Avionics

AVIONICS SYSTEMS

By

SREENIVASA RAO. D.Engineer(Aero)

Rotary Wing Research & Design Centre

&

N.S.VENKATESHASenior Manager(Design)

Rotary Wing Research & Design Centre

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AVIONICS SYSTEMS FOR HELICOPTERSAVIONICS SYSTEMS FOR HELICOPTERSBy N.S.Venkatesha, Manager(Design) & Sreenivasa Rao D. A.E.(Aero), Rotary Wing R & D Centre,

HAL, Bangalore – 560 017.

Technology concerned with the development and use of electronic devices for an aircraft called AVIation electrONICS also known as AVIONICS. Avionics Systems can be broadly defined as systems required to accomplish certain functions like communication, navigation, identification, weather avoidance, Display, Control, Recording, Electronic Counter Measurement, Electronic Counter Counter Measurement, target acquisition and other weapon related applications.

Avionics can be sub-divided into :

Basic Avionics Mission Avionics

Whereas certain communication and navigation systems are mandatory, other systems are defined and configured to suit a particular role helicopter is designed for, keeping in view, among other things, space and weight limitations.

Table 1 & 2 gives a typical avionics fit for Military & Civil Helicopters

TABLE – 1

AVIONICS ON MILITARY HELICOPTER

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BASIC AVIONICS MISSION AVIONICS

COMMUNICATION :

Intercom V/UHF HF

VHF(FM) Sighting System FLIR

Radar Sonar Weapon Electronics ESM

NAVIGATION :

Doppler GPS Radio Altimeter

ADF Homer Weather RadarIDENTIFICATION :

IFF (Identification Friend or Foe)

TABLE – 2 AVIONICS ON CIVIL HELICOPTER

Intercom VHF (2 Nos.) VOR/ILS/MB (2 Nos.) DME HF Heading Reference GPS ADF Weather Radar FDR / CVR

MILITARY STANDARDS:

Some of the Military Standards and specifications that define and regulate procurement, installation and qualification of avionics systems are given below:

MIL-E-5400T : General Specs, Electronic Equipment

MIL-STD-810D : Environmental TestMIL-STD-461B : EMI/EMCMIL-B-5087 : Bonding, Lightning Protection

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MIL-STD-704D : Electrical Power RequirementMS-25212 : Panel DimensionMIL-W-5088 : Wiring, Selection installationMIL-C-38999 : ConnectorsMIL-L-85762A : NVG CompatibilityDEF-STN-970 : Installation

For civil application the regulating standards are DO-160, FAR-29, JAR-29, DO-178.

GENERAL DESIGN CONSIDERATIONS :

Some of the design considerations that are general to Avionics systems are as follows:

EMI / EMC ASPECTS :

Creation of a zero potential difference reference base, which will dissipate all undesired interfering noise / signal

Single point grounding to avoid ground loop currents. This is especially important for lower frequency radio systems like ADF, HF

Separation of DC and AC grounds

Separation of power lines and signal lines

Bonding and shielding : A low impedance path must exist between all avionics equipment case and structure. The desired DC resistance value to be defined as per MIL-B-5087 (or DIN 29576). Long term effects of vibration, corrosion on bonding contact surface to be minimised. The length to dia ratio of bonding braid should not exceed 5:1

ENVIRONMENTAL ASPECTS :

The location of different LRUs have to be chosen keeping in view the environmental conditions of the locations and the environmental specification for which the equipment is cleared for operation. A temperature and vibration survey will be helpful before locations are finalised. If found necessary forced air-cooling is to be adopted to bring down ambient temperature if the avionics bay is not sufficiently ventilated. Equipment should be kept away from places where lubricating oil drip/splash is likely to occur.

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INTERCOM:

Intercom is the vital link between the pilots and all the communication and warning systems and some of the navigation systems. Intercom or Audio Management System plays a very important role in determining the quality and reliability of many avionics systems on the helicopter. Great care must be taken while selecting an intercom system with particular attention to the number of stations it can support, number of communication, navigation systems, CVR and Warnings. It should also meet certain operational requirements like call, private mode selection etc.,

Since intercom interfaces with a number of other on board equipment, the interface requirement should be properly spelt out. The following are the areas that require attention.

