DECU's, DDR's & Power Assurance!!

[Joker's Wild]

Situation:

Company aircraft all capable of performing power assurance checks using on board equipment.

Most of the time, on board equipment works just fine resulting in a pass or a fail being displayed on said equipment.

However, when a fail is displayed, data is immediately ignored and paper charts pulled out. Data recorded long hand is then plotted on paper charts which, more often than not, result in a pass.

What I want to know is, if the on board equipment (which is deemed suitable to perform the power assurance) is working just fine and a fail is displayed, would that data not take precedence over everything else???

[400 Hertz]

This is a common problem with automatic recording of engine parameters. Manufacturers are normally only interested in the 'eyeball' figures as there are errors normally associated with the airborne data gathering.

Most data gathering systems will only record figures to 12-bit accuracy, this leads to numbers being rounded up (or down) depending on the installation. Without going into the boring stuff, if your engine is doing 29,994 RPM gas generator, the recorded number might be anywhere from 29,982 to 30,006 RPM.

I know what you are going to say now "How come they don't accept those figure when my mark one eyeball is not accurate enough to tell the difference from 29,960 to 30,000 even on a good day?"

Another problem is hysteresis of the cockpit indicators. This is taken into account on the graphs, but the onboard equipment does not. Figures taken during a transistion where the engine is accelerating can mislead the automatic algorithms and give a fail.

In the end, the engine manufacturer always wins. After all, it's their train set, and your lives.

[TomBola]

The S76C+ seems to have a problem passing the automatic power assurance when it's operated in hot climates (above 25-30 degrees). The airframe and engine manufacturers both say that the definitive PA is the manually recorded one, using the FM graphs. There are some differences between the inputs from the engines to the DDR and the cockpit gauges and this can be the difference between a fail and a pass. When we were operating them in hot conditions and having a lot of problems with PA, Sikorsky tried to blame Turbomeca and said that they were supplying the better specification engines to Eurocopter for the EC155, whilst Turbomeca claimed that it was the fault of Sikorsky because the air intake system was not properly matched to the 2S1 engines. Maybe it's a bit of both!

[FlawTolerant]

As far as the C+ goes, the information that the computers use to calculate the PA data is not necessarily the same as what is displayed to the pilot.
That is why the RFM charts are plotted using the information you actually see on the guages.
Specifically, the T5 harness has redundant dual thermocouples. Half of the probes send information to the DECU for power assurance calculations, the other half is used for display of T5 information. A variation in the resistance between the two would cause the computer calculated T5 margin to show a fail but pass when using the displayed T5.
OAT, as mentioned in the previous post, gets carried out 4 decimal places for the computer but gets rounded for display on the panel.
There are a number of other parameters that can affect the computed PA data but what's important and takes precedence is the data the pilot is using to fly the aircraft, hence, paper over 'puter!

[Joker's Wild]

It does make for an interesting debate.

But if paper data can be assumed to take priority over certified on-board equipment, why doesn't it specifically say that in the RFM?

One could argue the manufacturer might be exposed on a liability angle because a valid fail result on the DDR was ignored in favor of a hand plotted power assurance.

It just seems to me to be fundamentally incorrect ignoring what are deemed to be valid numbers.

[chuckolamofola]

This is a common problem with automatic recording of engine parameters. Manufacturers are normally only interested in the 'eyeball' figures as there are errors normally associated with the airborne data gathering.

I don't know what system you are referring to, but most systems are very accurate. In fact the eyeball errors are just what the manufacutrers want to get rid of.


Most data gathering systems will only record figures to 12-bit accuracy, this leads to numbers being rounded up (or down) depending on the installation. Without going into the boring stuff, if your engine is doing 29,994 RPM gas generator, the recorded number might be anywhere from 29,982 to 30,006 RPM.

I think most if not all monitoring systems use Hertz as their units, then 29994 RPM would be read as 499.9 Hz and 30,006 RPM would be read as 500.1 Hz, their accuracy would be within the 12-bit accuracy you state.

I know what you are going to say now "How come they don't accept those figure when my mark one eyeball is not accurate enough to tell the difference from 29,960 to 30,000 even on a good day?"

I am not sure what you mean here.

Another problem is hysteresis of the cockpit indicators. This is taken into account on the graphs, but the onboard equipment does not. Figures taken during a transistion where the engine is accelerating can mislead the automatic algorithms and give a fail.

