Aircraft Screws

   Machine screws

Materials available include Steel, Stainless steel and Brass.

Countersunk machine screw (Measure: total length).
A Countersunk machine screws is designed to fit flush with the surface of the fastened material for a smooth safe finish.  

Raised Countersunk machine screw.
A raised head on a countersunk machine screw giving a slightly rounded top for a more finished look.

Pan head machine screw (Measure: from under head). A domed head machine screw sits on the surface of the material to be fastened, has a flat underside and can be used with washers.

Cheese Head machine Screw (Measure: from under head). Head style: Slotted. A Cheese head machine screw sits on the surface of the material to be fastened and have a flat underside, can be used with washers.
 
Round Head machine screw (Measure: from under head). Head style: Slotted. A Round head machine screw sits on the surface of the material to be fastened and have a flat underside, can be used with washers.
Shapes of screw head

(a)pan      (b)button     (c)round    (d)truss    (e)flat        (f)oval

Pan head: a low disc with chamfered outer edge.
 
Button or dome head: cylindrical with a rounded top.
 
Round: dome-shaped, commonly used for machine screws.
 
Truss: lower-profile dome designed to prevent tampering.
 
Flat or Countersunk: conical, with flat outer face and tapering inner face allowing
it to sink into the material, very common for wood screws.
 
Oval: countersunk with a rounded top.
 
Cheese head: disc with cylindrical outer edge, height approximately half the
head diameter.
 
Fillister head: cylindrical, but with a slightly convex top surface.
 
Socket head: cylindrical, relatively high, with different types of sockets (hex,
square Torx, etc.)
 
Mirror screw head: countersunk head with a tapped hole to receive a separate
screw-in chrome-plated cover, used for attaching mirrors.
 
N.B: Headless machine screws, called "setscrews" or "grub screws", are also used. They
either have a socket or a slot.

Thrust Reverser Systems

BASIC PRINCIPLE

To ensure a good braking effect for the aircraft also on contaminated runways (with water or slush) and for the reduction of brake wear, transport aircraft are equipped with thrust reversers. A thrust reverser allows the generation of a rearward-directed thrust force when deployed. To achieve this it redirects the exhaust gas flow at an angle of approximately 120 degrees. Figure 1 shows the direction of the airflow during reverse thrust operation. On turbofan engines with high bypass ratios, only the secondary gas flow is redirected by the thrust reverser because this gas flow generates the larger portion of the engine thrust. This results in a reverse thrust force high enough for braking purposes. To redirect the secondary gas flow only makes mechanical deflector components in the hot gas flow unnecessary. This results in a simpler reverser kinematics with less weight and costs.

          Fig . 1     Direction of the secondary airflow for the generation of the reverse thrust  force

Because the effect of the reverse thrust is independent from the tire friction, the thrust reverser ensures a good deceleration of the aircraft on a contaminated runway with reduced tire friction. During normal runway conditions the use of the thrust reverser requires less use of the wheel brakes for the same aircraft deceleration. The results are less wear of the wheel brakes and a longer operating life of the brake disks.

To have these advantages the higher weight of the engine nacelle must be accepted. To minimize the weight penalty caused by a thrust reverser the designers use a high percentage of composite material for the reverser

and nacelle structure. Usually all engines of an aircraft are equipped with a thrust reverser. But this is not a general rule in any case. An exception is the A380. For weight reduction purposes this aircraft has only two thrust reversers. They are installed on the two inboard engines.

REVERSER OPERATION 

A thrust reverser system is designed for the use on ground only. The system is equipped with safety features preventing the deploying of the reverser during flight. During landing the thrust reverser is deployed shortly after touchdown by selection of the pilot. The best braking effect is achieved at the higher speeds during the landing run because the propulsive efficiency for the reverse thrust has its highest values at the high forward

speeds. With the decreasing aircraft speed it also decreases. During typical flight operation the thrust reverser is used down to a speed of 80 knots. The pilot selects as much thrust as needed. This procedure

ensures an operation with the highest propulsive efficiency and prevents the ingestion of dirt at slow taxi speeds. It is the most efficient way of thrust reverser use in terms of fuel consumption and brake wear.