POWER SUPPLY : Intercom should work on dual power supply of 28 V DC, with one power line acting as standby. Both supplies should be from emergency bus. The power supply requirements should be as per MIL-STD-704D. If required appropriate filters should be housed in the junction box to minimise background noise carried on power supply lines.

HELMET / HEADSET : The helmet/headset selected should match with the intercom in terms of its electrical characteristics like mike and earphone impedance, mike output level, earphone output etc. This is to be ensured for both boom mic and mask mic. The mike should be noise-cancelling type typically of dynamic moving coil type

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not requiring DC bias. Since helicopters generally have high cockpit noise, noise-cancelling microphone alone may not be sufficient. In such cases, noise frequency and level and appropriate filters should be built in to the system to eliminate in the noise pick-up. This will enhance the quality of intercom and also that of radio transmission.

One more technique for reducing noise pick-up is by employing VOS (Voice operated switch) in intercom system. Some of the characteristics relevant for VOS are threshold of operation, frequency response and time lapse.

TRANSMITTERS: While interfacing with different radio systems it is important to check the input mic level and impedance requirements of each radio with the corresponding channel output level and impedance from intercom junction box. This has to be confirmed first by measuring the mic output for the rated input and further after connecting the corresponding T/R unit, by measuring the depth of modulation for the rated mic input, both for boom microphones and, mask microphones. Similarly audio output during reception and side tone output during transmission should be measured.

ANTENNAS:

The specification of an antenna is based on the requirements of the equipment for which is used.

Some of the considerations are:

1. Frequency Range2. Radiation Characteristics3. Polarisation4. VSWR

The frequency range of antenna determines its size. Normally monopole antennas are used. Where required dipole and conformal antennas can be used. VHF, V/UHF antennas have to be vertically polarised. HF Antennas are largely horizontally polarised. VSWR for all antennas should be typically 2:1 or less in order to minimise losses. VSWR can be calculated by the formula

VSWR = ( 1 + Pr / Pf ) / ( 1 - Pr / Pf )

Where Pr = reflected power & Pf = forward power

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Radiation patterns of the antenna indicate in which direction the energy is being radiated. Communication antennas are all omni-directional. Navigation systems such as Doppler require radiation in specific direction.

The airframe around the antenna directly influences the radiation by masking and re-radiation. The airframe can radiate energy coupled to it at frequencies where its dimensions match with wavelength.

The presence of rotor also affects the radiation pattern. The ‘rotor modulation’ effect is to modulate the field at frequency determined by number of blades and rotor RPM. The rotor modulation can be generally avoided by locating the antenna beneath the fuselage. Most of the modern equipment is immune to rotor modulation.

VERY / ULTRA HIGH FREQUENCY (V/UHF) :

External communication is achieved by means of radio-telephone (R/T) link while internal communication (intercom or audio integrating system) is by wire as opposed to wireless. The current situation is the V.H.F. is used for short-range communication while H.F. is used for long-range. An aircraft V.H.F. communication transreceiver is comprised of either a single or double conversion superhet receiver and an A.M. transmitter. Communication by V.H.F. is essentially ‘line of sight’ by direct (space) wave.

The range available can be approximated by 1.23(hr + ht) where ‘hr’ is height in feet. Above sea level of the receiver while ‘h t’ is the same for the transmitter.

A single V.H.F. installation consists of three parts, namely control unit, transreceiver and antenna. In addition crew phones are connected to the V.H.F. via selection switches in the Intercom. Transmission on set is initiated by PTT(Press To Talk) switch provided in Pilot’s control grip.

The T/R unit should be so located that the feeder lines do not cause power loss due to its length. In order to reduce the number antennas, broad band antenna covering 30-88 MHz, 100-156 MHz and 225-400 MHz can be used. Broadband tuned monopole antennas give better gain characteristics.

