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.

New Values for Existing Parameters

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, $)

New Parameters

Inputs

TripsPerYear
Residual
AnnualInterestRate
PaymentsPerYear
InflationRate

Outputs

Utilization
InitialLoan
PaymentSize
PresentValue
FinanceCost

Unused

The following values were provided by NASA-Langley but do not fit the ATA model used in PASS and may not be needed.
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

New Calculations

Depreciation & Utilization

The method of calculating physical depreciation of the value of the aircraft asset incorporates a value for Utilization, for which NASA-Langley provided an explicit valuein units of (trips/year). The ATA Utilization (block hrs./year) value is computed using a statistical relationship between an aircraft's typical block time for each trip and the number of such trips it makes per year. A curve fit to this statistical model is currently in PASS. However, since the number of trips per year is specified for the BWB it is possible to simply multiply this value by the block time to determine the ATA utilization; as follows:

	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.

Financing

The financing model assumes continuously compounded interest and payments of equal size using the Equal-payment-series capital recovery formulation (see Engineering Economy, Thuesen, G.J., Fabrycky, W.J., Prentice-Hall, 1989). The user may specify the number of payments to be made each year but the model assumes that all these payments will be equal. The model computes the present value of the payments over the life of the loan using the provided inflation rate (note that this annual InflationRate is distinct from the Inflation factor used in the ATA model to adjust 1967 values to reflect current values). The difference between PresentValue and the InitialLoan (which is assumed equal to the total initial purchase cost) is the total cost of the financing. This value divided by (Annualized Utilization * loan period * BlockV) equals the cost of financing per mile, FinanceCost.

New Subroutine

The new subroutine cost.f is listed here .