Category A Takeoff: Background
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JimL
The logical question to ask is, are engines actually more reliable these days than this "low standard" when the CAT A profile was thought up many moons ago?
If so the CAT A PC1 procedure could be due an update...
Engines should meet a reliability figure of 1 x 10**-5; in ICAO parlance, this qualifies them as very reliable (the reason for the low standard is that a failure, at worst, should only result in an outcome of 'Major' - i.e. 'physical distress including injuries).
Tail-rotors should meet a reliability figure of 1 x 10**-9 because a failure could result in an outcome of 'Hazardous' or 'Catastrophic' - i.e. a fatality or multiple fatalities.
Reliability targets for tail-rotors are therefore 4 orders of magnitude better than engines - i.e. 10,000.
Tail-rotors should meet a reliability figure of 1 x 10**-9 because a failure could result in an outcome of 'Hazardous' or 'Catastrophic' - i.e. a fatality or multiple fatalities.
Reliability targets for tail-rotors are therefore 4 orders of magnitude better than engines - i.e. 10,000.
If so the CAT A PC1 procedure could be due an update...
The data that has been collected over the time since 'Exposure' was first introduced to regulations (circa 1999), indicate that that reliability has improved slightly - but not by an order of magnitude. When the data was first analysed for the regulatory risk assessment, it was believed that a number of engine/helicopter combinations would not be able to achieve 1 x 10**-5. In the event, most have. That could have been as a result of control and monitoring systems such as FADEC or UMS.
It would not be appropriate to lose sight of the objective of Category A certification stated in the definition:
There is already a great deal of flexibility in the system even with the current reliability status of engines. If a State wishes to have a 'Code of Performance' where failure resulting in a 'Hazardous' or 'Catastrophic' outcome has been risk assessed (to a defined safety target) and found to be 'tolerable' to its population, and those taking part in the CAT activity, that can be done (and is ICAO compliant). However, for those who are not taking part in the activity - i.e. third parties (including property), or for a higher number of passengers (greater than 19) - the current standards of Category A are usually applicable.
The whole regulation of aviation certification and operations is based upon risk assessment (the targets are set down in regulations and guidance). Controls are only in place where failure results in an unacceptable consequence to some defined degree. Yes there should be flexibility and adjustment of the system when societal mores change or when necessity dictates - but that is already the case.
JimL
It would not be appropriate to lose sight of the objective of Category A certification stated in the definition:
Category A: With respect to helicopters, means a multi-engined helicopter designed with engine and system isolation features specified in Annex 8, Part IVB, and capable of operations using take-off and landing data scheduled under a critical engine failure concept which assures adequate designated surface area and adequate performance capability for continued safe flight or safe rejected take-off.
Category A departure and arrival procedures are deterministic - i.e. if there is a failure, safe flight can be continued. If engines ever reach a reliability figure approaching 1 x 10**-9, a rethink might be appropriate. (Under the current regulations, if OEI, HOGE is achievable, no additional procedures are required.)There is already a great deal of flexibility in the system even with the current reliability status of engines. If a State wishes to have a 'Code of Performance' where failure resulting in a 'Hazardous' or 'Catastrophic' outcome has been risk assessed (to a defined safety target) and found to be 'tolerable' to its population, and those taking part in the CAT activity, that can be done (and is ICAO compliant). However, for those who are not taking part in the activity - i.e. third parties (including property), or for a higher number of passengers (greater than 19) - the current standards of Category A are usually applicable.
The whole regulation of aviation certification and operations is based upon risk assessment (the targets are set down in regulations and guidance). Controls are only in place where failure results in an unacceptable consequence to some defined degree. Yes there should be flexibility and adjustment of the system when societal mores change or when necessity dictates - but that is already the case.
JimL
JimL,
When the analysis was being done re Engine Reliability.....was it a generic determination applied to the total universe of Helicopter Engines or was consideration given to particular makes and models.
Some engines have a better Failure Rate than others and if some consideration to that would seem to penalize the better engines by lumping them into the pot with the lesser reliable engines.
When the analysis was being done re Engine Reliability.....was it a generic determination applied to the total universe of Helicopter Engines or was consideration given to particular makes and models.
Some engines have a better Failure Rate than others and if some consideration to that would seem to penalize the better engines by lumping them into the pot with the lesser reliable engines.