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RADIO PTT

Pilot’s Control Grip

AVIONICS SYSTEMS

HIGH FREQUENCY (HF):

The use of long-range communications, i.e. H.F. (2-30MHz) carries for communication purpose greatly extends the range at which aircrew can establish contact with Aeronautical Mobile Service Stations. This being so, we find that H.F. Communication Systems are fitted to aircraft flying routes which are, for some part of the flight, out of range of V.H.F.service. Such aircraft obviously include public transport aircraft flying intercontinental routes, but there is also a market for general aviation aircraft. The pilot can choose the higher frequencies during day light (10MHz to 30 MHz) and lower frequencies during night (2MHz to 10 MHz) for the best possible HF communications. Because sunlight induces ionisation and increases the density of the electrically charged particles in the ionosphere during the day. At that time ionosphere becomes thicker and reflects the higher frequencies in the HF band. When the sun goes down the density of charged particles decreases and it can reflect only lower frequencies in the HF band.

The long range is achieved by use of sky waves that are refracted by the ionosphere to such an extent that they are bent sufficiently to return to earth. The H.F. ground wave suffers quite rapid attenuation with distance from the transmitter. Ionospheric attenuation also takes place, being greatest at the lower h.f. transmission is that it is subject to selective fading over narrow bandwidths (tens of cycles).

The type of modulation used, and associated details such as channel spacing and frequency channelling increments, have been the subject of many papers and orders from users, both civil and military and regulating bodies. The current and future norm is to use single side band (S.S.B) mode of operation for H.F. communications, although sets in service may have provision for compatible or normal A.M. i.e. carrier and one or two sidebands being transmitted respectively. Depending upon the feasibility wire or tube and grounded or open-ended antenna can be used. For grounded antenna the bonding resistance should be well controlled within 2.5 m Ohm.

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AUTOMATIC DIRECTION FINDER (ADF) :

An Automatic Direction Finder automatically indicates the direction from which the electromagnetic field arrives at the ADF aerials. This system works in the frequency range of 190 to 1860 KHz. This system employs a sense antenna and a loop antenna. Signals from these antennas are added vectorially and cardioid pattern in obtained. The systems always seeks the null of the cardioid and by calibration the pointer is made to show the relative bearing to the radio station to which it is tuned.

The sense antenna should be installed at the electrical centre of the helicopter. It should also be away from the rotor in order to prevent interference from static discharge noise.

The loop should be mounted along longitudinal axis. As the loop is affected by large masses, location chosen should be such that the resulting quadrantal errors do not exceed limits. The ADF receiver never is to be considered as precision equipment. It is versatile and its angular error varies inversely with distance and increases with atmospheric noise level. It is vulnerable to counter measures.

RADIO ALTIMETER :

Radio Altimeter is one of the navigational aids for measuring the altitude of the aircraft with respect to the immediate terrain on which it is flying. The Radio Altimeter gives accurate altitude information within 3%. Radio height is measured using the basic idea of Radio ranging i.e. measuring the elapsed time between the transmission of electromagnetic wave and its reception after reflection from the ground. The system consists of a transmitter/receiver, an indicator and two antennas, one for transmitting and one for receiving of radio altimeter signals. Each Radio Altimeter system comes with its own AID value set. Before locating the T/R unit and the two antennas AID should be calculated for a particular installation to determine the cable length and height of antenna above ground when helicopter is on ground. The separation between the two antennas should be about 0.6 to 1 m. While using indicator manufactured by a vendor different from the system vendor, care should be taken to check the compatibility. Also to check if the system can be switched ‘ON’ and ‘OFF’ through the indicator. Validity signal from the system to be checked. Check the suitability of audio warning for helicopter applications. For helicopter applications audio warning should come ‘ON’ on reaching the Decision Height (DH). Chapt

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DOPPLER NAVIGATION SYSTEM (DNS) :

In 1842, the Austrian Scientist Christian Doppler predicted the Doppler effect in connection with sound waves. It was subsequently found that the effect is also applicable electro magnetic waves. The Doppler effect can be described as the change in observed frequency when the source (transmitter) and observer (receiver) are in motion relative to one another. The noise of moving trains and road traffic is a demonstration of the effect commonly observed. The applicability to electro magnetic waves is illustrated by the use of police radar speed traps, to the cost of offenders. The Doppler Navigation system makes use of self-contained sensors for precise navigation.