TYPES OF THRUST REVERSERS


Thrust reversers can be differentiated by the types of their subsystems. These subsystems are the


• Airflow deflection system
• Actuation system
• Control system

The airflow deflection system comprises the structural components necessary for the deflection of the airflow during the operation in the reverse thrust position. For the change between the forward thrust operation and

the reverse thrust operation some components of the airflow deflection system are movable. To achieve its movement an actuation system is installed. This is controlled by the pilots via the thrust lever and the reverser

control system. It is designed to move the reverser components into one of the two end positions. These are the forward thrust (stowed) position or the reverse thrust (deployed) position.

Continued..........

Abbreviations of Aviation

AC...................Alternating Current
ACC................Active Clearance Control
ACOC.............Air-Cooled Oil Cooler
ADC................Air Data Computer
AFDX..............Avionics Full Duplex Switched Ethernet
AIMS...............Aircraft Information Management System
ALF.................Aft Looking Forward
ARINC............Aeronautical Radio Inc.
ASC.................Aircraft System Computer
ASTM..............American Society for Testing and Materials
ATA.................Air Transport Association of America
A/THR.............Auto thrust
AVM................Airborne Vibration Monitoring
CBP................Customer Bleed Pressure
CCS................Common Core System
CDP................Compressor Discharge Pressure
CIT..................Compressor Inlet Temperature
CONT.............Continuous
COTS..............Commercial Off The Shelf
CS...................Certification Standard
DAC................Dual Annular Combustor
DC...................Direct Current
DEU.................Display Electronic Unit
DMC................Display Management Computer
EASA...............European Aviation Safety Agency
ECAM..............Electronic Centralized Aircraft Monitor
ECM.................Engine Condition Monitoring
ECU..................Electronic Control Unit
EEC..................Electronic Engine Control
EFIS.................Elecronic Flight Instrument System
EHSV...............Electrohydraulic Servo Valve
EIA...................Electronic Industries Alliance
EICAS..............Electronic Indication and Crew Alerting System
EIU...................Engine Interface Unit
EIVMU.............Engine Interface and Vibration Monitoring Unit
EPR..................Engine Pressure Ratio
EUROCAE.......European Organisation for Civil Aviation Equipment
E/WD................Engine/Warning Display
FADEC.............Full Authority Digital Engine Control
FCU..................Fuel Control Unit
FDRV...............Fuel Diverter and Return Valve
FF.....................Fuel Flow
FIFO.................First In First Out
FLA..................Forward Looking Aft
FMC.................Flight Management Computer
FMU.................Fuel Metering Unit
FMV.................Fuel Metering Valves
FOB..................Fuel On Board
FRT...................Flat Rate Temperature
ft........................Feet
FWC.................Flight Warning Computer
FWD.................Forward
GRD..................Ground
HMU.................Hydromechanical Unit
HPC..................High Pressure Compressor
HPT...................High Pressure Turbine
IDG...................Integrated Drive Generator
IEEE..................Institute of Electrical and Electronics Engineers
IGN...................Ignition
IMA...................Integrated Modular Avionics
IPC....................Intermediate Pressure Compressor
IPT.....................Intermediate Pressure Turbine
ISA.....................International Standard Atmosphere
LLP....................Life Limited Part
LPC...................Low Pressure Compressor
LPT....................Low Pressure Turbine
LRM...................Line Replaceable Modules
LRU...................Line Replaceable Unit
LVDT................Linear Variable Differential Transformer
MCDU..............Multipurpose Control and Display Unit
MEC..................Main Engine Control
MEMS...............Micro electro mechanical Systems
Mn.....................Mach Number
N1, N2, N3.......Engine Rotor Speeds
OAT..................Outside Air Temperature
ODM.................Oil Debris Monitor
PMC..................Power Management Control
PPBU................Power Plant Build-Up
QEC..................Qick Engine Change
REV...................Reverse
RTD...................Resistive Thermal Device
RTDCA.............Radio Technical Commission for Aeronautics
RTOS................Real Time Operating Systems
RVDT................Rotary Variable Differential Transformer
SAL...................System Address Label
SDAC................System Data Aquisition Computer
SOAP................Spectrographic Oil Analysis Program
TAPS.................Twin Annular Premixing Swirler
TAT...................Total Air Temperature
TBV...................Transient Bleed Valve
TIA....................Telecommunication Industry Association
TLA...................Thrust Lever Angle
TOGA................Take-Off/Go Around
TRA...................Thrust Lever Resolver Angle
TSFC.................Thrust Specific Fuel Consumption
UART.................Universal Asynchronous Receiver-Transmitter
VBV...................Variable Bleed Valve
VSV...................Variable Stator Vane

