miscellaneous instruments

traffic alert and collision avoidance system (TCAS)

Two models of TCAS exist today for air carriers or aircraft owners and operators. Both TCAS systems are transponder based. If a nearby aircraft has a transponder that is not functioning or is in the "OFF" position, then the TCAS system will not detect the threat aircraft. Three basic components make up the TCAS systems. The first in the antenna system; the second the TCAS processor; and the third component is the instrument panel display.

TCAS I provides three levels of alert. The first level of alert is indicated on the display as an open diamond shape, with the altitude separation indicated between the host and threat aircraft and an arrow indicating if the threat aircraft is climbing, descending, or at the same altitude.

This is referred to as Other Traffic (OT). OT is not an immediate threat but is within the surveillance area and the pilot should be aware of existing traffic.

The second level of alert is Proximity Alert (PA) and is displayed with the same information as OT, with the exception that the diamond shape is now a solid shape on the traffic display. Both OT and PA alerts are white on a colour display or are not highlighted on a monochromatic display.

Traffic with a calculated intercept course for altitude and direction become a Traffic Alert (TA). When a TA is encountered, the intruder traffic is indicated as an orange colour circle, or becomes highlighted on a monochromatic display. The pilot is also alerted by an automated voice alert that says "Traffic, Traffic!"

This aural alert diverts the pilot’s attention to the panel mounted display to ascertain the location of the threat alert, determine the degree of threat, and decide what action may have to be taken to avert a mid-air collision. The alert is based on time to closure rate. The maximum alert is 30 seconds at a maximum 1,200-knot closure, which calculates to a ten mile distance.

TCAS II provides a similar level of alerts as the TCAS I with the addition of Resolution Advisories (RA). When the TCAS II system detects an imminent intercept course, the instrument panel display changes the intruder aircraft to a red colour circle and the pilot is instructed by the TCAS II system to "Climb" or "Descend" by the voice alerter to avert a mid air collision. Both aircraft must be equipped with TCAS II systems to experience Resolution Advisories.

TCAS I system

head-up display (HUD)

The head up display (HUD) was originally developed for use in the 1960s for combat aircraft. Today HUDs are also available in commercial aircraft. Cockpit displays were traditionally head down displays (HDD). These displays are mounted on an instrument panel below the level of the wind screen (wind shield) and pilots have to take their eyes off the outside environment and lower and refocus their vision to read the meters, dials and indicators. As aircraft became more sophisticated and electronic instrument landing systems (ILS) were developed in the 1930s and 1940s, it was necessary while landing in poor weather for one pilot to monitor the instruments to keep the aircraft aligned with radio beams while a second pilot watched out side of the windows to quickly take control of the aircraft as soon as the runway appeared through the weather. This is still the standard practice used for passenger carrying aircraft in commercial service while making ILS landings. As single piloted aircraft became more complex, it became very difficult for pilots to perform the workload. To overcome this problem the head up display (HUD) was developed in the 1960s by a number of companies. HUDs enable a pilot to monitor the external environment while simultaneously being able to monitor key instruments and information presented in their field of vision (FOV). The HUD information is also focused at infinity so that the pilots need not refocus their eyes while monitoring both the external world and the instruments.

Typical HUDs project information from a bright cathode ray tube (CRT) mounted vertically in the instrument panel. These CRTs project light upward onto the surface of a nearly transparent glass plate through which the pilot also continues  to see the outside world. Most often this transparent glass plate is placed in between the pilot and the wind screen and is tilted toward the pilot at about a 45 degree angle.

More recently helmet integrated displays (HID) have been developed so that the users can read instruments while turning their heads to view the world. In these systems one or two miniature CRTs are usually mounted inside of the helmet over the pilot’s ears and project forward to reflect off the inner surface of the helmet face plate back into the pilot’s eyes.

Two CRT or liquid crystal display (LCD) systems (one for each eye) are being developed to provide 3D stereoscopic  display of information to the pilot.

weather radar

The conical beam of the weather radar sweeps up targets left and right of the aircraft centreline. This positions the azimuth of the storm area, and that position is incorporated with the distance to “paint” a two dimensional image on the screen. Through the tilt control knob, the pilot can better examine the storm in the vertical, relating various degrees of precipitation to the aircraft’s current level. The tilt knob is arguably the most important control function available to the pilot. Tilt knob use is essential if ground returns are not to be painted at differing aircraft attitudes and altitudes.

