# Formulation to Estimate DOC

The DOC formulation is presented in this section, based on the AEA ground rules [1]. The formulae compute the component DOC per block hour. To obtain a trip cost, the DOC per block hour is multiplied by the block time. Aircraft performance calculates the block hour and block time for the mission range (see Section 13.5.6). The next section works out the DOC values, continuing with the Bizjet example used thus far.

Normally, the DOC is computed for a fleet of aircraft. The AEA suggests a ten – aircraft fleet with a 14-year lifespan and a residual value of 10% of the total invest­ment; these values can be changed, as shown in the next section. Fuel prices, insur­ance rates, salaries, and manhour rates vary with time. Engine-maintenance costs depend on the type of engine; here, only the turbofan type is discussed. For other types of power plants, readers may refer to [1].

Aircraft Price

Total Investment = (aircraft + engine price) x (1 + spares allowance fraction)

Readers must be sure to obtain the Standard Study Price from the manufacturer. The AEA uses the total investment, which includes the aircraft delivery price, cost of spares, any changes in the order, and other contractual financial obligations. In the example, the aircraft and engine price are taken as the total investment per aircraft.

Outstanding Capital = total capital cost x (1 – purchase down-payment fraction)

Utilization (per block hour per annum in hours/year)

TT. Tf 3,750

Utilization, U = —– — x t

(t + 0.5)

where t = block time for the mission.

For the flight crew, the AEA uses \$493 per block hour for a two-crew operation. For the cabin crew, the AEA uses \$81 per block hour for each crew member.

Trip-Cost Elements

• Landing fees = (7■8xMTOtWintons) where t = block time for the mission

• Navigational charges = (°-5xraJeinkm) x умтоуши™ where t = block time for the mission

• Ground-handling charges = (100xPaylotad in tons) where t = block time for the mission

The landing and navigational charges are MTOW-dependent and the ground­handling charges are payload-dependent. In practice, the crew salary is also MTOW – dependent but the AEA has kept it invariant.

• Airframe maintenance, material, and labor

(a) airframe labor

where Wairframe = the MEW less engine weight in tons

R = labor manhour rate of \$63 per hour at the 1989 level t = block time for the mission

(b) airframe material cost

where Cairframe = price of aircraft less engine price in millions of dollars

• Engine maintenance, material, and labor

(a) engine labor

0.21 x R x C1 x C3 x (1 + T)0 4

where R = labor manhour rate of \$63 per hour at the 1989 level T = sea-level static thrust in tons C = 1.27-0.2 x BPR02 where BPR = bypass ratio

C3 = 0.032 x nc + K where nc = number of compressor stages K = 0.50 for one shaft = 0.57 for two shafts = 0.64 for three shafts

(b) engine material cost

2.56 x (1 + T)0 8 x C1 x (C2 x C3)

where T = sea-level static thrust in tons

C2 = 0.4 x (OAPR/20)13 + 0.4

where OAPR is the overall pressure ratio; C1 and C3 are the same as before.

(c) direct engine maintenance cost (labor + material)

Ne x (engine labor cost + material cost

where Ne = number of engines

block fuel x fuel cost
block time

Table 16.14. Bizjet data for DOC estimation

 Aircraft details Turbofan details (two engines) Conversion factors MTOW – 9,400 kg TSLs/Engine -17.23 kN 1 nm = 1.852 km OEW – 5,800 kg Dry Weight – 379 kg U. S. gallon = 6.78 lb MEW-5,519 kg Bypass Ratio – 3.2 1 lb = 0.4535 kg Payload -1,100 kg[29] No. of Compressor Stages -10** 1 ft = 0.3048 m Range – 2,000 nm Block Time – 5.38 hr Block Fuel – 2,233 kg Overall Compressor Ratio -14 No. of Shafts = 2 Fuel Cost = \$0.75 per U. S. gallon 1 kg fuel = 0.3245 gallon

Notes:

* 10 passengers

** It has one high-pressure compressor, four-stage low-pressure compressor, and one fan. Aircraft price = \$7 million Engine price = \$1 million

Total aircraft acquisition cost = \$8 million (total investment per aircraft;price includes spares)

Then, DOC per hour = (fixed charges + trip charges)per_hour and DOC per trip = t x (DOC)per_hour and DOC per aircraft mile = DOCx10Q”~t’i°cktime and DOC per pas­senger mile per nautica. l rnle = гаП^х^^І^^^ ■