cfiaime (12/21/82)
This discussion of ILS procedures has been quite enlightening. There are several misconceptions which could cause some grief to someone someday. First, the approach catigory of an airplane is based on 1.3 Vs0. However, if you are maneuvering at a speed over 90 knots, your Cessna 172 (catigory A - 1.3 Vs0 = ~58 knots) will actually use catigory approach minima. Second, how much controlability do you have in gusty air at 58 knots in a Cessna 172. Not much, really. Consider, on short final, most pilots fly this same Cessna 172 at 65 to 75 knots. (Too fast, really!) On any actual ILS approach, you can expect some turbulence, so a speed adjustment should be made from the FAA approved 1.3 Vs0 anyway. Third, as was mentioned earlier, you need to consider braking action and hydroplaning. The hydroplane speed for an airplane is the square root of the tire pressure times nine. That gives 45 knots for basic hydroplaning in a tire with 25 PSI inflation. Too much speed can be a problem on touchdown. So, taking our mythical Cessna 172, let me suggest the following: 85 knots, 1700 to 1800 RPM and ten degrees of flaps (in newer Cessna 172 aircraft with the approach flaps at 100 knots). In older (pre 1981) 172 aircraft, a no flap approach may be called for. Matter of fact, you will have very good luck in any piston aircraft if the ILS is flown as follows: 1. Gear down at the LOM (FAF) 2. 1700 to 1800 RPM or 17 to 18 in. hg. 3. Center the ILS (or two dots high if following a larger airplane). This works on every aircraft I have flown from a Cessna 152 to a Cessna 310. This has been an interesting discussion. If you have any comments, please feel free to ask. Jeff Williams (CFIAIME, ATP Multi, AGI/IGI)