The key question is: what are those exact parameters — Page 474, Lesson 568

We're moving into a new chapter now — Chapter 35, the Airborne Collision and Avoidance System, which we call TCAS. Before we dive into the system itself, I want to close out the previous topic with one specific warning mode, because it's a classic exam point and it connects directly to what we just covered on position and flap indicators. That's Ground Proximity Warning mode 4A. This is the "Too Low Gear" warning. It gives you a visual and audible alert if the aircraft descends below a certain altitude with the landing gear still retracted. The key question is: what are those exact parameters? The correct answer is 500 feet radio altitude, with speed below Mach 0.35. Let me break that down carefully, because there are two traps in the options. First, it's radio altitude, not barometric altitude. Radio altitude is measured by the radio altimeter, which bounces a signal off the ground directly beneath you — it gives you your true height above the terrain. Barometric altitude is derived from pressure and gives you height above sea level. For a ground proximity warning, what matters is your actual height above the ground, so it must be radio altitude. Second, the threshold is 500 feet, not 200 feet. And the speed condition is below Mach 0.35, not M 0.28. So the full condition is: you get the "Too Low Gear" warning when you're below 500 feet radio altitude, with your speed below Mach 0.35, and the gear is still up. Why does the speed matter? Because at higher speeds, the aircraft is likely in a different phase of flight — perhaps climbing out or in cruise — whereas below Mach 0.35, you're in a configuration where you'd expect the gear to be down, typically on approach or in the pattern. If you're slow, low, and the gear is up, that's a genuine hazard, and the system is telling you to get the gear down now. So remember the pairing: 500 feet, radio altitude, Mach 0.35. That's mode 4A. Now, with that closed out, let's turn to the new chapter. This is the Airborne Collision and Avoidance System — TCAS. This is a big one, and it's all about preventing mid-air collisions. The chapter opens with an introduction, then takes us through TCAS I and TCAS II, the principle of operation, the aircraft equipment, how it operates, and the system interconnections. We'll also cover synthetic voice prioritization — that's the system deciding which aural warnings take precedence. Then we get into the two core advisory types: Traffic Advisories, which we call TAs, and Resolution Advisories, which we call RAs. There's a distinction between proximate traffic and other traffic, and we'll look at the data tag, which is the information label attached to a target on the display. We'll also cover off-scale traffic advisories — that's when traffic is detected but is outside the display range — and then the TCAS displays themselves. There's a combined TCAS and SSR control panel — SSR being Secondary Surveillance Radar, the transponder system. And we'll spend time on how TCAS advisories appear on the electronic VSI, the Vertical Speed Indicator. That includes preventative resolution advisories, corrective resolution advisories, the test format, and no-bearing advisories. Finally, we'll cover the action to be taken when you receive TAs and RAs — that's the operational procedure, what the pilot actually does in response. Let's start with the introduction and the fundamental principle, because everything else builds on it. TCAS is an airborne system — it's carried on the aircraft itself, not dependent on ground radar. Its job is to detect other aircraft that are equipped with transponders and to warn the pilot of potential collision threats. The system interrogates the transponders of nearby aircraft and uses the replies to determine their range, bearing, and altitude. Now, the two versions. TCAS I is the simpler system. It provides Traffic Advisories only — that's a warning that traffic is nearby, but it doesn't tell you what to do about it. TCAS II is the more advanced system, and it's the one that provides Resolution Advisories — those are the commands that tell you to climb or descend to avoid a collision. TCAS II is the standard for commercial air transport. Let me pause there. We've set the stage: mode 4A is behind us, and we're into TCAS. Next, we'll go deeper into the principle of operation and the equipment.