours tacho time would be passed during that flight. Is the engine going to become a block of molten metal for an hour or two?

 

 

 

Lots of people I suspect, because as I understand it, the rules say that is what you must do. No, the engine isn't going to become a block of molten metal. But you need to decide how much you care about following the maintenance schedule. If you want to follow it, it makes sense to use the method it specifies to record time.

 

 

[old man emu]

After a fair bit of trawling through the legislation, I finally found the definition of "Time in service" in the Civil Aviation Safety Regulations dictionary:

 

Time-in-service means:

 

(a) for an aircraft—each period starting when the aircraft takes off for a flight and ending when the aircraft lands at the end of the flight; and

 

(b) for an aircraft engine or propeller that is fitted to an aircraft—each period starting when the aircraft takes off for a flight and ending when the aircraft lands at the end of the flight.

 

If you do 1.0 VDO flights including 0.1 on the ground at both the start and end (which seems to be fairly common flight school pattern) your 100 hours airswitch is 125 hours engine running time. Is 25 hours on the ground "hours on end"?

 

 

 

Normally any small engine has an idle speed of around 750 to 850 RPM. At these engine speeds, not much torque is being generated, so the forces transferred between connecting parts (piston crown to piston pin to con rod to crankshaft) are minimal. Abrasion of piston rings and cylinder walls is minimal. For this reason it is practical, and economical, to ignore engine operation on the ground. Remember that if an engine is properly maintained it will continue to be serviceable at 110 hrs tacho time. Also, you must remember that the tachometer only matches the Hobbs meter when the aircraft is operating at cruise. When you pull back the throttle the RPM slows, which slows the "tachometer minute".  So, on the ground, or when on Base and Final, a tacho minute is less than a Hobbs minute. 

 

The tachometer minute is equivalent to the number of minutes of running at a certain, specific reference speed of rotation. If the reference speed of rotation is 2400 RPM then the timer runs in real time when the engine is running at 2400 RPM, half speed while the engine is run at 1200 RPM  or at 5/6ths real time at 2000 RPM (a slow cruise speed). At idle of, say 800 RPM each tachometer minute is only  800/2400, or 20 seconds real time. Taxying at 1000 RPM each tachometer minute is equal to 25 seconds real time. CAVEAT: The idle and taxi RPM values used here are for the likes of Lycoming and Continental type engines. If you have a Rotax or similar you'll have to play with the figures yourself.

 

When I said "Does anyone regularly sit in an aircraft on the ground and run the engine for hours on end?" I meant sitting in the parked aircraft with the engine running with no intention of going flying. Obviously that's a ridiculous thing to do.  Your 10 to 15 minutes on the ground before and after a flight will be done at less than 1000 RPM.

 

I agree with your calculation of the difference between the definition if TIS and tacho time. But how many of the aircraft we fly, RAA or GA non-commercial would clock up 100 hours in 12 months? Isn't the average somewhere less than 50 hours? There's more chance of metal corrosion (rust) under those circumstances than wear and tear due to use.

 

 

[kgwilson]

According to the RA-Aus website the average hour since 1/1/19 is 62.29 which I reckon is total BS. Students probably clock up the most but most aircraft owners fly way less that 50 hours annually. I fly regularly & fly around 50 hours/year. Of the 30 or so aircraft based at our aerodrome about 10 don't see the light of day, 10 fly 2 to 10 times a year and the others clock up 10 to 60 hours a year. The exception is the FTF Jabiru that would be well in excess of 200 hours a year.

 

 

[Downunder]

Normally any small engine has an idle speed of around 750 to 850 RPM.

 

Rotax idle setting is about 1800 rpm....

 

Significantly lower can damage gearbox gears......

 

 

[old man emu]

Rotax idle setting is about 1800 rpm....

 

Significantly lower can damage gearbox gears......

 

Thanks. I don't have any experience in operating a Rotax.

