Gross vs. Net takeoff performance

[Looking_4_Answers]

I'm fairly new with these two terms so forgive me if my question seems a little basic.

It is my understanding that in the event of an engine failure at V1 an aircraft is required to meet the gross climb gradients of positive rate, then 2.4%, and finally 1.2% ( if twin engine), and 0.5%, 3.0%, and finally 1.7% (if 4 engine). The Net flight path would then be 0.8% less (2 engine) or 1.0% less (4 engine). The net flight path gives you 35' clearance within 300' horiz either side of center. Hear is where I get a little lost.

If an aircraft is required to meet the Gross climb gradients One engine inop. then when would the Net climb gradient ever be used?

If net climb gradient is 0.8% less than Gross and the engine failure happened just after V1 it would seem the Aircraft would never make it through the first segment. As all that is required is positive rate.

I've read another thread on take-off performance but it didn't seem to clear this up for me. Any help would be appreciated.

[Max Angle]

I would not claim to be an expert, just a user of the figures and tables with a basic understanding but here goes anyway.

The gross climb performance is what the aircraft actually achieves at a given weight and the gross gradients are what it is required to achieve for certification and will have been demonstrated by test flying. Never done any but I guess that the aircraft is gradually loaded up to a weight during testing at which it just meets the minimum legal gradients with an engine out and this then becomes the base line maximum weight for that segment. I am sure there is much more to it than that but that’s the basic procedure. The gross performance is then degraded on paper by a certain amount to achieve the net performance figure and it is this performance data that is used to calculate the max. operating weights etc. This ensures that the even the worst aircraft on a fleet (old, dirty, perhaps not rigged quite right etc.) is statistically very unlikely not to meet the minimum GROSS performance that is required to comply with the regulations.

So in fact you never use gross performance, all the tables and figures are calculated on net performance. Not quite sure what you a driving at in your second question, perhaps a real expert like Alex W. (purveyor of fine performance courses) could help.

[Looking_4_Answers]

I think I understand what you are saying. So the Gross flight path is the required minimum flight path of the aircraft after an engine failure, and the Net flight path is a buffer zone to allow clearance even if the aircraft is maybe not as straight as it used to be and maybe the pilot is not reacting as quick as would be ideal.

Does this sound about right?

[john_tullamarine]

Gross is what you should reasonably expect to see on the gauges (ie ASI and IVSI) if the pilot is of reasonable competence and the conditions are reasonable

Net is what you do the sums on ....

[greengage22]

I seem to remember once being told that gross is what you expect to see from an average aircraft of your type, on an average day, with reactions from an average pilot.

Net is those conditions factored down to a probability of one in ten to the sixth - which won't happen so often - but which will cover (except for the ones which end in accident) the worst case.

I'm prepared to be corrected on points of detail - but, basically, net is a statistical extrapolation of gross - and I'm sure that one in ten to the sixth comes into it somewhere.

[Old Smokey]

Looking_4_Answers,

I think that a good approach to understanding of Net Vs Gross performance is to appreciate that Net performance is used to ensure obstacle clearance, and, once having satisfied obstacle clearance (with very small margins) Gross performance is used for flight planning. For example the Net 3rd segment acceleration altitude clears the obstacle by a mere 35 feet, but the acceleration altitude actually planned and used is factored by the Net Vs Gross difference, e.g. if the Net obstacle 3rd segment altitude was 300 feet, then the Minimum Acceleration Altitude (for a 2 engined aircraft at the minimum 1.6% gradient) would be

(300-35)*(1.6+0.8)/1.6+35 = 432.5 feet, i.e. 167.5 ft obstacle clearance in lieu of 35 feet.

Similarly, if discussing en-route OEI altitude capability, the Net Ceiling is used to comply with obstacle clearance, and, having done so, Gross performance at the higher altitude used for flight planning.

With regard to your 1st segment concerns, true, at first sight the requirement to merely have a positive rate of climb may seem alarming, but this is only a certification requirement. In actual runway analysis, the actual gradient for the weight and environmental conditions is computed against the known obstacles, and obstacle clearance must be ensured. In reality most twin engined aircraft achieve at least 0.8% net in the 1st segment (types WILL differ). If the 1st segment obstacles ARE limiting, then in most such cases the 1st segment limit will govern the Maximum Takeoff Weight. In extreme cases, it may be necessary to 'wear' a large Takeoff distance penalty and accomplish the 1st segment above the runway.

Regards,

Old Smokey

[Looking_4_Answers]

Thanks for the responses. That cleared it up nicely.