The algorithms should be using steady state and probably use averaging anyway. You are right about the hysteresis of the cockpit indicator. I have seen many times where gage accuracy has caused a failed power assurance indication due to Ng speed. A proper check of the gage showed it reading higher than it should have by a couple of percent. Many aircraft gages have a 2% tolerance because of hysteresis. This is where I agree that you can't set the limit on an onboard system to have a tighter tolerance than the gage as the gage is all the pilot can fly it to. However, if the onboard system differs than what the pilot reads from his indicators then a check should be made of both systems using a calibrated input to determine which system is in error greater than allowed.

Regards

[PVL-70]

Lets stay on track!
The C+ PA check in our company is performed on the ground, not in the Air.

If the DDR shows a PASS most people accept this and write it up in the Tech log Period !!
However is the DDR shows a FAIL then why do most people / companies choose to do it again and again or plot it on the graph in the RFM???

Should we not be using to DDR to record a Pass or FAIL if the DDR is serviceable in the first place!
:mad:

[400 Hertz]

Ah, if it was only that simple.

If you dig DEEPLY into the way of SOME installations, you will find the answers you are looking for.

It would be a waste to have the full 12 bit recorded accuracy used over the whole range of the ind, leading to less RECORDED accuracy at lower RPM's etc.

Of course the S76C+ uses ARINC 429 data and these figures are better than 12 bit, but still have errors when converted to 12 bit for the FDR, and IF you use FDR data for PA calc, that would be a problem.

Another problem is that EuroCAE ED55 recording params spec states the max reading to be recorded, for instance this is not the Max N1 of the eng e.g. 101.2% N1, it will be a lot higher like 120% etc, making the scale even less accuratley recorded.

Well, this thread has left the tracks and gone off into the deep and dark gloomy world of sadness...

[Joker's Wild]

Yes, once again a thread has completely gotten off track into the world of something that was not asked.

Let's say it this way. Company A has a general, albeit unwritten policy, of happily recording DDR data if it's a pass but discounting DDR data if it's a fail in favor of hand plotting long hand data on the chart from the RFM.

This goes on for a long time until the day something bad happens. Aircraft departs, one engine kacks just after CDP and the crew tries in vain to fly it away on the remaining engine. However, this engine has not been doing too well on the old power assurance checks and routinely failing according to DDR data. But, as chart data has been used to calculate results, a pass has constantly been achieved, thereby allowing that engine to remain in service.

Now here's where it gets sticky. Murphy being what he is, the crew tries in vain to fly away but find they just cannot get the damn thing to climb (remaining engine is just not up to the task).

After the aircraft is recovered and the inquiry begins, some clever aviation litigator keys on the unwritten policy of Company A and asks the million dollar question, "Uh sir, can you please show me where it states the procedure you have been using is legal?"

We can talk about the inner workings of the system and why they do what they do until we're blue in the face, but truth is it won't matter a flick after you've bend the bird.

Is the PIC going to cop it yet again, or is blame going to be laid at a higher level within Company A for having allowed what is essentially a non-documented procedure for recording and applying power assurance data?

At some point, there has to be one and only one way of recording and applying this type of data. If not, why even bother with power assurance in the first place.

[PVL-70]

YES now were back on track again!!
Is their an answer for this question
If the C+ has the equipment should we not follow / belive what it tells us!
:)

[chuckolamofola]

YES now were back on track again!!
Is their an answer for this question
If the C+ has the equipment should we not follow / belive what it tells us!


Does the flight manual or maintenance manual state that if a discrepency occurs then resort to the flight manual charts or are the charts there for aircraft without the onboard system? IMHO, If not and you read the charts and decide to dispatch by using the charts then you have taken all the responsibility as it is all based on your perceptions. If something happens and you are dispatching after repeated power assurance failures of the onboard system you will be deemed at fault.

The proper thing to do is to determine why a difference occurs between the onboard system and the manual. What if the gages are off just enough to show you to pass but the onboard system says it fails? It is still failing power assurance.

