Net flight path and obstacle clearance
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Net flight path and obstacle clearance
Friendly but heated discussion between self and old Air Force colleague needs resolution. Beers rest on this. Subject was the 0.8% factor applied to the one engine inop gross gradient of 2.4% at instant of gear retraction in a twin engine turbine type (737 for example).
Colleague states that the 0.8% degradation has nothing to do with obstacle clearance requirements, but is purely a figure that allows for less than optimum operation (worn aircraft and pilot etc)
I maintain that it has everything to do with obstacle clearance otherwise why have it in the first place?
My understanding is that when calculating obstacle clearance the net flight path (2.4% less 0.8%) which is 1.6%, is used to ensure all obstacles are cleared by at least 35 feet straight ahead or 50 ft in a curving take off.
In addition for the 737 the net gradient is further reduced by another 0.60% for more than 15 degrees angle of bank for the duration of the turn. Theoretically therefore the final net gradient could be just one percent during the duration of a curved take off (we are talking one engine inop of course).
Also if the screen height is reduced to 20 feet with a reduced V1 applied to wet runway, from where does the net flight path begin for obstacle clearance purposes - because normally the calculations start from the end of the first segment don't they (35 feet)
Colleague states that the 0.8% degradation has nothing to do with obstacle clearance requirements, but is purely a figure that allows for less than optimum operation (worn aircraft and pilot etc)
I maintain that it has everything to do with obstacle clearance otherwise why have it in the first place?
My understanding is that when calculating obstacle clearance the net flight path (2.4% less 0.8%) which is 1.6%, is used to ensure all obstacles are cleared by at least 35 feet straight ahead or 50 ft in a curving take off.
In addition for the 737 the net gradient is further reduced by another 0.60% for more than 15 degrees angle of bank for the duration of the turn. Theoretically therefore the final net gradient could be just one percent during the duration of a curved take off (we are talking one engine inop of course).
Also if the screen height is reduced to 20 feet with a reduced V1 applied to wet runway, from where does the net flight path begin for obstacle clearance purposes - because normally the calculations start from the end of the first segment don't they (35 feet)
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John,
If that's Milt, I'll take up the argument with him in Canberra next week ....
(a) gross is determined from flight test degraded to fleet average engine performance .. I would have to dig into the archives to get the delta used ... just can't recall it now.. a long time since I looked at that side of the table ..
(b) gross to net is to "cover" all the odd bits and pieces .. pilot variation, etc., which can be expected to compromise the gross assumption .. but beware turbulence ... although I have no idea whence he got it Roger G at Ansett used to quote a notional 1 in a quarter million probability of achieving net.
(c) gross to net delta applies throughout the takeoff from the end of the first segment .. with a corresponding reduction in acceleration capability for the third segment
(d) gross to net not specifically related to obstacle clearance but the net capability in the AFM is used in constructing the NFP profile which is then overlaid on the obstacle profile to juggle RTOW
(e) note that the certification 2.4/1.6 is a minimum WAT limit for maximum weight departures and may need to be increased to achieve whatever obstacle clearance is necessary for a given departure analysis.
(f) while the calculated clearance is of great interest in the early departure, by the time we get to the fourth segment, the gross to net delta has fed in a big margin of fat .. depending on what the aircraft is actually achieving.
(g) the delta for any given bank angle can be calculated simply but needs some knowledge of the aircraft drag characteristics to get a reasonable answer. 0.6, or thereabouts, appears in a number of AFMs. Just to convince myself that I don't have Alzheimers the DC9-33 manual quoted 0.57 ( .. or was that the price of a loaf of bread ... just can't quite remember ..)
(h) the obstacle clearance calcs start from the end of the TODA, while the gross to net delta starts from the end of the first segment. Although not usual to see, there is no reason why the first segment can't extend beyond TODA .. in which case, the first bit of the obstacle clearance calcs are based on the first segment capability. However, the first segment performance normally is so abysmal that it is often a toss-up whether one is better to pull the weight back to bring the first segment into the TODA.
... now waiting while Mutt and Old Smokey find the deliberate errors above ..
If that's Milt, I'll take up the argument with him in Canberra next week ....
(a) gross is determined from flight test degraded to fleet average engine performance .. I would have to dig into the archives to get the delta used ... just can't recall it now.. a long time since I looked at that side of the table ..
(b) gross to net is to "cover" all the odd bits and pieces .. pilot variation, etc., which can be expected to compromise the gross assumption .. but beware turbulence ... although I have no idea whence he got it Roger G at Ansett used to quote a notional 1 in a quarter million probability of achieving net.
(c) gross to net delta applies throughout the takeoff from the end of the first segment .. with a corresponding reduction in acceleration capability for the third segment
(d) gross to net not specifically related to obstacle clearance but the net capability in the AFM is used in constructing the NFP profile which is then overlaid on the obstacle profile to juggle RTOW
(e) note that the certification 2.4/1.6 is a minimum WAT limit for maximum weight departures and may need to be increased to achieve whatever obstacle clearance is necessary for a given departure analysis.
(f) while the calculated clearance is of great interest in the early departure, by the time we get to the fourth segment, the gross to net delta has fed in a big margin of fat .. depending on what the aircraft is actually achieving.
(g) the delta for any given bank angle can be calculated simply but needs some knowledge of the aircraft drag characteristics to get a reasonable answer. 0.6, or thereabouts, appears in a number of AFMs. Just to convince myself that I don't have Alzheimers the DC9-33 manual quoted 0.57 ( .. or was that the price of a loaf of bread ... just can't quite remember ..)
(h) the obstacle clearance calcs start from the end of the TODA, while the gross to net delta starts from the end of the first segment. Although not usual to see, there is no reason why the first segment can't extend beyond TODA .. in which case, the first bit of the obstacle clearance calcs are based on the first segment capability. However, the first segment performance normally is so abysmal that it is often a toss-up whether one is better to pull the weight back to bring the first segment into the TODA.
... now waiting while Mutt and Old Smokey find the deliberate errors above ..
Last edited by john_tullamarine; 31st Oct 2004 at 22:25.
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Thanks JT. But isn't the sole reason for laying on the 0.8% decrement is to run a worst case scenario for obstacle clearance planning? Sure, accelerate- stop performance and landing performance takes into account worn brakes, delayed crew actions and so on, but I cannot see what other event apart from obstacle clearance planning, the climb gradient reduction of 0.8% is meant to cover.
Moderator
You could well be 100% correct.
A definitive answer would require access to some historical data on the reasons for the introduction of the gross to net decrement .. unfortunately, this I don't have and can never recall having seen .... probably could hunt it down in the historical FARs, either explicity or by implication, but that would take more spare time than I have to spare...
Looking at the current FARs, for instance,
(a) 25.115 (as an airworthiness design standand) prescribes the decrement but makes no reference to obstacle clearance.
(b) 121.189 (as an operational standard) relates the NFP to obstacle clearance.
Perhaps your colleague and you are both correct but looking at the problem with different sets of spectacles ...
A definitive answer would require access to some historical data on the reasons for the introduction of the gross to net decrement .. unfortunately, this I don't have and can never recall having seen .... probably could hunt it down in the historical FARs, either explicity or by implication, but that would take more spare time than I have to spare...
Looking at the current FARs, for instance,
(a) 25.115 (as an airworthiness design standand) prescribes the decrement but makes no reference to obstacle clearance.
(b) 121.189 (as an operational standard) relates the NFP to obstacle clearance.
Perhaps your colleague and you are both correct but looking at the problem with different sets of spectacles ...
