Exam Prep Flashcards
What is a negative effect of pressurising a/c? Explain
How can we increase the safe life of an a/c?
It has detrimental effects on the fatigue life of an a/c. With each pressurisation cycle the a/c will initially stretch placing skin under tension and fasteners under shear stress, followed by depressurisation back to normal.
This cycle will slowly fatigue the a/c structure which will eventually terminate its “safe life”. We can increase its life by limiting the cruise altitude, flying within the envelope, avoiding turbulent air and avoiding heavy landings.
Describe in detail the structure of a semi monocoque skin
The metal (typically aluminium alloy) skin is riveted to longerons, bulkheads and other structural members and will carry part of the load. The thickness of the skin is determined by the load carried at a particular point and the stresses at this location, as the skin absorbs most of the loading hence “stressed skin”.
What are some advantages to the semi monocoque design?
Both the skin and the frame share the load. Ie) the bulkheads, stringers, frames and longerons are able to add strength and rigidity to the structure and help facilitate the shape/design of the structure.
What is a major disadvantage of using metal airframes?
What are some techniques to avoid this?
Corrosion.
Regular washing of a/c, hangering of a/c if not using a/c long term, not damaging painted surfaces, moisture removal chemicals to internal structures, and care with hazardous chemicals.
Mercury is VERY hazardous to a/c skin if metal and will instantly react and destroy the structural integrity.
What are the 4 types of fuel used in large jets?
Describe their different properties.
Jet A: same as JA1 but has a higher freeze point (-40*c), aka AVTUR40, is a low temperature fuel
JetA1: very common, aka kerosene/AVTUR50. FP -47 FP38
JetB: is used in cold climates as has a low freeze point, low flash point and low viscosity, aka wide cut fuel/AVTAG (30% kerosene, 70% gasoline) and has military uses.
JP5: has a high flash point so is used on a/c carriers for safety reasons.
What type of chemical are jet fuels fundamentally?
What additive does jet fuel sometimes consist of? Why?
Hydrocarbons, so therefore hydrogen and carbon.
Sometimes gasoline is added to reduce gas turbine fuels tendency to become viscous at high altitudes.
How are jet fuels designed (in terms of producing oxides)?
What colour are jet fuels?
Designed to produced gaseous oxides which keeps solid particles to a minimum which could block turbine blade nozzles/erode turbine blades.
Jet fuels have a natural straw colour.
What types of fuel tanks are there?
Rigid, bladder and integral.
Explain the rigid, bladder and integral fuel tank types
Rigid: require an open space in structure to install an AlAlloy tank that is welded/riveted to structure. Usually has shock absorbing rubber coating that is self sealing.
Bladder: requires a free space in structure to install a thin fabric of an impervious material (neoprene).
Integral: make use of spare space in wing (wet wing). Wing will be sealed to create fuel cell and can include baffles to reduce surging/increase strength.
-bladder and rigid type need restraining in wing
Why are fuel tanks pressurised?
Do gravity systems require additional pumps?
To create a positive pressure head to fuel system and reduce vaporisation.
Gravity fed systems require a venting system so the tanks don’t suffer caviatations.
What is the main purpose of a sump?
What is the main purpose of the fuel drain and how does it work?
A sump is a sediment trap at the lowest part of a fuel tank that allows impurities and water to accumulate so they can be drained, collected and inspected.
It has a sealing poppet that when depressed allows fuel to drain.
Main purpose of fuel drain is to collect water.
How can water be prevented from entering fuel tanks?
Is AVTUR or AVGAS more hygroscopic?
Water by condensation can be prevented by filling tanks full every night. Also by including a standpipe which prevents the last of the fuel (possibly contaminated) from entering the fuel system.
AVTUR.
Can engines tolerate water?
Yes, but only if it’s dissolved. Large slugs of water can cause engine damage or failure. Frozen water can block tubes/filters and lead to fuel starvation.
Water can harvest microbial growth and corrosion
What is vapourlock? What can cause it?
What can it cause?
A condition for which the fuel vaporises in the feed lines of the fuel system at the high point so that fuel flow is blocked.
Is caused by either high temperatures and/or low pressure.
Can cause fuel starvation.
What should be done to fix vapour lock on a ground start?
Fuel pump on (mags off), full throttle and full rich. Then wind the starter for a few seconds. Then leave for 10min and try hot start.