 

Now, what is the RPM setting for normal cruise for a Rotax? 

 

If we know that, then we can make the assumption that tacho time = real time at normal cruise RPM and can estimate the difference between tacho time and real time at other RPM.

 

Tacho minute = {Observed RPM/Cruise RPM} x 60 seconds

 

 

[skippydiesel]

When I drain my oil I always lift the right wheel on to a block so the oil is directed to the sump plug & in theory most of the goo if there is any. Dunno if it works but I feel better about it.

 

You & I must be "fellow travelers" - I too get an unreasonable level of satisfaction in draining all the oil.

 

I know full well that any small amount of oil left in an engine will have no detrimental effect whatsoever, being diluted by the fresh  but the satisfaction just  would not be there if I didn't try to wring the last drop out.

 

I own a Ford Ranger PX 1- some strange peccadillo of the engine designer, requires that oil changes (empty/refill) be completed in under 10 minutes, to prevent loss of oil pump "prime". Every time I change the oil, I  am frustrated by having to replace the sump plug while there is still a thin stream coming out, so as to complete that part of the service in under 10 minutes.

 

 

[onetrack]

Does anyone regularly sit in an aircraft on the ground and run the engine for hours on end?

 

The answer to that is - Yes. There are applications where aircraft engines are run for extended periods on the ground, for equipment testing purposes.

 

I well remember one crash of an Aero Commander, which was caused by fuel starvation, and related to unrecorded time running the engines on the ground.

 

https://www.atsb.gov.au/media/27392/aair198600131.pdf

 

The circumstances were that the Aero Commander was set up with extensive on-board aerial survey equipment. The aircraft was fully fueled by the pilot early the previous day to the flight - but unbeknowns to him, during the day, ground crew carried out intermittent ground testing of the onboard equipment, which involved running the engines for an extended period of time. They did not log this engine run time.

 

The pilot turned up at 08:00HRS to take the aircraft for a flight involving aerial survey, and he failed to physically dip the tanks, because he obviously believed they were full, as he last left them - and no aircraft use had been recorded.

 

In addition, the fuel gauges were inoperative, a fact that the pilot was apparently aware of.

 

He took off with tanks low on fuel, and ran out of fuel after around 90 minutes of operation, and crashed in the Darling Scarp near Canning Dam, above Perth, resulting in total destruction of the aircraft, 2 fatalities and 1 seriously injured passenger.

 

Re the measurement of engine hours done, I have seen engine manufacturers who measured wear rates and engine operating time, as the total number of revolutions of the crankshaft.

 

I believe TTIS should be measured by any aircraft actual movement, on the ground or in the air.

 

 

[skippydiesel]

".........................................................................

 

I believe TTIS should be measured by any aircraft actual movement, on the ground or in the air.

 

It may be pedantic of me but I would go one step further - all operational (engine on, systems operating) time should be logged, "movement" on ground or in the air is not sufficient

 

 

[facthunter]

 Engines wear most at the early start cycles and least in cruise which us usually below 75% power. High boost high rpms are also life reducing . Engines run to Mil specs/limits have reduced TBO's compared to the same engines in civil service. Often take off power is time limited as the engine never stabilises properly as it's used as a heat sink.  METO is maximum except take off. and still high load above normal climb. Engines used on long flights wear much less than short sector piston engined operations.

 

  Several ways are used to measure times. Actual start to stop  Times or RPM related airswitch etc Depending on which method you use the TBO figures are adjusted up or  down to allow for the obvious disparities. You MUST stick to a stipulated method that you have nominated. Airframes get worked on the ground as well as flight loads. Braking, turning,  Floor loads, tyres with flat spots from sitting for long periods and landing shocks to the Gear heavy braking and pressure cycles for  pressurised aircraft structure. Number of landings is recorded with RAAus and many other aircraft, in fact I would say ALL aircraft.