At another company I worked for years ago, we would fail power assurance by Ng speed so they would swap indicators side to side or get another indicator that would read good enough to pass the check. All they were doing was getting a gage that read lower than the others. Years later when I starting working with onboard systems and measuring the actual tach frequency to those gages on that aircraft model, I found out that they are quite often in error and the onboard system would indicate one reading and the gage another. First response by everyone was that the onboard sytem was in error, but in fact it was getting the same signal input as the gage and then when using a third source to read the signal it proved the onboard system to read correctly and the gage was in fact in error.

C-Y-A

[400 Hertz]

Many cockpit indicators used for PA calc are on-condition items, and not calibrated at all. Quite funny when you see the ITT/T4/T5 inds calibrated regularly and the Ng/N1 ind is left to its own devices for years. And don't forget that OAT ind as well.

One aircraft automatic PA plotting system that I know of, uses an approximation of the actual graphs, and already has errors built-in. Another reason for using the eyeball/graph job, and another reason that the results from the auto calc are 'advisable'.

[Arm out the window]

Electronic monitoring equipment is all well and good, but there's nothing like the thickness of a grease pencil line on a laminated chart to bring out of limits back into limits again!

[PVL-70]

400 hertz I think your battery is LOW.
This is about the C+ PA Check, is it not!!
:o

[GLSNightPilot]

OAT gauges give only an approximation of the actual temperature. After a question about a power assurance check being close to limits once, & discussing it with maint, we went out & checked the temps on several ships out on the ramp. We couldn't find 2 gauges that gave identical readings - some high, some low, none agreed exactly with what the weather observer got with his mercury thermometer, when converted to Celsius.

[chuckolamofola]

400 Hz

If you have a engine failing power assurance the proper method is to check the accuracy of the indication whether it has a calibration requirement or not. Failure to check for instrumentation errors can lead to an early removal of a marginal engine.

GLSNightPilot

The OAT gage at the aircraft can be expected to be different than the wet bulb temperature used at the weather observers station. The are two different measurements with the most difference is that the weather observers are done in the shade and not out on a hot concrete ramp. The other is they don't use a dry air temp but use a wet bulb thermomenter.

The OAT gage should read within the allowable tolerance at the aircraft when compared to a calibrated source held next to it. This also applies to the temperature measurement of the onboard system.

PVL-70

While I am not directly familiar with the C+PA Check here is my opinion, don't forget, you get what you paid for :-)

The onboard system is certified to perform this operation, unless the flight manual specifically states that you can perform a manual check in lieu of using the onboard system then you can not use the manual check. It could also be said that by not using the onboard systems data that you are in fact stating that it is inoperable and therefore are flying around with inoperable equipment. Do you then make a log entry and does the MEL allow this to be inoperable and deferrable?

You are asking for a silver bullett answer. There is not one unless there is a specifc procedure to perform in case this situation arises. IMHO if something happens and you have an incident that is related to not making power then the burden of proof is up to you and you have one strike against you when the data is reviewed that the onboard system is showing a fail.

I don't think any agency will back you up for using the system when it only tells you good news. If you deviate from using that system then you should document why.

[FlawTolerant]

From the C+ Maint. Manual:

Power Assurance Input Signals.
Outside Air Temperature (OAT), displayed as 1 OAT and 2 OAT on IIDS display units, is referred to as T0 in the following description. The T0 readings displayed on cockpit instrumentation are derived from a dual element probe located under the aircraft nose.
Indications from element No. 1 are displayed on the copilot IIDS display unit and supply the No. 1 DECU. Indications from element No. 2 are displayed on the pilot IIDS display unit and supply the No. 2 DECU. Since the IIDS display unit displays only the whole number with any decimal rounded up or down and the DECU receives 4 decimal places, there can be as much as a 0.5°C difference between the indicated rounded-off temperature and the exact temperature used in determining automatic target torque. This would cause a 0.5% difference between power margins obtained using automatic and manual target torque look-up. If manual target torque is being established and copilot T0 display has been rounded up 0.5°C while pilot T0 display has been rounded down 0.5°C, there can be a 1.0% power margin difference depending on which data has been entered.
Pressure Altitude (Hp), displayed as 1 PO/FEET and 2 PO/FEET on power assurance and performance pages on IIDS display units, is referred to as P0 in the following description. The P0 signal is derived from the static reference static pressure port located in the tailcone as part of the RDAU. The same signal feeds both the IIDS displays units and the DECUs. The only difference here would
be as a result of the rounding off to a whole number on the IIDS display units. The P0 readings can be verified by using the N1 test switch. This is because a comparison is made between the primary biased N1 signal from the DECU using T1 and DECU P0 and the backup biased N1 signal displayed by the IIDS using cockpit T0 and P0. The maximum allowable tolerance of 6 0.2% ensures that the T1/T0 and the DECU P0 and cockpit P0 readings are close together.
The T5 signal displayed on the IIDS display units is conformed T5 and a secondary source of T5 information, separate from the source used by the DECU. The T5 information displayed on the IIDS display units is derived by using the direct chromel/alumel signal to the RDAU from the second T5 harness in the engine and incorporating the conformation resistor values transmitted over the ARINC data bus to the IIDS. The DECU uses the primary source of T5 information from the other T5 chromel/alumel harness and also calculated conformed T5, but is completely separate from the IIDS display signal. The two values of T5 should be fairly close but may be a few degrees different
due to thermocouple wire tolerances,. To avoid any power assurance differences between automated DECU power assurance and manual reading/checks done by the pilot from his indication, the IIDS calculates the final automated T5 power assurance margin by using the pilot indication of T5 and then subtracting the DECU value of specification T5. This way there will be no difference in automated and manual T5 margin as the same source of indicated T5 is used. Roundoff error in OAT is still a factor, however.