Name characteristics of aluminium in its pure state. What is the major disadvantage of AlAlloys?
Does composite plastics have this problem?
Al in its pure state is lightweight, lustrous, corrosion resistant, ductile, malleable and are non magnetic.
AlAlloys although stronger, are corrosive. This has a relatively predictable fatigue life and will notice cracks in inspections. Plastics don’t necessarily corrode but they will have a structural failure immediately and not small cracks beforehand.
What purpose are booster pumps? What type of pump are they typically? Are they submersible?
AKA auxiliary pumps are not required necessarily on high wing a/c.
On low wing a/c is used to provide a positive fuel pressure head, to aid starting, cross feed systems, t/o and landing to prevent vapour lock, jettison fuel.
Are generally electrical or centrifuge type.
Yes, to help draw all the fuel from the tank.
Describe the operation of an electronic capacitance type fuel indicator. What are some advantages/disadvantages?
They are several metal tubes placed in fuel. Air and fuel act as dielectrics and the metal tubes read the change in fuel volume by the different capacitances, so the total capacitance across the probes will change.This info is sent to a bridge circuit which then drives an indicator. It is used to calculated fuel MASS.
A: There are no moving parts. Not affected by attitude changes as total capacitance across probes remains the same.
D: Density affects fuel dielectric, however a compensator unit is included to counteract the differences.
What are some other forms of fuel quantity indicators?
Float type (pointer or rod), electrical ratio meter, basic type
What is the purpose of a cross feed system? Draw a schematic
They are used to correct fuel imbalance due to engines burning unequally (different TBO age), heaters using fuel, asymmetric, fuel leaks, shift for W+B, or shift fuel in case of engine failure.
GO LEARN THE SHIT OUT OF THE DIAGRAM
What is an advantage of hydraulic systems over mechanical systems?
What PSI does an average a/c operate?
Low volume, low initial cost, high reliability, low weight per unit power, easier construction/design, reduced maintenance cost, efficient (little losses due friction), small pipe diameter (flexibility in construction)
Can use a small force to produce a larger force using pascals principal. 3000PSI
What are the different types of pumps that can be used in hydraulics?
Hand pump (emergencies)
Power driven
Electrically driven
RAT (Ram air turbine)
Constant delivery/displacement include (output per cycle constant)
- gear type (low/med Q 1500PSI)
- vane type (high Q, low px)
- radial/axial Piston (high px 3000-3500)
Variable displacement include:
-axial Piston
Draw diagrams and give a brief explanation on gear type, vane type, axial Piston and Piston type pumps
Go to pictures on iPad.
What is the purpose of a pressure regulator/relief valve?
How can they be operated?
Draw a relief valve
Hydraulic will not deliver pressure until actioned to. If not required the fluid will flow back around through to the reservoir. Pressure will build up once required too, but to protect the system and its components from damage will have a pressure regulator to direct excess fluid back to the reservoir. The relief valve is a back up to the pressure regulator and prevent excess pressure due temperature rise.
Manual, electric, mechanical, hydraulic or a combination of multiple
What is the purpose of an actuator?
Draw a linear actuator
To convert the hydraulic pressure in a system into mechanical movement to do useful work.
What is the purpose of an accumulator?
How does it work? Draw a diagram to explain (Piston/bladder type)
They smooth out shocks and store fluid to aid supplying peaks in emergencies.
Is a pressure vessel divided into 2 by a flexible diaphragm or bladder, one space connected to hydraulic system and the other to N2 (px N2 1/2 system operating px). Fluid is pumped in which compresses the gas and can act like a spring. This means it can smooth out shocks (variations in system pressure) and also readily supply fluid when demand peaks.
Explain selector valves, check valves, sequencing valves, shuttle valves
Check valves: prevent back flow is a system. They provide some measure of protection of a leak of px upstream of valve. Can be a simple ball and spring. Aka non return valves.
Selector valves: used to initiate actuators and their direction of movement by controlling the path of hydraulic fluid.
Shuttle valve: allows flow from one or more inputs of pressure reduces.
Sequencing valve: ensures the correct order of valves (ie landing gear)
What types of hydraulic fluids are there? What are the basic ideal properties of hydraulic fluid?
Vegetable, mineral and synthetic.
Fire resistant, low viscosity and chemically stable, operate high temp/px