In an airborne Doppler radar we have a transmitter that, by means of a directional antenna, radiates energy towards the ground. A receiver receives the echo of the transmitted energy. Thus we have the situation where both transmitter and receiver are moving relative to the ground; consequently the original frequency transmitted is changed twice. The difference between transmitted and received frequencies is known as the Doppler shift and is very nearly proportional to the relative motion between the aircraft and the ground along the direction of the radar beam.

Doppler System measure velocity only, Doppler effect in which radiation from a source in motion relative to the viewer is displaced in frequency. In practice, this means comparing the frequency of the returned echo with a stable reference frequency; the difference between the two is direct measure of the relative velocity. Accuracy thus depends upon the echo quality. Echo quality from water, for example, often is poor.

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Doppler systems determine location relative to the point of flight origin by integration of measured velocity vectors, Doppler accuracy represents an improvement over airspeed-clock-compass dead reckoning because the velocity vectors measured are relative to the ground. Generally, the vectors in the direction of flight and normal to it (x and y) are measured. The system accuracy is expressed as a percentage of the distance travelled.

The ground speed and drift angle information is normally presented to the pilot but in addition is used, together with heading information, to give the aircraft position relative to a destination, or forthcoming waypoint. To achieve this the pilot must set desired track and distance to fly before take off.

GLOBAL POSITIONING SYSTEM (GPS) :

The Doppler/GPS Navigation system is a self-contained navigation system for providing navigation and steering information like present position, bearing and distance to go, to selected waypoints, velocities in 3 axes of the helicopter etc.,

The GPS consists of 3 segments.1. Space segment, 2. Control segment 3. User segment

C/A = Coarse Acquisition code

P = Precision code1. Space segment : It

consists of constellation of 24 satellites. All the satellites are placed in 6 orbital planes at an altitude of 11,000 miles from the earth inclined at 55 to the equator. Each satellite continuously broadcasting two signals namely the “Coarse Acquisition”(C/A) code and “Precision”(P) code signals. The ‘C/A’ code signals are for worldwide civilian use and ‘P’ code signals are for U.S. military use only.

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2. Control segment: It consists a system of tracking stations located around the world. Master control station is located at Colorado springs and other monitoring stations are located various places in the world which will monitors and controls satellite function.

3. User segment : It consists of a GPS receiver. A GPS receiver requires GPS signals from three satellites for two-dimensional positioning and four satellites for three-dimensional positioning (latitude, longitude and altitude).

The receiver uses the time interval between the transmission and reception of the GPS signal to calculate distance from each satellite and use these distance to compute the position. At present GPS receivers with C/A code can provide position information with an error of less than 25 meters and velocity with an error of less than 0.5 m/sec.

VHF OMNIDIRECTIONAL RANGE(VOR)/INSTRUMENT LANDING SYSTEM (ILS):

Directional radio beams, modulated so as to enable airborne equipment to identify the beam centres, define the correct approach path to a particular runway. In addition vertical directional beams provide spot checks of distance to go on the approach. The total system comprises three parts, each with a transmitter on the ground and receiver and signal processor in the aircraft. Lateral steering provided by the localizer for both front-course and back-course approaches; the glide scope provides vertical steering for the front course only while marker beacons give the distance checks.

