DECU's, DDR's & Power Assurance!!
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
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
Last edited by John Eacott; 19th Sep 2002 at 11:21.
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.
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.
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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. 
Cheers
Randy_G
1013.2 on my altimeter ?? Our scale only goes to the mid 30's. Cheers
Randy_G
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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?
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?
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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!!
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!!
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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.
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.
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?
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, 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.
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.
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.
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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.
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.
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CDN's went metric ??
When did that happen ?? Okay the country went metric, but aviation is still run on 'merican. 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
Randy_G
Okay the country went metric, but aviation is still run on 'merican. 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
Randy_G
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.
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.
