Every time you fly a visual or instrument approach you will be flying a nominal-3 degree flight path. This podcast covers several techniques to fly a 3-degree final approach, whether you have glide slope guidance, such as an ILS, or simply referring to visual cues.Since the glideslope on most ILS installations and the desired visual glide path is 3 degrees, we will look at ways to easily fly a 3-degree glide path.
A 3-degree glide path is equal to an altitude loss of 300 feet per mile. Considering that a nominal threshold crossing height (TCH) is 50 feet, the proper glide path would be an altitude of 350 feet above ground level (AGL) at a distance of one mile from the runway, 650 feet AGL at 2 miles, and 1000 feet AGL at 3 miles (I'm a pilot, so I try to simplify things!). If you know your distance from the runway and the elevation of the airport, it's fairly easy to keep yourself on the right path. You can determine your distance from the runway using GPS, VOR/DME or visual references.
The vertical speed (VSI) (in feet per minute - FPM) to arrive at a 3-degree flight path is one-half your groundspeed in knots times 10. For example, if your groundspeed is 100 knots, your VSI for a 3-degree flight path would be 500 FPM. It's important to note that this is GROUNDSPEED, not airspeed.
You can determine your groundspeed from your GPS (if you have one) or by calculating your true airspeed (TAS) and subtracting your headwind. To calculate your TAS, you can estimate it by increasing your indicated (or calibrated) airspeed by 2 percent for every 1000 feet of altitude. So if your IAS is 100 knots indicated airspeed (KIAS) and you are at 5000 feet MSL, your TAS would be 110 KIAS. You can estimate your headwind by taking the headwind component at the runway and increasing it by about 20 percent. In this example for a 100 KIAS approach flown at 5000 feet MSL with a 20 knot headwind, you have a groundspeed of 90 knots, and would descent at 450 FPM.
If you do not have an ILS receiver and are approaching a runway served by an ILS, you can fly toward the runway in level flight, configured and at final approach airspeed, until reaching the outer marker (OM), then simply lower the nose 3 degrees.