 

  Idling some engines too slow is not good for them and many pistons get somewhat flooded during starting. jet Engines get their most severe thermal shocks when starting. Not everything is as straightforward as it might seem.  Liquid cooled engines had more strife overheating on the ground than the aircooled ones. Merlins were quite restricted on ground time in the tropics. Nev

 

 

[old man emu]

You MUST stick to a stipulated method that you have nominated.

 

There's no arguing with that statement. 

 

The argument is "Which method do you stipulate?"

 

 

[Thruster88]

There's no arguing with that statement. 

 

The argument is "Which method do you stipulate?"

 

The one that floats your boat

 

 

[spacesailor]

Airframe LOG.

 

Engine LOG

 

Propellor LOG

 

Pilot LOG.

 

Fuel LOG. (for the missus ).

 

spacesailor

 

 

[facthunter]

 When a choice is available, YOU make the choice. if you wish to. You can't expect to fudge the system in an obvious way as "everyone would be doing it". Perhaps now, THEY do "stipulate" more often, as many engine makers do/have done in the past, and some type of consistency has to happen or record keeping has little to recommend it. Cowboys have often done/flown hours not logged and that might save money in the short term, but well documented and appropriately serviced aircraft command better prices on a world market and AD compliant aircraft are probably the only legally airworthy aircraft. Nev

 

 

[djpacro]

"You MUST stick to a stipulated method that you have nominated."

 

There's no arguing with that statement. 

 

The argument is "Which method do you stipulate?"

 

When I bought the airplane it was using tacho so I continued with that for a short while until I bought a flight switch. The flight switch was U/S for a short while so I used time on my wristwatch from take-off to landing.

 

 

[Bruce Tuncks]

I reckon Mike Busch is right and that time in service is a terrible indicator of when an engine needs service. There is no comparison between a carefully-operated engine and one that is often operated near its limits . The more surprising thing, at least to me, is how there are many more engine components which do NOT exhibit the " increased risk of failure with usage" which is the unsaid but false assumption behind all this TTIS stuff. Many components become more reliable with age.

 

Personally, I reckon that changing the oil frequently and avoiding disuse corrosion are big and worthwhile things. The rest of the stuff is mainly  to keep you out of trouble and to deny the insurance company one more detail which they could use to avoid a payout.

 

( Disclaimer: There are some other worthwhile maintenance jobs,  for example control-surface free play, which need doing regardless of TTIS.  The daily pre-flight inspection is what counts most. )

 

 

[facthunter]

Never heard of components that get "more reliable with age." Some new engines need bedding in but an aero engine has to cop full chat on the first take off.  All ball races have a design life and all things exhibit metal fatigue with use. An aero engine has to be built as light as possible so the concept of design life is" built" into it. You don't want old fatigued components in any part of an aeroplane  Seals and gaskets harden and degrade. NOTHING lasts forever. Some metals even age harden and get really brittle. A manufacturer is not going to put his company at risk by making over the top claims when safety and lives are at stake if they are reputable.. Your old car can potter around town for 40 years but it's not the same thing with an aeroplane. A rusty valve spring can kill you.  Nev

 

 

[Bruce Tuncks]

I don't really disagree with you Nev, and I don't want to attribute things to Mike Busch that he wouldn't like to own.

 

But if you read his stuff on " reliability centered maintenance" you will find many comments about how most failures are due to reasons other than the age of the component. In fact there are many components where " infant mortality" failures are  the most common mode.

 

This is not to say that he advocates sloppy maintenance. He is very big on things like borescope examination and oil analysis trends. ( He is disparaging about leakdown tests, somewhat to my surprise )

 

To take that rusty valve-spring example, that hypothetical engine had been neglected such that internal corrosion was taking place.  The corrosion was the big thing, and not the TTIS. The corrosion should have been prevented from happening in the first place.

 

As a philosophy, it is hard to disagree with the idea that we should be scientific and be swayed more by hard evidence than by our intuition.