VOR operates in the frequency range of 108 – 118 MHz with channels spaced at 50 KHz and requires horizontally polarised omni directional antenna. This band is shared with ILS localizer the VOR being allocated to 160 of the 200 available channels. ILS operates in range of 108 – 112 MHz and uses the same antenna. The location of VOR antenna is critical due to rotor modulation effects. The crew of an

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appropriately equipped aircraft can tune into a VOR station within range and read the bearing to the station and the relative bearing of the station. Should the flight plan call for an approach to, or departure from, a station on a particular bearing, steering information can be derived from the received VOR signals.

DISTANCE MEASURING EQUIPMENT (DME) :

Distance measuring equipment (DME) is a secondary radar pulsed ranging system operating in the band 978-1213 MHz. Distance measuring equipment (DME) consists of an airborne transponder that sends out a signal that triggers a ground transponder into sending back another signal to the aircraft on a slightly different frequency.

The system provides slant range to a beacon at a fixed point on the ground. The difference between slant range and ground range, which is needed for navigation purposes, is small unless the aircraft is very high or close to the beacon. The airborne transponder measures the total elapsed time, divides by two and converts this figure into miles.

The distance then is presented to the pilot by means of a digital display. The antenna should be isolated from other antennas as much as possible due to the pulsed characteristic of the output. The same control unit as VOR controls distance-measuring equipment. The DME antenna should be isolated from other antennas of similar frequency. Normally a suppression pulse is provided by DME to IFF and vice-versa in order to prevent operation of other.

WEATHER RADAR:

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Weather forecasting is by reputation and, until the introduction of satellites, in fact, notoriously unreliable. What is required is an airborne system capable of detecting the weather conditions leading to the hazards of turbulence, hail and lightning. It is alight weight ‘X’ band radar operating in the frequency range of 9375 5 MHz and provides a peak power of 10KW.

The primary purpose of the system is to detect storms along the flight path and give the pilot a visual indication in colour of their intensity. It provides five primary mode of operation i.e. three air to surface search and detection modes, two conventional weather avoidance modes. The system also has the capability to receive and decode beacons.

Weather Radar operation depends on three facts :

1. Precipitation scatters r.f. energy. 2. The speed of propagation of an r.f. wave is known. 3. R.F. energy can be channelled into a highly directional beam.

Pulses of r.f. energy are generated by a transmitter and fed to a directional antenna. The r.f. wave, confined to as narrow a beam as practicable, will be scattered by precipitation in its path, some of the energy returning to the aircraft as an echo. Since the pilot needs to observe the weather in a wide sector ahead to the aircraft the antenna is made to sweep port and starboard repetitively, hence we use the term scanner for a weather radar antenna. The beam will effectively slice any storm cloud within the sector of scan so that a cross-section of the cloud is viewed.

Display of three quantities for each target is required: namely range, bearing and intensity of echo. Plan Position Indicator (P.P.I.) display is invariably used since this allows the simultaneous display of the three quantities and is easy to interpret. A cathode ray tube (c.r.t.) is used in which the beam of electrons is velocity modulated in accordance with the received signal strength. The beam is made to seep across the screen in synchronism with both the time of transmission and the antenna position.

Although the primary function of a weather radar is to detect conditions likely to give rise to turbulence, various other uses for the system or part of the system have been, and continue to be found. Virtually all weather radars offer a mapping facility. At its most crude, selection of mapping merely removes s.t.c. whereupon the pilot can tilt the beam down to view a limited region of the ground. When selected to mapping, rivers, lakes and coastlines are clearly identified, so allowing confirmation of position With Chapt

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downward tilt the returned echo is subject to a Doppler shift due to the relative velocity of the aircraft along the beam. The spectrum of Doppler shift frequencies is narrowest when the beam is aligned with the aircraft track.

The transmitted pulse from the weather radar can be used to trigger a suitably tuned beacon(transponder) on the ground. The weather radar indicator is increasingly used for purposes other than the display of weather or mapping information.

IDENTIFICATION FRIEND OR FOE(IFF) :

Purpose of the IFF system is to respond an interrogation whether Friend or Foe. ALH equipped with an IFF system on all military versions.

