777newbie,
The OPT (Onboard Performance Tool) component of the EFB is a relatively new thing. (Bloomin Marvellous!
) I've seen you on these pages for some time, so I'm assuming that you're well familiar with the "old" (but still very much in use) paper Airport Analysis RTOW charts and their associated OEISIDs. These were all created by Performance Engineers (as was the OPT) who could not incorporate the numerous variables, and often had to consider the worst case for some environmental conditions, lest they would have to produce many more volumes of the Airport Analysis. The OPT designers do have the luxury of considering everything. Here's a few examples, using the parameters given by you :
TEMPERATURE : Keeping things simple, consider a TRUE Minimum Acceleration Altitude (MAA) of 1000 feet above the runway. That would ensure a mere 35 feet clearance of the Net flight path above the Critical Obstacle. What happens if the ambient airport Temperature is -20C? (I suffered this not so long ago in Moscow). 1000 feet of INDICATED Altitude suddenly became a TRUE 878.5 feet, and to achieve a TRUE MAA to guarantee obstacle clearance, the MAA must be bumped up to 1139 Ft INDICATED Altitude. Conversely, on a hot ISA+20C day, we could allow MAA of 935 ft and retain the same safety margins.
The OPT has the luxury of manipulating the TRUE MAA to an Indicated Altitude for your cockpit use. Thus, variation in Temperature will definately affect the MAA provided by the OPT. The Performance Engineer, on the other hand, when producing paper RTOWs assumes the worst case, and provides one constant MAA. I provide one MAA for the lowest permissible environmental temperature for the aircraft (Others may do it differently). Do you want one more chart for MAA correction with temperature variation from ISA? I suspect not.
QNH CORRECTION : (Note - This will not apply if Obstacle Limited). The formula for ascertaining MAA is to take the Lowest 2nd segment Gross Gradient achievable, dividing it by the Lowest 2nd segment Net Gradient achievable (0.8% less for a twin), multiplying it by the height above the lowest point of the runway, adding 35 feet screen height, and then adding it to the highest point of the runway. Let's say that the highest obstacle in the Takeoff area was 500 feet AFL (but not limiting in the 1st/2nd segment), and your B777 weighs 280 Tonnes. You ascertain that at this weight the aircraft can achieve a 2.0% 2nd segment gradient (i.e. 2.8% Gross), thus the MAA becomes -
2.8 / 2.0 X 500 + 35 = 735 Ft above Field level.
You will be aware as a B777 operator, that performance degradation occurs at the rate of 250 Kg per hPa (Mb) below Standard Pressure (above also but you're not allowed to use it). If the QNH was 1003, i.e. 10 hPa below standard, the penalty would be 10 X 250 Kg = 2.5 Tonnes. Thus, your 280 Tonne aircraft will perform as though it were at 280 + 2.5 = 282.5 Tonnes. At this higher "effective" weight, you ascertain that the aircraft can achieve a 1.9% 2nd segment gradient (i.e. 2.7% Gross), thus the MAA becomes -
2.7 / 1.9 X 500 + 35 = 745 Ft above Field level, same ATOW, different QNH, new MAA.
I'm beginning to assume that you don't want a second QNH correction page for MAA if using 'paper' data, and would prefer one worst case MAA.
(We account for the performance aspects anyway by applying the QNH correction, but not to MAA)
WIND COMPONENT : Borrowing a few figures from the previous example, let's assume that the 500 Ft obstacle lies in the 2nd segment, and is LIMITING.
B777 'A' takes off in NIL wind and requires 3000M of Takeoff Distance to screen Height. A 2nd segment gradient of 2.0% (2.8% Gross) is required to clear the obstacle. As earlier described, MAA will be 735 Ft AFL
B777 'B' takes off in 20 Kt Headwind and only requires 2500M of Takeoff Distance to screen Height. Because aircraft 'B' is airborne 500M more distant from the obstacle, the gradient required is
LOWER (and allows a greater Takeoff Weight). A 2nd segment gradient of 1.9% (2.7% Gross) is required to clear the obstacle. As earlier described, MAA will now be 745 Ft AFL.
Thus, variation in Wind Component will definately affect the MAA provided by the OPT. It has the luxury of considering EVERYTHING!
The Performance Engineer, on the other hand, when producing paper RTOWs does make full consideration for Wind Component and the resultant OCGs required, but assumes the worst case (the lowest gradient), and provides one constant MAA. By now I'm assuming that you definately don't want a third wind correction correction page to MAA for wind component if using 'paper' data, and would prefer one worst case MAA.
TOW & TODA CHANGES : Similar effects to the "Wind Component" discussion. Weight changes affect the TODR and Distance to obstacle (and therefore gradient). TODA changes (e.g. Intersection Departure or WIP at the far end of the runway) have a similar effect upon Distance to and Gradient required to clear the obstacle.
The OPT (and other on-board performance lap-tops) are truly marvellous, until the day when the Captain's and the F/O's figures disagree for the same input (happened to me once). Then it's time to get out the well worn and trusted Airport Analysis).
WARNING - All of the numbers except for Temperature correction and the 250 Kg per hPa correction are "made up" for the example here. Don't go looking for verification in the AFM. The relationships, however, are very real!
I hope that that clears the air.
Regards,
Old Smokey