When training for your Private Pilot license or simply refreshing your foundational skills, you encounter various terms related to speed. Among these, airspeed is pivotal, but it comes in different forms: indicated airspeed, calibrated airspeed, true airspeed, and ground speed. In this post, we’ll unravel these concepts, explore how to calculate them, and understand why they matter to pilots.
Ground Speed: The Basics
Let’s kick things off with ground speed, which is arguably the easiest to grasp. Ground speed refers to how fast you travel from point A to point B on the ground. Think of it like the speedometer in your car. To determine ground speed, you can use a combination of a chart, a timer, and some visible landmarks.
For example, if we’re flying north along the Maryland Coast, we start our timer when we are abeam a bridge on Route 50 into Ocean City. As we calculate, it’s essential to focus on the vertical lines of longitude on the chart. Avoid using latitude lines for distance calculations, as these lines shrink as you approach the poles, leading to inaccurate measurements.
Measuring distance provides context. For instance, from the Route 50 bridge, we can identify various landmarks: about 3.5 miles to the next prominent bridge, 8 miles to the northern end of Ocean City, and 10 miles to the start of Bethany Beach, Delaware. Our goal is to find out how far we travel in a set time.
Ground speed is calculated in nautical miles per hour. If we fly a distance of 100 nautical miles in an hour, we’re traveling at 100 knots. However, it’s not practical to fly for a full hour just to figure out our ground speed. Instead, we can opt for a simpler mental math method: fly for a tenth of an hour, which equals 6 minutes.
During those 6 minutes, if we maintain a consistent speed and heading, we can take the distance flown and simply add a zero to get our ground speed. For example, if we cover 10 miles in that time, our ground speed would be 100 knots. If we prefer a quicker method, we could fly for 3 minutes, double the distance flown, and then add a zero. If we cover 5 miles in that time, we’d again arrive at 100 knots.
In our example, at the 6-minute mark, we find ourselves about a mile short of the reservoir we noted earlier. If we flew 12 miles in that 6-minute stretch, our ground speed is 120 knots.
Airspeed: What You Need to Know
Now, let’s shift our focus to airspeed. The most common reference for airspeed is what’s displayed on the airspeed indicator. In our case, we’re maintaining a steady 100 knots indicated airspeed. However, it’s crucial to understand that indicated airspeed is not the same as true airspeed.
True airspeed is the actual speed we’re moving through the air surrounding us. The reason indicated airspeed and true airspeed differ is due to how indicated airspeed is determined. It relies on the pitot-static system, which compares ram and static pressure to indirectly measure speed. This system can be affected by installation errors, leading to discrepancies in indicated airspeed.
Calibrated airspeed comes into play here. It’s calculated by adjusting the indicated airspeed for instrument and positional errors. For instance, at an indicated airspeed of 100 knots, the calibrated airspeed might read 98 knots if we had a perfect system.
However, a more significant difference arises because the pitot-static system and airspeed indicator operate under the assumption that temperature and pressure are standard—conditions that rarely occur in reality. For example, flying in hot summer weather results in less dense air, meaning fewer air molecules enter the pitot tube. To achieve the same reading of 100 knots on the airspeed indicator, we must fly faster, thus increasing our true airspeed.
To correct for non-standard pressure and temperature, we need the pressure altitude. Typically, unless conditions are extremely unusual, the indicated altitude will closely match the pressure altitude. For our calculations, we can use an altitude of 3,500 feet and an outside temperature of 26°C.
By adjusting the airspeed indicator to line up 3,500 feet with 26°C, we find our true airspeed is approximately 110 knots. Why is this important? This true airspeed represents our speed through the air, while the indicated airspeed informs us of how fast the aircraft feels it’s moving from an aerodynamic standpoint.
The Importance of True Airspeed
Understanding true airspeed is crucial for determining wind effects. For instance, if we have a true airspeed of 110 knots and a tailwind of 12 knots, our ground speed would be 122 knots. This ground speed is critical for flight planning and navigation.
Indicated airspeed is also vital for airspeed limitations. All speeds related to stall, maneuvering, and other operational limits are referenced against indicated airspeed. This is why it’s essential to keep an eye on this reading during flight.
If you use a Garmin 430 or similar device, it can simplify these calculations for you. By navigating to the auxiliary menu and entering the necessary data, the unit can quickly provide you with outputs for true airspeed and density altitude, enhancing your situational awareness.
Conclusion: Why All This Matters
Understanding the various types of airspeed—indicated, calibrated, true, and ground speed—is integral to flight safety and efficiency. Each type plays a distinct role in how we navigate, plan, and execute our flights. Whether you’re calculating your ground speed for time estimates or ensuring your indicated airspeed remains within safe limits, these concepts are foundational to your success as a pilot.
So, as you continue your aviation journey, keep these airspeed definitions and calculations in mind. Mastery of these elements will not only enhance your flying experience but also contribute to the safety and efficiency of your flights.
For more in-depth discussions on these topics, check out our online ground schools today. Happy flying!
You forgot equivalent airspeed. Although ir is only applicable to high and fast airplanes it is important to understand. It corrects calibrated airspeed for compressibility error.