IFF system has the following LRUs :

1. Dedicated control unit for the system

2. Transponder unit having compatibility with MIL-STD-1553B dual bus.

3. A Switching unit switching signals between the 2 antennas.4. 2 antennas (top & bottom) per system those are compatible with the

system.

IFF operates as follows :

Receives interrogations from ground interrogator in 1030 MHz and transmits replies on 1090 MHz

Five modes of operation :

Mode-1 : General IdentityMode-2 : Personal IdentityMode-3 : Air Traffic Identity Mode-4 : Automatic Altitude reportsMode-5 : Selective addressing

EMERGENCY LOCATOR TRANSMITTER (ELT) :

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The ELT offers full frequency operation at 121.5 MHz, 243 MHz and 406.025 MHz with optional inclusion of transmission of last known GPS co-ordinates. The ELT complies with COSPAS/SARSAT specifications. The ELT automatically activates during a crash when used in conjunction with the Bi-directional G-Switch module or can be operated manually via the Remote Controller situated with the cockpit.

The ELT also has the ability to be used portably as a Personnel Locator Beacon when released from the mounting tray. The ELT can achieve a range of 100 nM on 406.025 MHz and 40 nM on 121.5 MHz & 243 MHz.

SEARCH & RESCUE HOMING SYSTEM :

121.5MHz System Operation :

With over 330,000 121.5 MHz beacons in the field, most of the operational experience acquired by COSPAS-SARSAT relates to the 121.5 MHz system.

406 MHz System Operation :

The November 1986 exercise the first world wide test of the 406 MHz global mode involving the 4 operational satellites, all operational LUTs and MCCs, the associated ground communication networks and 26 beacons distributed in 15 countries.

The operational availability of the 406 MHz system is described as follows :

Detection probability : All beacons detected

Location probability (for single satellite pass)=95% (99% when excluding situations of known interference at several sites)

Location accuracy : better than 10 km with 82.6% probability

In addition to the traditional 121.5 and 243 MHz SAR frequencies, the new 406 MHz transmits a 450 m/sec digital pulse every 50 seconds. Traditional Chapt

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systems rely on a continuous swept tone from the beacon to ensure reliable homing, but the digital pulse from 406 MHz beacons is too brief for the traditional systems to secure a lock. In 406 MHz mode, either 121.5 or 243 MHz can be selected to automatically cover the 49.5 second lapse time between two digital pulses.

The need to locate and rescue personnel from emergency locations has never been greater. The increasing use of both SARSAT/COSPAS and Global Positioning Systems (GPS) has highlighted the need for SAR systems to be able to pin-point and effect immediate recovery and rescue of personnel from emergency.

FLIGHT DATA RECORDER / COCKPIT VOICE RECORDER :

Advanced Light Helicopter (ALH) is equipped with the Fairchild model FA2100 family of solid state flight recorders comprises a Cockpit Voice Recorder (CVR), a Flight Data Recorder (FDR), and a combination Cockpit Voice and Flight Data Recorder (CVDR). The main purpose of the FDR/CVR is to acquire and record all the essential flight data and also voice data for the purpose of accident / incident prevention and investigation. FDR is an equipment installed in the aircraft for the purpose of recording the parameters required to determine accurately the aircraft flight path, speed, altitude, engine power, configuration and operation for adjudging the aircraft performance and complementing accident / incident investigations. CVR is an equipment installed in the aircraft for the purpose of recording the aural environment on the flight deck during flight time for the purpose of accident / incident prevention and investigation.

It records all the parameters just like the normal recorder.

It consists of the following LRUs :

1. Modular Data Acquisition Unit (MDAU)

2. Helicopter Voice Data Recorder (HVDR)

3. CVR Control Unit4. Area MIC (Remote Microphone)

1. MDAU acquires flight data and voice data from various sensors and systems. MDAU provides data output to FDR/CVR unit at a data rate of 256 w.p.s.

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2. HVDR is a crash survivable recording device. Recorder retains upto 25 Hrs. of flight data and 30 minutes of voice data on 4 channels. It uses non-volatile memory device, which will retains data without the power applied.