All helicopter engines were analysed but the data-set was skinny; up to that time there was no requirement to collect usage data, and report failure - except when a reportable incident or accident occurred. (The reliability figure was for each engine/helicopter combination.)
No engine/helicopter combination was excluded from assessment. The system of reporting (both of usage and failure) was defined and signed-up-to by the Manufacturers (and they have been reporting reliability rates, for every engine/helicopter combination, ever since).
The safety targets for acceptance into the 'Exposure' regime were realistically set - the first for helidecks (where a problem existed) and then for elevated heliports. The safety target established for these operations was 5 x 10**-8; which permited 'Exposure' of 9 second for a twin and 18 seconds for a single . That target followed an analysis based upon profiles and data supplied by the Manufacturers.
Having set a pragmatic safety target, there were no immediate assessment failures (although the S61 came close to it later in its life, as did the S92 in its early traumas). A notable failure in the system did occur fairly recently with the S76 (the fatal accident in Myanmar) when a known bearing failure was not flagged for an AD because the regulators, unknowingly, left them out of the population for early rectification. In their view, they 'always flew in accordance with the Category A requirements' - i.e. with engine-failure accountability.
JimL
No engine/helicopter combination was excluded from assessment. The system of reporting (both of usage and failure) was defined and signed-up-to by the Manufacturers (and they have been reporting reliability rates, for every engine/helicopter combination, ever since).
The safety targets for acceptance into the 'Exposure' regime were realistically set - the first for helidecks (where a problem existed) and then for elevated heliports. The safety target established for these operations was 5 x 10**-8; which permited 'Exposure' of 9 second for a twin and 18 seconds for a single . That target followed an analysis based upon profiles and data supplied by the Manufacturers.
Having set a pragmatic safety target, there were no immediate assessment failures (although the S61 came close to it later in its life, as did the S92 in its early traumas). A notable failure in the system did occur fairly recently with the S76 (the fatal accident in Myanmar) when a known bearing failure was not flagged for an AD because the regulators, unknowingly, left them out of the population for early rectification. In their view, they 'always flew in accordance with the Category A requirements' - i.e. with engine-failure accountability.
JimL
Last edited by JimL; 6th Nov 2018 at 05:11. Reason: Correction of safety target for helideck operations.
No doubt the hardest part of the process is to develop reliable Data by which to assess the Engines/Aircraft combinations....and probably some thought as to the actual operators and locations where they fly and what kind of work they do.
SASless,
The old adage that if you can't count it you can't manage it holds true.
With respect to the first part of your post: yes, but as soon as it was flagged that usage and failure data had to be collected, the system became more and more effective.
Category A and the Performance Classes (and controlled exposure) are applicable only to Passenger Carriage - although reliability figures are dependent upon the collection of data over the whole spectrum of operations. However, the manufacturer has some discretion over the inclusion of failures where failure results from "unsuitable or non-representative use (operation or maintenance)".
JimL
The old adage that if you can't count it you can't manage it holds true.
With respect to the first part of your post: yes, but as soon as it was flagged that usage and failure data had to be collected, the system became more and more effective.
Category A and the Performance Classes (and controlled exposure) are applicable only to Passenger Carriage - although reliability figures are dependent upon the collection of data over the whole spectrum of operations. However, the manufacturer has some discretion over the inclusion of failures where failure results from "unsuitable or non-representative use (operation or maintenance)".
JimL
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SAS;
As part of current congested area operations where Class 1 can't be assured we as operators are required to provide the data as part of our risk assessments. For the 135 we can guarantee Class 1 all the time, the S76 has a problem with Battersea as the FATO is too small for S76 Class 1. Sikorsky collate the information continually and provide it to us on request, the data is based around the helicopter/engine combination and is very detailed. Certainly it means we can mitigate the risk right down to an insignificant level.
SND
As part of current congested area operations where Class 1 can't be assured we as operators are required to provide the data as part of our risk assessments. For the 135 we can guarantee Class 1 all the time, the S76 has a problem with Battersea as the FATO is too small for S76 Class 1. Sikorsky collate the information continually and provide it to us on request, the data is based around the helicopter/engine combination and is very detailed. Certainly it means we can mitigate the risk right down to an insignificant level.
SND
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Engines should meet a reliability figure of 1 x 10**-5; in ICAO parlance, this qualifies them as very reliable (the reason for the low standard is that a failure, at worst, should only result in an outcome of 'Major' - i.e. 'physical distress including injuries).