[PVL-70]

chuckolamofola

Thanks for comments,
It seems that the C+ RFM needs more to be specific in this area.:)

[GLSNightPilot]

The wet bulb thermometer is used for relative humidity/dew point information. The dry bulb is used for temperature. It's normally in a little ventilated box, but we took it out on the ramp that time. I don't know the tolerances for aircraft OAT gauges, so they may have all been within tolerance, but they were all slightly different. They are also highly susceptible to parallax error, so the indication depends on the angle of the eye to the degree markings, which are rather close together, & only indicate even degrees.

My only point is that OAT gauges aren't precision instruments. I'd much prefer to trust a digital gauge.

[400 Hertz]

All these years in aviation, and only NOW somebody tells me to cal check the gauges when an engine is marginal.

I wish I'd known that a lot earlier, it would have saved our company millions.;)

[John Eacott]

400,

Not always. We had an early 76A which had an engine failing Power Assurance, but a calibration check gave us an almost identical temperature overread (about 60C, IIRC) on each engine. Same calibration error with our other S76, so we were instructed by our black hand gang to fly to a higher T5 than indicated on all four engines.

Needless to say, after a month or so of bickering, the test equipment was found to be faulty, and the duff donk was removed post haste. The implications of constant overtemping was thankfully not an issue, since we ignored the engineer's instructions just this once ;)

[[email protected]]

What is wrong with dialling up 1013.2 on the altimeter and reading the PA straight off the most accurate instrument, then read off the oat gauge (note spelling) and apply 120' per degree difference from ICAO to give you a very accurate DA. It rather sounds like things have got too complicated in the civvy world, probably to help operators squeeze the last hour of flying out of a shagged engine.
The wet bulb temperature is not a dew point indicator - it shows the lowest temperature that the air can be cooled to by evaporation, which is not the same thing.

[Randy_g]

crab methinks that PA in this case stands for Power Assurance, and not Pressure Altitude. :) 1013.2 on my altimeter ?? Our scale only goes to the mid 30's. :D :p

Cheers

http://randyspics.tripod.ca/gifs/naughty.gif Randy_G

http://randyspics.tripod.ca/gifs/man_grilling_hamburgers_sm_wht.gif

[FlawTolerant]

If the description from the Maint Manual is this confusing for the pilots to understand, for the mechanics who don't deal with this type of information as part of their normal routine it must be downright jibberish.
Hp (or Po) is Pressure Altitude
Pa is Power Assurance
To (or OAT) is Outside Air Temp measured from a probe under the nose.
T1 is Compressor inlet temperature measured from a probe in the inlet plenum chamber
T5 is Sikorsky Interstage Turbine Temperature
T4.5 is TurboMeca Interstage Turbine Temperature
IIDS is Integrated Instrument Display System
RDAU is Raw/Remote Data Aquisition Unit

Did I miss any?:confused:

[400 Hertz]

The Turbomeca Makila engine uses T4 displayed in the cockpit, but this is really a 'derivative' of T3. A resistor selected during eng manufacture adjusts the cockpit ind to read whatever the manufacturer wants, according to his measurements found during testing.

