Take-off Minima( RVR/Visibility)

[subsidence]

hi

I`m struggling right now with the Appendix JAR-OPS 1.430, relating the take-off minima:

There is the quote:

" For multi-engined aeroplanes in the event of a critical power unit failure, there may be a need to re-land immediately and to see and avoid obstacles in the take-off aera. Such aeroplanes may be operated to the following take-off minima provided they are able to comply with the applicable obstacle clearance criteria, assuming engine failure at the height specified. The take-off minima established by an operator must be based upon the height from which the one engine inoperativee net take-off flight path can be constructed. "

The table is divided in two parts:
1.Assumed engine failure height above the take-off runway
2.RVR/Visibility


How can you know your assumed engine failure height, before departure, that doesn`t make any sense to me?

And what is the net take-off flight path?

Maybe anybody can help me,
thanks
subsidence

[Alex Whittingham]

I can't find those exact words. I have Appendix 1 to JAR OPS1.430 (a)(3)(ii) and it reads:

(ii) For multi-engined aeroplanes whose performance is such that they cannot comply with the performance conditions in sub-paragraph (a)(3)(i) above in the event of a critical power unit failure, there may be a need to re-land immediately and to see and avoid obstacles in the take-off area. Such aeroplanes may be operated to the following take-off minima provided they are able to comply with the applicable obstacle clearance criteria, assuming engine failure at the height specified. The take-off minima established by an operator must be based upon the height from which the one engine inoperative net take-off flight path can be constructed. The RVR minima used may not be lower than either of the values given in Table 1 above or Table 2 below.

Sub-para (a)(3)(i) refers to “multi-engined aeroplanes, whose performance is such that, in the event of a critical power unit failure at any point during take-off, the aeroplane can either stop or continue the take-off to a height of 1 500 ft above the aerodrome while clearing obstacles by the required margins”, in otherwise those in Performance Class A or Class B commuter category or some Class C aircraft. This means that the paragraph you quoted refers to the rest of Class B and Class C.

JAR OPS 1.535 (a)(3) says that, for Class B at least, “Failure of the critical engine occurs at the point on the all engine take-off flight path where visual reference for the purpose of avoiding obstacles is expected to be lost” which is pretty much the same as the cloudbase.

Class C is a bit more complex, it refers to big piston aircraft and there is more latitude in performance requirements because the aircraft are invariably quite old. Most Class C aircraft assume an engine failure on the runway but not all, it depends how they were originally certified by the manufacturer.

In summary, this appendix is referring to Performance Class B or C aircraft where the engine is assumed to fail at the cloudbase or, in the case of some Class C aircraft, at a fixed specifed height.

The net take-off flight path (NTOFP) is an imaginary flight path constructed using a pessimistic view of life (net performance) and assuming an engine failure at some stage in the process. If the net take-off flight path requirements are complied with the theory is that the chances of coming within 35ft of an obstacle are in the range of 1:100,000 to 1:10,000,000 and the chances of hitting an obstacle are in the range of 1:10,000,000 to 1:100,000,000, assuming the aircraft is flown as recommended. A NTOFP calculation should be made before each public transport take-off.

Why on earth are you studying this section of JAR OPS in such detail?