Radar provides slices of storm characteristics

When radar energy encounters a precipitation particle some of the energy is absorbed, some returns to the receiver, and some is scattered in various directions. Different types of precipitation have widely differing rates of reflectivity. Weather radar does NOT detect non rain bearing cloud, fog, or clear air turbulence (CAT). It does detect wet hail and large raindrops very well. A large amount of microwave energy is reflected by wet hail and large raindrops, whereas dry snow does not reflect energy, scattering it instead, mainly away from the receiver.


This describes degrees between the edges of the beam. At 60 nm distance from aircraft a 3 degree beamwidth is 3nm across, and at 120 nm it is 6 nm across. This can lead to two distant storms that are close together being painted as one large storm. As the aircraft gets closer to the storms, they eventually resolve into two separate storms, once the distance between them is greater than the beamwidth at that range.

Beamwidth can effect storm resolution.


Stormscopes are ultra sensitive receivers set up to display electrical discharges. When a lightening bolt releases it's energy it sends out a signal at a frequency that the stormscope is set up to receive. It then measures that signal for strength and duration in order to display proper distance information. The antenna (usually mounted on the bottom of the aircraft) has coils that cross each other at a 90-degree angle. When a strike is received the antenna senses the direction by feeling which part of the coil had the strongest pulse.

mode 'S' transponder

The Mode S (S for Select) Transponder has been designed as an evolutionary addition to the Air Traffic Control Radar Beacon System (ATCRBS) to provide the enhanced surveillance and communication capability required for Air Traffic Control (ATC) automation. While providing the usual 4096 Identification Code and pressure altitude information to ATC interrogation, Mode S also provides for selective interrogation to enhance surveillance functions and data link communication functions.

Mode S performs all the functions of Mode A and C transponders, and has data link capability. Mode S transponders are an integral component of all Traffic Alert and Collision Avoidance System (TCAS) II installations and replace the Mode A and C transponder for TCAS II equipped aircraft. A Mode S transponder may be installed to replace a Mode A or C transponder without necessitating the installation of TCAS.

One new feature of the Mode S Transponder is that each aircraft is assigned a unique address code, which is broadcast in unsolicited “SQUITTER” transmissions occurring approximately every second. ATC or another Mode S equipped aircraft will use this address for interrogation or communication purposes.

Mode S Transponder specific advantages are as follows:

  1. Surveillance of a large number of aircraft with better accuracy and increased surveillance reliability;

  2. High degree of data integrity in ground-to-air, air-to-ground and air-to-air data link; and

  3. In TCAS equipped aircraft, the TCAS transmits coordination/interrogations to the other aircraft via the Mode S link in order to ensure the selection of complementary Resolution Advisories.

E. ground proximity warning system (GPWS)

With the growth of air transport operations after World War Two, there was an alarming increase in controlled flight into terrain (CFIT) accidents, where a perfectly good aircraft was inadvertently flown into the ground. To counter this trend a Ground Proximity warning system was devised, and introduced in air carrier aircraft in 1974. After that time there was a dramatic decrease in these type of accidents. The rate of CFIT accidents is still alarmingly high as a proportion of total accidents though. In air transport jets and turboprops, more than 50% of all accidents involve CFIT. The figures are even more alarming if you consider hull losses in the corporate jet sector, where about 72% of all accidents involve CFIT.

There are six different types of protection given, dependant on the conditions the aircraft is subject to. These are called modes, and they are ..

Mode 1

Excessive rate of descent with respect to the aircrafts height above the ground. Two different warnings can be given. First, an advisory (sometimes called a soft warning) of "SINKRATE", repeated every 3 seconds, and illumination of the amber GRND PROX light. This can be inhibited by pushing the GND PROX switch. Second, an aural warning (sometimes called a hard warning) of "PULL UP", together with illumination of the "Pull Up" light.Both alerts stop when the aircraft exits the respective warning envelopes (ie: reducing rate of descent to one outside of the warning envelopes).

