This must be my night for not being sure about who is confused about what… I’m going to go back to AirbusA380
’s original question – and this looks like its likely to be another “rant” – sorry Becky!
…when my instructor showed me a flapless landing, the "picture" did look shallower, but we still maintained a 2 reds 2 whites glideslope? He said in a flapless landing, just like in a flap extended landing, it is still possible to maintain a VASI glideslope. This totally confused me: if the approach is shallower with a flapless, why didn't I see 3 whites and one red or maybe all whites, as this would have been consistent with a shallower approach? Please unconfuse me.
: your instructor is correct … you should maintain on or above the glide slope (electronic or visual) for any landing – with or without flaps. Recall that approach speed should be 30% above the stalling speed for the configuration used – plus one-half of the wind component and all of the gust component (and the additions shouldn’t be changed). So to calculate your reference speed with flaps and gear extended, compute your stalling speed for that configuration and add 30% to that figure, and in a no wind condition, that is the speed you will fly; and you will fly on (or above) the glide slope. To calculate your reference speed with the gear extended and the flaps/slats retracted, compute your stalling speed for that configuration and add 30% to that figure, and in a no wind condition, that is the speed you will fly; and, again, you will fly on (or above) the glide slope.
The things that will “change” in these cases are the configuration (flap settings), the pitch attitude, the airspeed, and, very likely, the power required. The point that may be confusing is that the approach gradient should not be substantially different – not if you fly the ILS glide slope as an example. In fact, maintaining the same descent path will require you to descend at a slightly higher rate with no flaps – why? Because you are coming down the final approach at a higher airspeed, and you have to loose the same altitude in the same horizontal distance. If you’re covering that horizontal distance at a higher rate, guess what … you’ll have to lose that altitude at a higher rate or you won’t wind up at the same place.
Let’s use some numbers so you can see that I’m not handing you a line of the proverbial “bullstuff.” The typical FAF is about 4 nautical miles (or 24,304 feet) from the runway end. Add 1,000 feet to the touchdown point and the distance from the FAF to TD should be 25,304 feet. The typical FAF is about 1500 feet above the ground. Let’s say that we have an airplane with a gross weight that would give us a clean wing stalling speed of 120 knots and a full flaps stalling speed of 94 knots. These are realistic numbers for a smaller transport category turbo-jet like the B737 or the DC9. This would mean that the flaps retracted approach speed would be 120 knots + 30% of 120 knots (which is 36 knots) or a total of 156 knots. The full flaps approach would be flown at 94 knots + 30% of 94 (which is 28.2 knots, rounded up to 29 knots) or a total of 123 knots.
How long will it take to fly from the FAF to the TD in each case?
The clean wing case, 156 knots on final approach, will take 1 minute 36 seconds to cover the 25,304 feet to the TD point. To lose 1,500 feet in 1 minute 36 seconds you will have to descend at an average rate of 938 feet per minute.
The full flaps case, 123 knots on final approach, will take 2 minutes 2 seconds to cover the 25,304 feet to the TD point. To lose 1,500 feet in 2 minutes 2 seconds you will have to descend at an average rate of 739 feet per minute. I don’t know if your instructor has ever talked to you about standard rates of descent on an ILS, but for a reasonably wide range of turbo-jet airplanes, choosing an initial rate of descent at the FAF of about 750 fpm will put you in the “ballpark” for more easily maintaining the glide slope.
The other differences will be the pitch attitude and power settings you will have to hold to maintain those airspeeds and rates of descent. For the clean wing case, the pitch attitude will be noticeably higher – putting your aim point much lower in the forward windscreen. This tends to give you the impression that you are flying a “flatter” approach – but, we’ve just shown you that it is not flatter – it is, actually, quite considerably steeper in terms of rate of descent – 200 feet per minute steeper. With the full flaps approach, the pitch attitude will be much lower, putting your aim point much higher in the forward windscreen, tending you give you the impression that you are flying a much steeper approach, but you now know differently. However, in each of these cases, the airplane should be on, or very close to, the normal, descending flight path – that is, on or slightly above the desired "glide path" or electronic "glide slope" – giving you the appropriate visual indication, just like you saw – 2 reds and 2 whites.
In these cases the additional problems that might be encountered during the clean wing approach, is that you will use substantially less power and will likely be on the verge of being in, or very nearly in, idle thrust. It is not good to make an idle thrust approach in a turbo-jet airplane – just in case you need the engines to spool up for some reason – they take longer from idle than they do from a slightly higher power setting. The other side of the coin is that if you allow the speed to increase while on final, you’ll have to increase your rate of descent to get the airplane down where you want it down. Also, on most airplanes, there is a maximum tire speed – meaning that landing with a higher speed, may result in tire failure.
Anyway, while you may very well NOT be talking about your efforts in a turbo-jet airplane – it is possible that you were using a smaller aircraft. I’m sorry that I can’t give you a similar dissertation for a smaller airplane – its just that its been a very long time since I instructed in anything but a turbo-jet – and habits are hard to break; but the premise is the same in either case.
Let me know if I can attempt to answer any additional questions you might have.