January 2002 Issue
Another ILS Trick
Knowing winds can also help you fly
I have some comments on “Nailing The Needles” [Instrument Check, November]. I am a CFII and already made copies of your article for my students, as I think the information may clarify some things to them or at least bring up questions (both will be good).
I’d like to add what I consider another major component of an ILS (or any approach): the winds aloft and on the ground. These are taken too lightly prior to starting an approach, but I’ll stick to an ILS since that was the subject. Winds aloft provide a lot of information, including preferred altitudes, freezing levels, potential areas of turbulence and an idea of how you will be correcting for the wind.
The enroute portion of the flight should give you an idea as to whether the forecast has held up or not, based on what you’ve been doing to correct for the wind. Knowing that and the winds on the ground at your destination will also help you set up your ILS.
The wind will determine when you start to turn as the localizer comes alive. If you’re into a 25-knot headwind, for example, you don’t want to start turning as soon as it comes alive. Even before that, the winds on the ground will tell you whether you’re going straight in or circling and what your DA or MDA may be. I can’t tell you how many times I have asked a pilot what his DA is, knowing it would likely be a circling approach, and have the student spit out the straight-in DA because they didn’t consider the wind or the possibility of a circling approach. Wind will also determine what initial heading you will turn to fly the desired track.
Another important aspect is knowing how you will nail the glideslope. I teach a descent rate of five times the ground speed as a rule of thumb for a three-degree glideslope, but you need to know the winds to guesstimate what they will do to your ground speed. This is even more important during a non-precision approach but that’s for another day.
Pitch/Power Not That Simple
I read with concern Raymond Leis’ assertion in “Pitch Vs Power” [Instrument Check, November 2001] that “pitch controls altitude and the power is used to control airspeed.” I agree that, regardless of the aircraft type, this becomes the preferred method as the aircraft nears the touchdown zone and a given glideslope needle deflection corresponds to a relatively small absolute altitude error. However, my concern is that, at a given distance from the touchdown zone, an aircraft that flies the ILS at a relatively slow airspeed is required to make much higher proportionate changes to that airspeed to correct a given glideslope error than are required of an aircraft that can fly the ILS at a higher airspeed.
The reason for this is rooted in the “conservation of energy principle,” which most pilots understand as the notion that airspeed can usually be exchanged for altitude. If power is left unchanged and the change of drag with airspeed/angle of attack is neglected, then “conservation of energy” dictates that the change of airspeed in knots required to correct a given altitude error is approximately 11.3 times that altitude error in feet divided by the starting airspeed in knots. [This relationship is only good for approximating airspeed changes of up to about 20 percent of the starting value. Beyond that, it breaks down in the direction of predicting a too-small required airspeed change.]
For example, if a Skyhawk flying an ILS with a nominal three-degree glideslope at 75 knots to a 200-foot decision height experiences an 0.7-degree glidepath error (a 47-foot altitude error, corresponding to exactly a full “fly-up” or “fly-down” needle deflection), it need only change its airspeed by 7 knots, or 10 percent, to correct this error without changing power. A 747 encountering the same error at the same location at 150 knots need only change its airspeed by 3.5 knots, or less than 3 percent, to correct the error. This is a comfortable airspeed change for both aircraft types.
However, if the Skyhawk encounters the full “fly-up/fly-down” needle indication while still two miles from the touchdown zone, corresponding to a 148-foot altitude error, it must change its airspeed by well over 22 knots, or 30 percent, to return to the glide path. The 150-knot 747 can perform the same correction with only an 11 knot, or 7.5 percent, airspeed change. Comfortable for the 747 captain, but a little frightening for the Skyhawk pilot.
Perhaps this is why “autoland systems and superbly precise autopilots” work better on 747s than on Skyhawks. I suspect this is also why many pilots of small propeller airplanes are taught to fly the ILS at speeds much too fast to be practical for landing, forcing them to scramble to power back and trim up if they break out at ILS minimums.
Granted, the Skyhawk pilot can use power to salvage such extreme airspeed variations. However, light airplanes make pitch excursions when power is applied, leaving the pilot who tries to “pitch for glide path and power for airspeed” in a constant battle not only with the glideslope needle, but also with his aircraft’s own inherent pitch stability.
My own rule of thumb is to pitch for glide path only when close enough to the touchdown zone to do so with less than a 10 percent airspeed variation. Otherwise, I power for glide path.
Why All This Baby Talk?
I’ve just read why we should be careful about sudden rpm changes [Risk Management, November].
