May 2000 Issue

The Numbers Game

Airspeed calculations show not all pilots know what their airplanes can do

I have two questions regarding the article “Landing at the Max” [Airmanship, March] You state:” … with a 46 KCAS stall speed. Enter the flare in a 172 at 65 knots instead of 60 and you have 2.25 times as much kinetic energy to dissipate before the airplane will stop flying.”

My first question is: “How did you compute the factor of 2.25?” Here is how I analyze this problem. Kinetic energy is proportional to the square of ground speed. For simplicity I’ll assume that CAS is equal to GS. The airplane stops flying at its stall speed of 46 knots, therefore the KE (kinetic energy) that must be dissipated before the airplane will stop flying is the difference between the KE at entry and the KE at stall speed.

With entry speeds of 65 and 60 knots and a stall speed of 46 knots, these energy differences are proportional to 65x65-46x46=2109, and 60x60-46x46=1484. 2109/1484=1.42.

You also state: “Likewise, in a normal 5 to 6 degree visual glide path …” Where is a 5 to 6 degree glide path defined as normal?”

Paragraph 2-1-2 of my AIM states: “ Two bar VASI installations provide one visual glide path which is normally set at 3 degrees. … normal glide path angles are three degrees, angles at some locations may be as high as 4.5 degrees...”

-John Lawton
Via e-mail

Mr. Levy replies: Your calculation is true only if you fly the landing all the way to a full stall, which almost no one does in conventional gear planes. Rightly or wrongly, virtually all landings are made with the airplane still flying, usually about 5-10 knots over stall speed, with the stall horn just starting to sound as you touch, if it does at all.

You are completely correct in your methodology. Given the 5 to 10 knots above stall speed most folks achieve at touchdown (from just a bit after the horn first sounds to a solid indication, depending on how lazy the pilot is in holding it off), the answer would be about 1.63 to 2.35 times the energy to bleed off. But for a well-trained taildragger pilot who makes full stall landings, your number is correct.

As for the glide path, you are correct as regards standard instrument approach procedures and for large aircraft. However, light aircraft in the VFR pattern normally fly a much steeper approach, typically turning 3/4 mile final at about 400 feet agl, which is a 5 to 6 degree glide slope depending on whether your mile is nautical or statute.


When No Calibration is Indicated
Have been reading my trial subscription, particularly the lead article “Landing at the Max” [Airmanship, March].

My trainer is a Cessna 152, and its 1984 info manual lists the flaps up KIAS as 40 and KCAS as 46, the lowest speeds shown in the table. Vso is shown elsewhere as 35 KIAS. How do I determine KCAS for the 35 KIAS so as to be able to follow the procedures in the article?

-Mikhail J. Brown
Huntsville, Ala.

Our 1981 info manual on a Cessna 152 shows zero-bank, flaps up stall speeds at max gross as 36 KIAS (46 KCAS) with an aft cg and 40 KIAS (48 KCAS) with a forward cg. As your manual had different information, we wouldn’t necessarily apply these earlier-model numbers.

Does your plane actually stall at 35 KIAS? Try a stall series and see where the break actually occurs. If it’s a bit higher than 35 KIAS, say at 37 or 38, then the difference between the 40 KIAS given in the manual and your real life experience is negligible.

If it’s less than 35 KIAS, then the indicator may need to be calibrated anyway.


Calculation Information
I enjoyed Ron Levy’s excellent article “Landing at the Max,” but question his calculation of corrected airspeed [Airmanship, March]. Mr. Levy discusses the actual weight of the Grumman Cheetah after two hours of flight and its effect on the stall speed. He states: “The airplane is now down to 1,850 pounds, only about 84 percent of its 2,200-pound max gross weight. Since stall speed changes by the square root of the gross weight, the stall speed is now 92 percent of the book value.”

I would like to know how he arrived at the value of 92 percent. When I take the value of 84 percent and subtract this from 100 percent I get a 16 percent change in gross weight. If the stall speed changes by the square root of the change in the gross weight, the square root of 16 is 4. Therefore, the stall speed should be 96 percent of the book value (not 92 percent), correct? Am I calculating this correctly?

-Angelo Petropolis
Sierra Vista, Ariz.

Almost. Instead of taking the square root of the difference and subtracting from 100 percent, you have to take the square root of 84 percent, which is 91.7 percent. See the next letter for more info.