Tail-rotors should meet a reliability figure of 1 x 10**-9 because a failure could result in an outcome of 'Hazardous' or 'Catastrophic' - i.e. a fatality or multiple fatalities.
Reliability targets for tail-rotors are therefore 4 orders of magnitude better than engines - i.e. 10,000.
(A probability does not mean that a failure will occur after the reliability number has been reached, it can occur at any time but it should only occur once in the period.)
....
JimL
Tail-rotors should meet a reliability figure of 1 x 10**-9 because a failure could result in an outcome of 'Hazardous' or 'Catastrophic' - i.e. a fatality or multiple fatalities.
Reliability targets for tail-rotors are therefore 4 orders of magnitude better than engines - i.e. 10,000.
(A probability does not mean that a failure will occur after the reliability number has been reached, it can occur at any time but it should only occur once in the period.)
....
JimL
Does that change your other maths? (why not?)
How many people died while using in upwards and backwards and PC type takeoffs?
There's the Norway 135 backing up into the real world wires maybe 4 or 5 other major accidents? (S76, 2x135, 139, 169, 902 others?)
10^-9 targets? really?
Catestrophic is having your rotorhead fall off, major is having to glide to a street/park/tennis court/carpark/sea/hillside etc
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What are the other major accidents then??
Are you trolling me AnFI?
The fact that tail rotors were not meeting their expected reliability led to CAA action resulting in a CAP with recommendations to address the issue. It does not change the mathematics but when reliability does not reach expected levels, it triggers a review process.
Hughes500: Perhaps that question should be addressed to AnFI, he might have relevant experience in that field.
Jim
The fact that tail rotors were not meeting their expected reliability led to CAA action resulting in a CAP with recommendations to address the issue. It does not change the mathematics but when reliability does not reach expected levels, it triggers a review process.
Hughes500: Perhaps that question should be addressed to AnFI, he might have relevant experience in that field.
Jim
Best response ever Jim!
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Are you trolling me AnFI?
The fact that tail rotors were not meeting their expected reliability led to CAA action resulting in a CAP with recommendations to address the issue. It does not change the mathematics but when reliability does not reach expected levels, it triggers a review process.
Hughes500: Perhaps that question should be addressed to AnFI, he might have relevant experience in that field.
Jim
The fact that tail rotors were not meeting their expected reliability led to CAA action resulting in a CAP with recommendations to address the issue. It does not change the mathematics but when reliability does not reach expected levels, it triggers a review process.
Hughes500: Perhaps that question should be addressed to AnFI, he might have relevant experience in that field.
Jim
No I am definately not trolling you. I only want a valid discussion. A "well balanced debate".
Has the CAA study you refer to actually led to an improvement in tail rotor reliability of a factor of 50000 !?
How have they acheived that? Seems a little unrealistic. Especially in light of Leicester. A tail rotor is an assembly of parts each made with the target 10^-9 in mind, how many critical components are there in a TR assembly?
You can't make a TR with a reliability rate of 10^-9, these are not realistic numbers. A simple TR is a more inherently reliable assembly.
You say it does not change the maths, but if you fix the cause of 1% of fatal accidents by increasing complexity and that results in other accidents more often and of a more catestrophic nature, then the maths are changed. You can't take the upside without accounting for the downside.
You have an odd idea of catestrophic.
This is catestrophic:
so is this
and this
and this !
and this !
These on the other hand are MAJOR/inconvenient ? :
That's not to say 3rd party fatal consequences are impossible from engine failure in urban areas, there's a recent exception
in Tampa (and Sao Paulo), very unusual and unlucky, even in a gentle landing like this it can go wrong, hitting wires / poles can do that.
Any landing poorly executed can go wrong.
Wait for the report but it looks like it'll be an answer to Hughes500 question, reports suggest it was known in advance that the engine was sick.
BUT nothing like as bad as this would be in an urban area:
Is it really justified to make the world perform to the engine-centric North Sea 'special case' - can you justify the safety yeild?
How much safer does 2 engines make it? Some/none/less ? Sometimes in specific scenarios?
What are the numbers?
99% of fatal accidents are from causes other than engines.
Even the slightest increase in the frequency of the 99% completely negates any benefit.
I don't expect serious answers because I don't think there are any serious answers to that.
If there are it would be great to hear them.
Would you be happier if we just made all the manufacturers put two tail rotors on every helicopter?
On a lighter note; love the way the KC Police helo just nonchalantly slides into view.
