Airborne Collision and Avoidance System — Page 484, Lesson 578

I want to walk you through the Airborne Collision and Avoidance System — TCAS — and I want to start with the very first idea on this page, because it's the foundation of everything else. The relative height of an intruder — that's another aircraft that's a potential collision threat — is found by comparing its Mode "C" height with the TCAS-equipped aircraft's height. So Mode "C" is the altitude-reporting transponder mode. Every aircraft with a Mode "C" transponder is broadcasting its pressure altitude. Your TCAS aircraft knows its own height, and it receives the intruder's Mode "C" height. The difference between those two numbers is the relative height — how far above or below you the intruder is. That's the vertical picture. Now, the bearing — the direction to the intruder — is determined separately, and that's what Figure 35.2 shows you. The bearing is found by comparing the times of arrival of the signal at the different antennas on your aircraft. That's the principle behind the bearing determination — the direction is derived from the timing difference of the received signal across your antenna array. Let's move to the system interconnections, because this is where you see how TCAS actually lives inside 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 all three jobs — it receives the transponder signals, it transmits its own interrogations, and it runs the collision-avoidance logic. That unit is controlled by a combined ATC/SSR/TCAS control panel. ATC is Air Traffic Control, SSR is Secondary Surveillance Radar — that's the radar system that interrogates transponders — and TCAS is our collision avoidance system. So one panel controls all three of those functions. Now, the displays. In this particular installation, the TCAS displays are a dedicated TCAS Plan Position Indicator — that's a PPI, a radar-style screen showing a plan view of the traffic around you — and the red and green sectors on the vertical speed tape of the Primary Flight Display, the PFD. The PFD is also called the Electronic Attitude Director Indicator, the EADI. The red and green sectors on the vertical speed tape are the Resolution Advisory commands — green is the safe rate to fly, red is the rate to avoid. On other aircraft, the symbols may be displayed on an electronic VSI — a Vertical Speed Indicator — or on the Electronic Horizontal Situation Indicator, which is the EHSI, also called the ND, the Navigation Display. 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. That means they have a preferred direction of sensitivity — the upper one looks up, the lower one looks down. But the Mode "S" antennas are omnidirectional — they radiate and receive equally in all directions. Mode "S" is the selective addressing transponder mode, and those antennas just need to hear everything around them. Here's a critical safety feature. The TCAS also has feeds from the radio altimeter to modify the Resolution Advisories — the RAs — when you're in close proximity to the ground. The radio altimeter measures your actual height above the terrain. And there are hard limits. 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 system will never command you to descend into the ground — it knows your height above the terrain and it refuses to give you a descent instruction that would be unsafe. The system will also take 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. It won't ask you to climb if you're dirty — gear down, flaps out — because you physically can't climb well in that configuration. 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. If you're near your maximum altitude, a climb command would be impossible, so the system suppresses it. And a further feed from the IRS — the Inertial Reference System — gives inertial vertical acceleration. That tells the system how the aircraft is actually accelerating vertically, so it can assess your real performance. Now, 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 synthetic voice is prioritized in a strict order. First, Stall Identification and Stall Prevention — that's stick-shake and stick-push. The synthetic voice is inhibited during stick-shake and stick-push operation. If the stick is shaking or pushing, the voice is silenced — the stall protection takes absolute precedence. Second, Windshear. The detection of performance-decreasing windshear takes first priority with the synthetic voice, inhibiting both GPWS and TCAS warnings. So if windshear is detected, the windshear voice warning overrides everything else — GPWS and TCAS are both suppressed. Third, the Ground Proximity Warning System — GPWS. Detection of approach to terrain takes priority over TCAS announcements. So if you're getting a terrain warning, that comes before any TCAS advisory. So the priority order is: stall protection first, then windshear, then GPWS, then TCAS. That's the hierarchy — the most immediate, most critical threats to the aircraft's survival get the voice first, and TCAS is at the bottom of that list.