Airborne Collision and Avoidance System — Page 484, Lesson 578

I want to walk you through the Airborne Collision and Avoidance System — TCAS — and I'll start with how the system figures out where an intruder is in three dimensions, then we'll look at how it's wired into the aircraft, and finally how its synthetic voice fits into the priority ladder with other warning systems. First, the vertical picture. TCAS doesn't just know an intruder is out there — it needs to know how far above or below you that aircraft is. The relative height of an intruder is found by comparing its Mode "C" height with the TCAS-equipped aircraft's height. Mode "C" is the transponder mode that reports pressure altitude. So the intruder squawks its altitude, your own aircraft knows its own altitude, and TCAS subtracts the two to get the relative height. That's the vertical separation picture. Now, the horizontal picture — the bearing. Take a look at Figure 35.2, which shows TCAS bearing determination. The bearing to the intruder is found by the TCAS directional antenna measuring the time difference of arrival of the reply signal across its elements. The antenna has multiple elements, and the reply arrives at slightly different times at each one. That time difference tells the system which direction the signal came from — that's the bearing. So you have range from the timing of the interrogation-reply cycle, bearing from the directional antenna, and relative height from the Mode "C" comparison. That's your full three-dimensional picture of the intruder. Now let's look at how the whole thing is interconnected on the aircraft. Figure 35.3 shows a TCAS installation in a commuter or feeder airliner. The heart of the system is the TCAS receiver-transmitter-computer unit. That's one box that does the receiving, the transmitting, and the computing — it's the brain. It's controlled by a combined ATC/SSR/TCAS control panel. ATC is Air Traffic Control, SSR is Secondary Surveillance Radar — that's the transponder side — and TCAS itself. So one panel controls all three. The displays in this particular installation are a dedicated TCAS Plan Position Indicator — that's the PPI, a radar-style display showing a plan view of surrounding traffic — and the red and green sectors on the vertical speed tape of the Primary Flight Display, the PFD, which is also called the Electronic Attitude Director Indicator, the EADI. The red sectors show you the vertical speeds to avoid, the green sectors show you the vertical speeds to fly for a resolution advisory. On other aircraft, the symbols may be displayed on an electronic VSI — a vertical speed indicator — or on the Electronic Horizontal Situation Indicator, the EHSI, also called the Navigation Display, the ND. And a synthetic voice issues the TCAS commands — you'll hear spoken instructions like "climb" or "descend." Now the antennas. The TCAS upper and lower antennas are directional — they're the ones that measure bearing. The Mode "S" antennas are omnidirectional — they transmit and receive in all directions for the transponder side. So you have two different antenna types doing two different jobs. Now, the critical safety feeds. TCAS has feeds from the radio altimeter to modify the resolution advisories — the RAs — when you're in close proximity to the ground. And here are the hard limits, and I want you to remember these exactly. There are no instructions given at all when the aircraft is below 400 feet AGL — AGL means above ground level. No descent RAs are given below 1000 feet AGL. And no increase rate of descent commands below 1450 feet AGL. So the lower you are, the more TCAS restricts what it will tell you to do — it won't tell you to descend into the ground. The system also takes aircraft configuration and performance into consideration when deciding an avoiding action. When the aircraft has gear and/or flap deployed, its climb performance will be poor, so TCAS will avoid giving climbing demands for an RA. A feed from the ADC — the Air Data Computer — tells the ACAS the aircraft's height, so that at high altitudes it will not give a climbing RA if the aircraft is close to its performance ceiling. And a further feed from the IRS — the Inertial Reference System — gives inertial vertical acceleration. So TCAS knows your actual vertical motion, not just your indicated rate. Now let's talk about the synthetic voice prioritization. Modern aircraft use a synthetic voice to give warning advice to the crew. That voice is used for various systems — windshear detection, ground proximity warnings including height call-outs, and TCAS. And the voice is prioritized in a strict order. First priority: Stall Identification and Stall Prevention — that's the stick-shake and stick-push. The synthetic voice is inhibited during stick shake and stick push operation. So if the aircraft is stalling, the voice goes silent — the stick shaker and pusher take over completely. Second priority: Windshear. The detection of performance-decreasing windshear takes first priority with the synthetic voice, inhibiting both GPWS and TCAS warnings. So windshear beats everything except the stall protection. Third priority: the Ground Proximity Warning System, GPWS. Detection of approach to terrain takes priority over TCAS announcements. So if you're about to hit terrain, that warning comes before any TCAS command. So the ladder is: stall protection first, then windshear, then GPWS, then TCAS. That's the order in which the synthetic voice will be used, and each higher priority inhibits the ones below it.