Mode 1 Excessive descent rate

Mode 2

This supplies warning protection when the terrain is rising dangerously fast. These warnings are given well ahead of the aircrafts projected collision with terrain. A gain in barometric altitude is required to stop the alert. Mode 2 is in fact split into two separate sub-modes, mode 2a being if the flaps are NOT in the landing position, and mode 2b if they are in the landing position.

Mode 2a has an advisory aural of "Terrain, Terrain", coupled with the illumination of the GND PROX G/S INHB switch light. This can be inhibited by pushing the GND PROX G/S INHB switch.

If the aircraft radio altitude, speed and rate of closure with the ground are within the warning envelope, an

aural message of "Pull Up, Pull Up", preceded by a whooping sound will be heard. This warning can NOT be inhibited.

Mode 2b provides monitoring when the flaps are in the landing position. Entering the envelope with the landing gear extended would cause the repeated message of "Terrain, Terrain", coupled with the illumination of the amber GND PROX G/S INHB switch light. Can be inhibited by pushing the GND PROX G/S INHB switch.

Excessive rate of closure with terrain

Mode 3

This mode warns pilots of an excessive altitude loss after takeoff or go-around. Mode 3 monitors the amount of radio altitude gained. If the barometric altitude lost equates to approximately 10 % of the radio altitude gained, the "Don’t Sink" aural warning will sound, coupled with an illumination of the amber GND PROX G/S INHB switch light.

A second aural advisory of "Too Low Terrain" will occur if the original radio altitude is greater than 150 ft AGL, and the radio altitude then decreases by more than 25% of that radio altitude. As with a "Don’t Sink" aural, the amber GND PROX G/S INHB switch light will illuminate.

Altitude loss after takeoff or go-around

Mode 4

 This is divided into two submodes, mode 4a, and mode 4b. Both identify to insufficient terrain clearance

during the climbout, cruise, descent and approach phases of flight. This protection is especially valuable when the aircrafts flight path is too shallow to develop excessive closure rates with terrain (Mode 2), or excessive descent rates (Mode 1).

Mode 4 has three different alerts, depending on the phase of flight and configuration of the aircraft.

A caution when the aircraft is near to the ground, and the landing gear is NOT down, or the flaps are NOT in the landing position. Aurals will be either "Too Low Gear", "Too Low Flap", or "Too low Terrain", dependant on aircraft speed. The voiced warning will be repeated until the flight condition is corrected. Once the landing gear and flap are set to the landing position, the aurals will cease.

Unsafe terrain clearance with landing gear/flaps not in landing position

Mode 5

This concerns itself with an excessive deviation below the glideslope whilst engaged in an ILS approach. It will be be active whenever the runway being approached is equipped with an instrument landing system, and the aircrafts navigation radio is tuned to the correct ILS frequency. In other words, mode 5 is not going to provide a caution if your navigation radio is tuned to a VOR, whilst engaged in a VOR approach.

This aural warning will warn the flight crew if the aircraft descends to a position 1.3 dots or more below the ILS glideslope. As mentioned, mode 5 has two volumes, soft and loud. The repetition rate is increased as the deviation from the glideslope increases, and radio altitude decreases. The aural warnings are coupled with anillumination of the amber GND PROX G/S INHB switch light.This mode is only armed when a valid ILS signal is being received, the radio altitude is 1, 000 ft or less, andthe landing gear is down.

The envelopes have two aural warning volumes, both of which are "Glideslope".

This aural warning will warn the flight crew if the aircraft descends to a position 1.3 dots or more below the ILS glideslope. As mentioned, mode 5 has two volumes, soft and loud. The repetition rate is increased as the deviation from the glideslope increases, and radio altitude decreases. The aural warnings are coupled with an illumination of the amber GND PROX G/S INHB switch light.

Below ILS glideslope

Windshear mode

The GPWS provides aural and visual warnings of significant windshear conditions. The aural warning consists of a two tone siren, plus the words "Windshear, Windshear, Windshear". The aural warning is given once only during a windshear encounter, and is associated with the illumination of the master warning "Windshear" light. Some EFIS equipped aircraft have a feature which gives pitch and wing levelling guidance to pilots through the flight director bars on the ADI.