Magnetic Chip Detector & SOAP System

The magnetic chip detectors for debris monitoring are installed in the
scavenge pump inlets or in the scavenge oil lines upstream of the pumps
where easy access is ensured.
The filter system is a very important element for the reliability of a recirculatory
lubrication system. Because the oil has to pass through small
holes and passages, even very small particles contaminating the oil could
block the oil flow resulting in a lubrication failure. The normal contaminant
is abrasive material and is released by the bearings and gears during
their normal operation. It is flushed away from the bearings and gears by
the oil and carried with the scavenge oilflow away from the sump. In the
filters of the system the contaminants are removed nearly completely from
the oil. Thus the oil can be supplied again to the bearings and gears. If a
bearing or gear failure develops, larger than normal particles will be found
in the filters and on the magnetic chip detectors.

Fresh oil filled into the lubrication system will gradually dissolve contaminants

and holds microscopic particles in suspension. Bearings, seals and

gears wear, erode and corrode introducing traces of these components into

the oil flow. Thus the condition of the oil, which has been circulated in the

lubrication system for some time, very exactly reflects the condition of the

system. If the system operates normally, the oil contains the amount of

particles, which is typical of the system. The size of the particles is typical

of abrasive contamination. During the development of a bearing or gear

damage the size and the amount of the particles become larger. With the

detection of these particles in the oil a bearing or gear damage can be

recognized in its early stage. These particles are also collected by the oil

filter, but the filter inspection intervals are too long to detect a damage

earTlyo. facilitate the check of the oil system for particles in shorter intervals,

magnetic chip detectors are installed in the scavenge oil flow of each sump

or as a master chip detector in the common scavenge line downstream of

the scavenge pumps. Because the gears and bearings are made of steel, the

magnets of the chip detectors are able to collect the particles (or chips) of

these parts. These chip detectors collect particles from 0.02 to 1 mm in

size. The whole arrangement of magnetic chip detectors is often called debris

monitoring system. In its simplest design the magnetic chip detector is

a tiny bar magnet which protrudes into the scavenge oil flow. Figure 3.9

shows such a chip detector. The check of the chip detectors for collected

particles in fixed time intervals is part of the maintenance checks. To facilitate

the check of these detectors they can be removed from their housing

without a tool. If particles are found on the chip detector, they can be

analyzed in a laboratory to exactly determine the component releasing

these particles into the oil.

The more sophisticated variant of the magnetic chip detector is the electrically

monitored chip detector as shown in Above Figure. Such a chip detector

comprises a set of two magnets. The FADEC ( Full Authority Digital Engine Control ) computer
monitors the resistance between these two magnets. A check and removal of the electrical

monitored chip detector is not necessary until the FADEC computer

sends the corresponding maintenance message.

For the particle monitoring of modern engines Oil Debris Monitors

(ODM) are used. These sensors are based on an inductive measurement

technique which enables the system to detect, count and classify wear

metal particles by size and type (ferromagnetic or non-ferromagnetic).

This allows the system to determine the trend for the amount of particles in

the oil. The ODMs are connected to the FADEC computer or another

computer assigned to this function.

Another tool for the monitoring of the oil-wetted parts is the Spectrographic

Oil Analysis Program (SOAP). Through this analysis the concentration

of particles of the size from 0.001 to 0.02 mm and its specific elements

can be identified. The metal type and concentration may indicate to

the analyst and engineers which part of an engine is failing if a direct assignment

to an engine part is possible. SOAP is used if this program is part

of the engine maintenance schedule. It may be used temporarily if uncertainties

exist about the reliability of engine bearings or gears. Some turbine engine manufacturers mandate SOAP in certain turbine engine maintenance

schedules.

For the monitoring of particle concentration in the oil a periodic analysis

of oil samples, taken from the engine, is made in a laboratory. When the

amount of particles for an element or an element combination increases,

this indicates an increase in wear. If the trend continues, the development

of a damage is imminent. In this phase the particle size also increases and

the presence of the particles can be verified by the magnetic chip detectors.

The verification with the help of a chip detector is important if an assignment

of the analyzed elements to an engine part is not possible.

If SOAP is used, the airline engineering has more time for the observation

of a failure development. This allows a longer planning period for the

engine removal necessary in such case.