Let’s pick this up right where the previous excerpt left off — Page 474, Lesson 568

Let’s pick this up right where the previous excerpt left off. We were working through the Ground Proximity Warning System, and the last item we looked at was the question about which inputs feed the GPWS. Now we have one more question to finish that section, and it’s a good one because it tests a specific warning mode. The question asks: The Ground Proximity Warning mode 4A provides a visual and audible warning called ‘Too Low Gear’ to the pilot if the aircraft descends below a certain altitude with the landing gear retracted. And then we have four options. Let me walk you through the logic, because this is exactly the kind of thing you need to know cold for the exam and for the aircraft. First, the warning is called ‘Too Low Gear’. That name tells you the problem: the gear is not down, and you are getting too low. So the system is watching your altitude and your gear position. Now, the key detail here is the type of altitude. The options give you a choice between radio altitude and barometric altitude. Radio altitude is measured by the radio altimeter, which bounces a signal off the ground and gives you your actual height above the terrain. Barometric altitude is measured from the static pressure and gives you your height above a pressure datum, like sea level. For a warning that is about getting too close to the ground, you need radio altitude, because that is the true height above the terrain. Barometric altitude can be wrong if the pressure setting is off, and it doesn’t tell you how far you are from the ground directly below you. So, for mode 4A, the ‘Too Low Gear’ warning, the system uses radio altitude. That eliminates the options with barometric altitude. Now, the altitude value. The options give you either 200 feet or 500 feet. For the ‘Too Low Gear’ warning, the threshold is 500 feet radio altitude. So the warning triggers when you descend below 500 feet radio altitude with the gear still retracted. And then there is the speed condition. The options give you a speed below M 0.28 or below M 0.35. M here is Mach number, which is the ratio of your true airspeed to the speed of sound. For this warning, the speed condition is below M 0.28. So the warning only activates if you are below Mach 0.28, which is a relatively slow speed, typical of the approach and landing phase. So, putting it all together: mode 4A gives you the ‘Too Low Gear’ visual and audible warning if the aircraft descends below 500 feet radio altitude with the landing gear retracted, and the speed is below M 0.28. That is the correct answer: 500 feet radio altitude with speed below M 0.28. Now, let me just make sure you understand why the speed matters. If you are going fast, above M 0.28, you are probably not in a landing configuration, and the system might not want to give you a gear warning because you are not necessarily trying to land. But if you are slow, below M 0.28, and you are that low with the gear up, you are in a dangerous situation, and the system warns you. So, remember: mode 4A, ‘Too Low Gear’, 500 feet radio altitude, gear retracted, speed below M 0.28. That is the complete condition. Now, that finishes the Ground Proximity Warning System section. The next chapter is Chapter 35, which is the Airborne Collision and Avoidance System, or TCAS. That is a whole new topic, and we will start it fresh.