Mechanical Engineering Question Bank

Civil Aviation Authority of Bangladesh (CAAB)- 2026

Post: Airworthiness Inspector (Aerospace)

Exam Date: 04.04.2026

Total Marks: 80 (Bangla 10, English 15, General Math 15, Mechanical 40)

Mechanical:

1) Describe a closed cycle gas turbine with a suitable schematic diagram.

Answer: Q4, QB-1, Ashuganj Power Station Company Limited (APSCL)-2010.

2) Compare centrifugal and reciprocating pumps in terms of pressure, flow, efficiency, head, etc.

Solution: Repeated question (Mechanical Engineering Question Bank).

S.No.	Centrifugal Pump	Reciprocating Pump
1.	It works on the rotodynamic (kinetic) principle; the liquid is lifted due to centrifugal force.	It works on the positive displacement principle; the liquid is sucked and then pushed by the piston.
2.	The discharge is large, continuous and smooth.	The discharge is small and pulsating.
3.	It develops a low to medium head; a high head requires multistage pumps.	It develops a high pressure at a low rate of flow.
4.	Its efficiency is high at the design point but falls at off-design conditions.	Its efficiency remains high over a wide range of operation.
5.	Priming is required before starting.	Priming is not required, as it is self-priming.
6.	It is compact, cheaper and requires less maintenance.	It is bulky, costly and has more wearing parts.
7.	It is not suitable for highly viscous liquids.	It is suitable for viscous liquids.

3) Two identical reversible engines work in series: the first between a source at 1600 K and an intermediate reservoir at T₂, the second between T₂ and a sink at 400 K. Each engine receives the same quantity of heat and rejects the same quantity of heat. Determine T₂.

Solution: Equal heat received and rejected means equal work and equal efficiency:

$1 - \frac{T _{2}}{1600} = 1 - \frac{400}{T _{2}} \Rightarrow T_{2}^{2} = 1600 \times 400 \Rightarrow T_{2} = 800 K$

∴ Intermediate temperature T₂ = 800 K (Answer)

4) 1 kg of a gas is heated at constant volume from 300 K to 800 K. If c_v = 0.487 kJ/kg·K, calculate the heat supplied.

Solution: At constant volume W = 0, so

$Q = Δ U = m c_{v} (T_{2} - T_{1}) = 1 \times 0.487 \times (800 - 300) = 243.5 kJ$

∴ Heat supplied = 243.5 kJ (Answer)

5) Water flows through a pipe of 5 cm diameter at 2 m/s. Determine the Reynolds number and state whether the flow is laminar or turbulent.

Solution: Taking ν = 1 × 10⁻⁶ m²/s for water at about 20°C:

$R e = \frac{V D}{ν} = \frac{2 \times 0.05}{1 \times 1 0 ^{- 6}} = 1 \times 1 0^{5}$

∴ Re = 10⁵ (> 4000); the flow is turbulent (Answer)

6) A heat engine has a thermal efficiency of 30%. If the same machine is run in reverse as a refrigerator between the same temperature limits, find its COP.

Solution: For a reversible machine, T_L/T_H = 1 − η = 0.70.

$CO P_{R} = \frac{T _{L}}{T _{H} - T _{L}} = \frac{1 - η}{η} = \frac{0.70}{0.30} = 2.33$

∴ COP of the refrigerator = 2.33 (Answer)

7) A heat engine receives 800 kJ of heat and rejects 500 kJ. Calculate its thermal efficiency.

Solution: $η = \frac{Q _{1} - Q _{2}}{Q _{1}} = \frac{800 - 500}{800} = 0.375$

∴ Thermal efficiency = 37.5% (Answer)

8) Explain the HVAC system used in an airport terminal building.

Solution: An airport terminal has large, high-ceiling glazed spaces, highly variable passenger load and 24-hour operation. Its HVAC system consists of:

Central chilled water plant: water-cooled centrifugal/screw chillers (often with thermal storage) and cooling towers supplying chilled water at about 6–7°C.
Air handling units: filters, cooling and heating coils and variable-speed fans; fresh air controlled by CO₂ sensors (demand-controlled ventilation).
Air distribution: ducts with diffusers, long-throw nozzles or displacement ventilation in high halls; VAV boxes for offices, lounges and gates.
Heating and humidity control: boilers or heat pumps, reheat coils, dehumidification by cooling coils or desiccant wheels.
Zoning: separate zones for check-in, security, lounges, baggage claim, offices and control rooms; pre-conditioned air units at boarding bridges for parked aircraft.
Building management system (BMS): monitors temperature, humidity, CO₂ and equipment status and optimises energy use.
Safety: smoke extraction and stair pressurisation fans, fire dampers and standby power.

9) Describe the maintenance activities carried out on a cargo handling unit.

Solution: Cargo handling equipment (conveyors, roller decks, elevating transfer vehicles, cranes, hoists, forklifts) is maintained as follows:

Daily (pre-use) checks: leaks, guards, rollers, belts and chains; emergency stops, limit switches and alarms; tyres, brakes, battery and fluid levels.
Preventive maintenance: lubrication of bearings, chains and gearboxes; tensioning or replacement of belts and chains; oil and filter changes; inspection and load testing of wire ropes, hooks and lifting gear; checking motor current, insulation and vibration.
Predictive maintenance: vibration analysis, thermography and oil analysis to detect wear before failure.
Corrective maintenance: fault diagnosis, repair or replacement and root cause analysis of repeated failures; stock of critical spares.
Safety and records: calibration of scales and sensors, lock-out/tag-out during work, use of PPE and maintenance logs for audit.