Old versions of PASS use the ATA cost model. The Cost subroutine is updated to permit comparison with results produced at NASA Langley. The modifications fall in three categories: new values for existing parameters, additional parameters, and additional calculations. Details for each of these categories are provided in this document. Information used in making these adjustments was obtained from Marcus McElroy at NASA-Langley.
Parameter Old New Units
Fuel-$perGal 0.75 0.70 ($/U.S.Gal) LaborRate 30.0 25.0 ($/Man Hour) InsureRate 2.0 0.35 (% of Initial Aircraft Cost, $)
TripsPerYear Residual AnnualInterestRate PaymentsPerYear InflationRate
Utilization InitialLoan PaymentSize PresentValue FinanceCost
Fuel Density 6.70 (lbs./gal) Maintenance Overhead Rate 200 (%Direct Labor) Spares -Airframe 6% of initial aircraft cost -Engine 23% of initial aircraft cost
Tcr = ([D+Ka+20] - [Dc+Dd]) / Vc Dc =(.25hrs) * (774 ft/sec) = 696600 ft = 114 n.mi = 99 st.miles Dd =(.25hrs) * (774 ft/sec) = 114 n.mi = 99 st.miles Vc = 774 ft/sec = 527.7 st.miles/hr Ka = Airway Distance Increment = 0.02*D = 0.02*(4000) = 80 n.mi = 70 st.miles Tcr = (4000+70+20 - (99+99))/527.7 = 7.37 hours Tblock = 7.37 hours + Tgm + Tcl + Td + Tam = 7.37 + .25 + .25 + .25 + .1 = 8.23 hours
Annualized Utilization predicted by ATA method for BWB: 4,525 hours/year Annualized Utilization based on given trips/year for BWB: 8.23*480 = 3,950 hours/year
To use Utilization value provided by NASA, the cost subroutine must be modified to allow the number of trips per year to be explicitly entered. To do this a variable named TripsPerYear was introduced into PASS. This is an input only value. If it remains at its default zero value then the Utilization variable remains calculated by statistical fit. If the TripsPerYear value is non-zero then it is calculated by multiplication with the block time.
The previous PASS depreciation formula is expressed as a function which includes the depreciation period (Da) and the ATA Utilization (U) in the denominator:
Cam = f(1/(Da*U))
This formulation assumes that residual aircraft value is 0.0% of initial aircraft cost. NASA reports that for the BWB the residual is expected to be 10.0% after a depreciation period of 15 yrs. To account for this the above formulation is modified as follows:
Cam = f( 1/(Da*U) - (Residual * Ct)/Da )Where Residual is a new variable to be added to the database and Ct is total aircraft cost including engines.