Miscellaneous Group – Civil Aircraft

Carefully examine which structural parts are omitted (e. g., delta fin). Use mass per unit area for a comparable structure (i. e., a lifting surface or a body of revolution; see Section 8.4). If any item does not fit into the standard groups listed herein, then it is included in this group. Typically, this is expressed as:

Mmisc — 0 to 1% of the MTOM

8.10.2 Power Plant Group – Civil Aircraft

The power plant group consists of the components listed in this section. At the con­ceptual design stage, they are grouped together to obtain the power plant group mass. It is better to use the engine manufacturer’s weight data available in the public domain. However, given here are the semi-empirical relations to obtain the engine weight.


(1) Equipped dry-engine mass (ME)

(2) Thrust-reverser mass (MTR), if any – mostly installed on bigger engines

(3) Engine control system mass (MEc)

(4) Fuel system mass (MFS)

(5) Engine oil system mass (MOi)


(1) Equipped dry-engine mass (ME) – includes reduction gear mass to drive pro-


(2) Propeller (MPR)

(3) Engine control system mass (MEc)

(4) Fuel system mass (MFS)

(5) Engine oil system mass (MOi)

Piston Engines

(1) Equipped dry-engine mass (ME) – includes reduction gear, if any

(2) Propeller mass (MPR)

(3) Engine control system mass (MEc)

(4) Fuel system mass (MFS)

(5) Engine oil system mass (MOi)

In addition, there could be a separate auxiliary power unit (APU) – generally in bigger aircraft – to supply electrical power driven by a gas turbine.

Engine manufacturers supply the equipped dry-engine mass (e. g., fuel pump and generator) and the engine thrust-to-weight ratio (T/Mdry. engine’, thrust is mea­sured in Newtons) as a measure of dry-engine weight in terms of rated thrust (TSLS). Typically, T/Mdryengine varies between 4 and 8 (special-purpose engines can be more than 8). For turboprop engines, the mass is expressed as (SHP/Mdryengine); for piston engines, it is (HP/Mdry. engine).

The remainder of the systems including the thrust reverser (for some turbofans), oil system, engine controls, and fuel system are listed here. The total power plant group mass can be expressed semi-empirically (because of the similarity in design, the relationship is fairly accurate). The power plant group mass depends on the size of the engine expressed by the following equations:


Civil aircraft power plant (with no thrust reverser):

MENG-tf = 1-4 x MdryENG per engine Civil aircraft power plant (with thrust reverser):

MENGjtf = 1-5 x MdryENG per engine


Civil aircraft power plant:

MENG-tp = ktp X MdryENG per engine

where 1-4 < ktp < 1-5-

Piston Engine

Civil aircraft power plant:

MenGp = kp X MdryENG per engine

where 1-4 < kp < 1-5-

APU (if any)

8.10.8 Systems Group – Civil Aircraft

The systems group includes flight controls, hydraulics and pneumatics, electrical, instrumentation, avionics, and environmental controls (see Section 8.6.1). At the conceptual design stage, these are grouped together to obtain the power plant group.

Msys = 0.1to0.11 x MTOWfor large aircraft > 100passengers MSys = 0.11 to 0.12 x MTOW for smaller transport aircraft of < 100 passengers

Msys = 0.05 to 0.07 x MTOW for unpressurized aircraft

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