Joined: 11 Dec 2005
|Posted: Thu Mar 22, 2012 9:26 am Post subject: Basic Concepts
| The performance requirements are in CS 25 and CS 23
In 2003 the European Aviation Safety Agency (EASA) assumed responsibility for regulating airworthiness within the EU member states from the JAA and many JAR (Joint Airworthiness Requirements) documents have been transposed into EASA documents. In performance (after year 2003) JAR 25 and JAR 23 have been re-titled Certification Specification (CS) 25 and CS 23. The requirements are identical in CS and JAR.
The pre-production aircraft are extensively tested to make sure they meet the certification standards. Apart from certification standards, they must also be used to supply the data for the performance section of the flight manual (performance manual). Subsequent aircraft that come off the production line only require limited testing. Operating Regulations (EU OPS) also ensure that the aircraft is safe to operate in the public transport role.
Measured Performance: The data produced from pre-production aircraft is known as "measured performance". This is invariably better than the fleet average. The measured data must be factored to convert it into data which is more representative of an average aeroplane in service.
Gross Performance: The average performance that a fleet of aeroplanes should achieve if satisfactorily maintained and flown in accordance with the techniques described in the manual, is the fleet average performance, called "gross performance". Regarding aircraft performance in a fleet, some achieve a better than average, some worse but most cluster around the average - the gross performance.
Net Performance: The gross performance does not provide an adequate safety margin for public transport operations as half of the fleet will not attain gross performance standards. Once an adequate safety margin is provided, we get the net performance. Net performance is the gross performance diminished to allow for various contingencies that cannot be accounted for operationally e.g., variations in piloting technique, temporary below average performance, etc. It is improbable that the net performance will not be achieved in operation, provided the aeroplane is flown in accordance with the recommended techniques.
Higher the safety margin - Higher the cost of air transport. To bring down the cost you have to compromise on safety. The assessment of acceptable risk involves judgements about the cost of safety measures balanced against the likely benefits. Risk level considered acceptable in this case is that there should be roughly a one in a million chance of a system failure, followed by a failure to achieve the required performance standard.
Likely events have a big safety margin, unlikely events a small one.
If an event is relatively unlikely, for instance an engine failure at an exact point in the take-off run, then the difference between gross and net performance will be very small. Conversely if an event is highly likely, a Climb conducted without an engine failure, there will be a larger margin between gross and net performance.
If an event was unlikely then the improbability of the event is used as part of the safety factor e.g. If the risk of an engine failure is assessed as 1 in 100,000 then the only extra margin needed to make the total safety factor of 1 in a million is another 1/10th (1/100,000 x 1/10 = 1/1000,000).
If an event is so unlikely that the probability of it happening is assessed as being already less than one in a million then the safety margin between net and gross reduces to zero e.g. possibility of a double engine failure in the cruise on a three or four engine aircraft. The chances of an unrelated double engine failure are considered so low that, if it did happen, there would only be a 50:50 chance of stabilising with the required terrain clearance.
Net performance standards keeps the risk of accident to an acceptable and very low level, but does not reduce it to zero, even if the correct techniques have been followed.
1) Class A
All multi-engine jets and turboprops with more than nine passenger seats or a maximum take-off mass greater than 5700 kg e.g. B737 would be certificated under CS 25 or equivalent and operated in performance Class A.
2) Class B
Small propeller driven aircraft (piston or turboprop) with nine passenger seats or less and a maximum take-off mass of 5700 kg or less.
Propeller driven twin engine aeroplanes in the commuter category with nineteen passenger seats or less or a maximum take-off mass of 8618 kgs.
The Class includes both singles and twins and would apply to aircraft certificated under CS 23 or equivalent.
CS 23 describes four sub-categories of normal, utility, acrobatic and commuter.
Apart from the commuter category (which has rules very similar to Class A), this Class does not assume an engine failure until you enter cloud.
3) Class C
Large piston aircraft with more than 9 seats or those that have a maximum take-off mass of more than 5700 kg. Performance Class C is certified under CS 23.
4) Unclassified Class
For aircraft that cannot comply with the Class A, B or C requirements e.g. seaplanes, concorde.
Operational mass limits for CS 25 aircraft are determined from:
• Take-off field lengths
• Climb limits
• Tyre speed limits
• Brake energy limits
• Net take-off flight path-obstacle limits
• En-route requirements
• Landing climb limit
• Landing field length limit for both destination and alternate