3. The control unit has the facility for ‘self test’ and has facility for ‘erasing’ voice data of previous flight when the helicopter is on ground.

4. Area MIC is to transfer the audio conversion taking place in the cabin to voice data recorder.

The cockpit voice recorder (CVR) function simultaneously records two separate channels of cockpit audio, high quality voice(HQV) and high quality combined (HQC), converts the audio to a digital format, and stores the data in a solid state memory. This configuration records a minimum of 30 minutes of high quality audio for the following recommended stations; Pilot, Co-Pilot, Public Address or Third Crew Member, and Cockpit Area Microphone (CAM). The audio inputs are conditioned, amplified, and equalized as necessary. The resulting signals are converted to digital pulse code modulation (PCM) data. The unit can record Greenwich Mean Time (GMT) Time via the ARINC 429 GMT input and can record rotor speed data via the Rotor Speed input.

In ARINC 757 installations, the CVR Fault line provides continous status of CVR and the FDR Fault line provides continous status of FDR operation. These fault lines may be connected to an annunciator or to an aircraft central maintenance computer. The Flight Data Recorder function receives flight data from an ARINC 573/717 FDAU at 64, 128, or 256 words per second(wps).

The FDAU data is received, buffered and returned to the FDAU as part of its Built-in-Test (BIT) function. The flight data is stored in flash memory segregated from the cockpit voice data. The CVDR is capable of storing a minimum of 25 hours of flight data that can be downloaded in less than 5 mins.

LIGHTNING PROTECTION :

The basic approach adopted for implementing lightning protection scheme on a helicopter is to provide a ‘Faraday Cage’ on the outer shell of the helicopter. ‘Faraday Cage’ is formed by conducting areas of the outer surface of the structure providing EMI/EMC and lightning protection for the system within the aircraft. When lightning strikes an aircraft at the attachment point the Chapt

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high current flow chooses the least resistance path from there to the point of exit.

If various paths of same impedance level are available between these points then the current trends to choose two symmetrically opposite outermost paths. So, if there exists a low impedance path placed symmetrically at the outer surface then that will be chosen path for the current. Such a path will be prevent the current from getting deeper into the helicopter thereby protecting sensitive equipment, looms etc. from damage due to direct effects.

On modern helicopters, which are composite intensive, there is a need to use diverters. These diverters have to be interconnected and well bonded to metal structures, so that an uninterrupted low impedance path is available throughout the length and breadth of the helicopter. The area of cross section of, say aluminium diverter, should be 26 sq. mm, which is sufficient to handle full lightning current of 200 KA as per DEF STAN 970. If radome is an attachment point, since solid diverters cannot be used, RF transparent, segmented diverter strips can be used.

GLASS COCKPIT :

Glass cockpit is a state-of-art technology which has become a global trend on military and civil helicopters : Eg. EH-101, NH-90, Tiger, Apache, Bell-430, Rooivalk etc.,

Obsolescence :

The environmental electromechanical instruments are likely to be phased out and become obsolete in the course of time. Further, their cost is expected to increase as they are more labour intensive, requiring skilled man power.

Glass cockpit consists :

Common Control Unit Built-in NAV processing including GPS card Data transfer device for uploading mission plan & downloading

mission events Moving map facilities

Advantages : Chapt

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Pilot workload for normal monitoring and system management are reduced as the display is ‘by exception’

Increased system redundancy enhances mission completion probability subsequent to a single system / unit failure

Facilitates quicker assimilation of system status by the flight crew Reduces inventory Due to reduced no. of LRUs, results in reduction in interconnecting

cables, connectors and loom accessories

COCKPIT INTERFACE UNIT (CIU):

The CIU provides a display and keyboard for control and display of navigation data. It is fully MIL-STD- 1553B compatible and is based on PC architecture. It also contain a Data Transfer (DTS) capability.

MULTI-FUNCTION DISPLAY (MFCD) :

The MFCDs provide the color displays for the selected modes and for the Digital Moving Map display.

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