P+W do a similar thing on the PT6, with their own tweek using an adjuster (NEVER used to take out gauge errors at all).

A good company has calibrated OAT inds in their aircraft, even better if they have 2 fitted. The biggest problem with the mechanical ind is the very handiness of the probe to act as a handle while somebody cleans the windscreen! When the probe is bent, the needle sticks. Most pilots I know seem to take ages to spot the stuck needle.

If we keep chatting like this, I can see pilots tut-tutting at the slightest ind problem over the next few weeks. The poor avionics guys will have to come out of the crew room.

I can hear those anorak zips going already!!

[Shawn Coyle]

There's another completely different reason for the use of paper vs. electronic stuff and it has to do with certification.
The power available charts in the FM are in the 'approved' section of the FM - checked by the certification agency and approved as being accurate.
The electronic one is not checked - this is a whole ball of wax (or is it worms) that the certification agencies don't want to get too deeply into. How good is the software in the electronic system? Is it certified to DO-178 Level whatever? How do you check to make sure it is valid in all conditions, etc.
Real problem in the fixed wing world for takeoff performance, etc. You can use electronic stuff all you want, but you still need to verify it with the old paper - until very recently, this was the case.
I understand the FAA has now allowed electronic data, but with some sort of approval process. Wheels in this area turn very slowly.

[[email protected]]

Randy, Doh!!!!! That'll teach me not to pay attention properly when I'm reading posts!
I also forgot you use inches on your side of the pond for altimeter settings instead of Mb/hpa....The Canadians went metric, why not the US?

[John Bicker]

Crab,

The Canadians went metric, why not the US?.................. or the UK?

Mind the gap or is this Brit-bashing?

[[email protected]]

John Bicker, No I'm not going there again, although it did generate some excellent posts. As for the UK, most of everything is metric here although we have held off Brussels from making us use Km instead of miles for distance for the meantime. Unlike the rest of Europe, we Brits slavishly enforce all the Eurolegislation that Brussels spouts (or should that be sprouts?). Maybe we should take a leaf out of the French or Italian book and just say we are complying whilst in fact completely ignoring the rules.

[Rob_L]

I seem to remember that if an S76A failed a PA check the usual first fix was to swap engines 1 to 2 and 2 to 1. A second test would often pass. I can only assume that airflow differences caused a difference between the engines. So does the S76C automatic PA system compensate for engine position? If not then there is a built in annomally.

A few years back I spoke to the CAA powerplant people ref an Allison C20b with a PA problem. They suggested that for accurate PA figures the engines must be heat stabilised. The result was that we checked again after a longish flight and the engine passed easily.

[Joker's Wild]

I can say with considerable certainty that there is a large difference in the results of a power assurance check accomplished in the C+ between the number 1 and number 2 engines.

You can take time of year, wind speed and temperature out of the equation and most times the number 1 engine will check out rather strong while the number two engine's numbers are decidely less.

I've experimented with nose into wind, out of wind, offset slightly; it just doesn't seem to matter. The airflow has got to be different around the number 2 intake, because I can see no other reason for the engine on that side to always come out a lot lower in PA checks than the number 1 side.

What gets me is the sheer amount of energy most people are willing to put into fudging these checks (DDR versus chart) instead of intelligently trying to figure out what's wrong in the first place.

[Flight Safety]

Shawn, thanks for the clarification. I enjoyed the book BTW, wouldn't do without a copy in my library.

[Randy_g]

When did that happen ?? :D :D Okay the country went metric, but aviation is still run on 'merican. :D Some of the airliners have metric altimeters, but I would have to guess that almost all of the civil a/c are still in inches here.

Cheers

http://randyspics.tripod.ca/gifs/naughty.gif Randy_G

http://randyspics.tripod.ca/gifs/man_grilling_hamburgers_sm_wht.gif

[Rob_L]

If you really want to see the airflow differences between the port and stbd sides of an aircraft fly with the doors off.
Years ago I flew in the back of a Scout doors off. On one side you could hardly breathe due to the force of the rotorwash added to the fwd airspeed, on the other side a hell of a lot less.
Must have the effect of "supercharging" one side on a 76.
The intake position must be an important factor. No doubt the aerodynamacists can give us an answer.