My Jeep owner’s manual doesn’t warn me of this, and its rpm is easily twice that of my l82.
My l99l Alfa’s manual doesn’t suggest any cautions. Red line is 6,000 rpm. What is going on? Why in the world – after all of these years – do we have to baby aircraft engines?
It’s not a matter of babying them. Aircraft engines are different than car engines and to compare them isn’t fair to either. Aircraft engines spend most of their life at high power settings, typically 65 to 75 percent. Try running your Alfa at 4,500 rpm all the time (even at stoplights) and see how long it lasts.
Full-Motion Works for GA
I very much enjoyed your “Risky When Real” [Risk Management, November]. We couldn’t agree more that simulators are the best place to practice emergencies, and static flight training devices don’t have the same impact as full-motion simulators.
Fidelity Flight Simulation pioneered the affordable full-motion flight simulator and sells both 6 and 3 degree-of-freedom simulators with wrap-around external visual systems for about the same price as static FTDs. In fact, we founded this company on the concept that flight training can be safer and more effective if the general aviation pilot has access to the very same motion-based flight training tools the airlines use.
Fidelity Flight Simulation Inc.
Full-Motion Works for GA
I want to tell you about a product we have at our flight school that addresses an issue raised in the article “Risky When Real” [Risk Management, November]. There are now full-motion flight simulators for general aviation training and we have one at Windsong Aviation.
The Motus flight simulator we have in our facility has a six degree-of-freedom motion base. Motion in pitch, roll, heave, yaw, surge and sway axes provide for an exceptionally realistic flight environment.
Simulated engine failures are easily demonstrated and practiced in our simulator and are becoming a regular part of our flight training. Because we are able to realistically simulate both single and multi-engine aircraft, the flight simulator is a true asset to our training programs.
While we understand that there are very few full motion flight simulators in flight schools today we want to be sure that your readers know that this valuable tool is available.
Keeping It Clean
While reading “Snowhere to Hide” [Weather Tactics, November], I was struck by a thought – if frosty wings reduce lift, how about dirty or gritty wings? Being cheap, I don’t get my plane washed until it’s fairly filthy – say two or three months of weekend flying. I have noticed that after washing, the wings are much smoother, presumably because of removed dust and grit. What does that do to lift?
Wing contamination reduces lift. But you already knew that. We’re cheap, too. We’ve found a good solution to dirty airplanes is to offer one of the neighbor kids a ride if they help wash the airplane, then get them to do the belly. It works once, anyway.
A Lonely Voice on Concorde
John Lowery’s article about the crash of the Concorde [Reality Check, October] came just as the news media reported that the newly modified aircraft was to resume service. The metal strip on the runway is still being reported in the media as the cause of the crash. I’ve heard Continental has already sent Air France a check in an out-of-court settlement.
I am a pilot for American Airlines, and our safety publications haven’t reported any of the info in your article either. How come Continental is being blamed, when it seems that Air France was guilty of gross negligence in several areas? How come the media isn’t making a big deal of this? I know that Gary Condit and now the events of 9/11 have been grabbing all the headlines, but it seems that the news media has been given a load of bovine excrement instead of the true information.
It seems to me that Air France should be taking more responsibility in this matter, and that everyone who flies should be made aware of the real story. Thanks for making your readers aware, at least.
Other aviation publications have reported on this issue as well, but it seems curiously absent from the mainstream media. Although the events of 9/11 and the war in Afghanistan doubtless have distracted them, we can only guess why this information has not gotten wider exposure. It will, though, and when it does remember where you read it first.
Keeping It Clean, Reprise
Each month, Aviation Safety comes into my home as a guest. And when it arrives it is as though you yourself were personally there.
When you are there you are, in my case, in mixed company with people of varied ages and genders. It is disappointing, actually offensive, to have my daughter, who is learning to love flying, show me literature that reads “...pilots are beating the snot out of airplanes...” or “...if you do decide to have a pissing match...” This is bringing the literate level of what is supposed to be a professional publication to the lowest common denominator; it is crude and vulgar.
I don’t know why you allow this. It is not necessary to use this language in order to convey a message promoting safety and suggests that the writer is unable to better express his thoughts. Remember that when reducing thoughts to text it is not correct to do so with the same sentence structure you use when conversing with a colleague or friend.
In the case of widely dispersed material it is also appropriate to consider that the audience may include people of varied ages and genders. I hope you take this constructively.
Thanks. We washed our keyboards out with soap.