 

The account of how Waddington overcame the "experts" to achieve great things in WW2 is also worth reading. Google up " Waddington ww2 aircraft maintenance ".

 

 

[skippydiesel]

I reckon Mike Busch is right and that time in service is a terrible indicator of when an engine needs service. There is no comparison between a carefully-operated engine and one that is often operated near its limits . The more surprising thing, at least to me, is how there are many more engine components which do NOT exhibit the " increased risk of failure with usage" which is the unsaid but false assumption behind all this TTIS stuff. Many components become more reliable with age.

 

Personally, I reckon that changing the oil frequently and avoiding disuse corrosion are big and worthwhile things. The rest of the stuff is mainly  to keep you out of trouble and to deny the insurance company one more detail which they could use to avoid a payout.

 

( Disclaimer: There are some other worthwhile maintenance jobs,  for example control-surface free play, which need doing regardless of TTIS.  The daily pre-flight inspection is what counts most. )

 

Terrible ? No ! - Not great !  True ! but like so many aspects of engineering the question is what cost effective system would you (or anyone) replace it with ?

 

The system we have, assumes that you the operator run the engine/aircraft somewhere between sub optimal and optimal - its an arbitrary line (a bit like making kids legally adult at 18). Does not suit every aircraft/situation but does capture the majority.

 

Like engineers setting a service/replacement interval, insurance companies must make a decision/draw a line that suits most (not all) eventualities.

 

I replace my "rubbers" as per the Rotax 912 manual every 5 years (dont do enough hours to go by time in service). I like to compare the condition of the rubbers old - new. The old are only slightly stiffer than new, still look great inside & out  (possibly because I try to purchase top quality hoses etc - ) they could definitely go for quite a lot longer (I actually reuse them on land bound equipment) - but how much longer is not something I can calculate/assess, so I stick with the conservative Rotax advice.

 

It is interesting that you challenge  "time in service as an indicator of service need" but then go on to recommend frequent oil changes (I do not disagree) seems like you are a little conflicted.

 

 

[Bruce Tuncks]

Skippy, I have no argument about doing effective maintenance on your plane. With regards to oil changes, I really don't know enough chemistry to have an expert opinion, but my amateur knowledge on the subject goes like this:

 

By-products of combustion are water and acids. ( remembering high-school chemistry, hydrocarbon combustion yields water, plus organic oxides.  Now the oxide of a non-metal in aqueous solution is acidic). Now an internal combustion engine is "washed" internally by the oil. Good oils contain additives which are used up counteracting  these combustion products.

 

Some other additives are there to keep the oil film there during periods of disuse, to the extent that we are advised to change the oil BEFORE a lay-up and not after.

 

Remember the Cam-guard story? Adding this is quite expensive, and we have been told on this site that the additive is probably already in W100. So not adding cam-guard and reducing the oil-change interval to make the same dollars per year seems to me ( in my ignorance ) to be a prudent thing to do.

 

If you read the Waddington story, you will see that he did not ban all maintenance, only the stuff which could not be shown to have any positive reliability outcome. I bet that oil-changes were not banned.

 

 

[Bruce Tuncks]

On the subject of Rotax rubbers, I think this is a mandated thing and you have no choice about doing it.

 

Well you could choose to fly illegally, but I for one would not recommend that . But it would hurt to pay   exorbitant money to replace sound rubber hoses  years before they needed it.

 

How long do those discarded hoses last on the ground vehicles? Many years is my guess.

 

What if the risk of a hose replacement job exceeded the risk of running with an older but still sound hose? On my cars, I squeeze the hose and look for micro-cracking of the rubber surface  in the area where it is under tension from the squeeze deformation.  This is an " on condition" test.

 

There was a Jabiru which force-landed on the Stuart Highway. The owner had conscientiously replaced the hoses before his big trip. The new hose slowly vibrated up into a tapered zone and got shut off. That guy illustrated how reliability can be LESSENED by too much maintenance. The old hose would have been fine.