Gross Weight Reductions
Thanks for reminding us of the change in max glide speed as weight changes [Instrument Check, March]. My POH is mute on this point, but I know a lot of the other speeds are for MGW, and change with a formula as the weight decreases below MGW. Is max glide one of them, and if so, tell us the formula so we can make some tables/graphs.

-Drew Doorey
Via e-mail

A good rule of thumb is to take one-half of the percentage below max gross weight, and subtract that from the published max gross glide speed. For example, flying two people in our Lance means we’re flying 14 percent below max gross. Divide by two to get 7, subtract from the published best glide of 92 knots, and you get 85 knots.

To be more accurate, take the square root of your percentage of gross weight and multiply that by the max gross speed. In the example we used, that would yield 85.3 knots.


How Serious is Serious?
In “Those Other Instruments” [Systems Check, March] the statement was made that “On carburetor-equipped engines, a sudden loss of fuel pressure that returns to normal with the boost pump on usually indicates a broken fuel line.”

That is an extremely broad generalization without enough information given for the scenario.

Perhaps Mr. Leis meant to write “a power loss accompanied by a sudden loss of fuel pressure that does not return to normal probably indicates a broken fuel line,” or “a normally running engine with a drop in fuel pressure that returns to normal with the boost pump on may indicate a fuel leak,” or “loss of fuel pressure accompanied by increased fuel flow (if your airplane is equipped) may indicate a fuel leak.”

If people take the advice given in that paragraph to shut off the fuel and prepare for a deadstick landing, there are likely to be some off airport landings in otherwise perfectly good airplanes.

I think better advice would be that any abnormal fuel pressure indication should cause you to be alert to the possibility of a fire and/or engine failure. Proceed to the nearest airport and make a power off landing, but be ready to turn off the fuel selector and make a deadstick landing somewhere else if there are any indications of fire.

In any case, refer to your POH and see if it addresses the issue. Be aware that if there is a fire you may have very little time. Twenty-five years ago we lost an Aztec and everyone on board when its wing separated less than two minutes after an engine fire started.

-Chuck Daly
Via e-mail

If you want to try to get to an airport, that’s your decision. But remember that a fuel-fed in-flight fire is far more dangerous than a deadstick landing.

You seem to want a less-extreme response to this emergency, but prudence demands otherwise. Both our diagnosis and opinion are backed up by Continental’s publication “Tips on Engine Care.”


Jargon Alert
In “Those Other Instruments” [Systems Check, March], you use the term “oversquare” several times. Can you tell me what it means?

-Mark Sheeter
Via e-mail

“Oversquare” refers to a kind of power setting in a piston plane with a controllable prop. If you’re not accustomed to flying a complex airplane, it may seem mysterious. With a fixed pitch prop, you control power by controlling RPM. With a constant speed prop, you control power by both manifold pressure, which is determined by throttle setting, and RPM, which is set by prop speed. Typically, most approved power settings are “square” or less. That is, the manifold pressure setting will be equal to or less than the RPM divided by 100. So a cruise setting may be square – 25 inches and 2,500 RPM – or less, such as 23.5 inches and 2,400 RPM. Oversquare is a realm of power settings where the MP is higher than the RPM divided by 100, such as 25 inches and 2,300 RPM. Many engines have an allowable range of so-called oversquare operation, but getting out of that range puts stress on the engine similar to driving a car with a manual transmission slowly in 5th gear.


Pulse Oximeters and CO
I read with interest the article by Bruce Chien regarding the use of pulse oximeters by pilots [Medical Matters, February]. While he did mention that “smokers lose up to 10 percent of their train cars from carbon monoxide tying up the hemoglobin,” he did not cover the fact that pulse oximeters can only indicate the saturation of the hemoglobin. They cannot differentiate between oxygen or CO tying up the hemoglobin. Thus smokers or anyone suffering from CO poisoning will show a high saturation (number) on the pulse oximeter. This could be significant for the very reasons he recommends using one, as the user could be becoming hypoxic while showing good numbers on the machine. Pilots still need to know and recognize the signs and symptoms of hypoxia, even with an aid such as the pulse oximeter.

-Keith Walker
Via e-mail

It’s true that pulse oximeters can’t tell you anything about carbon monoxide in your blood. A well-equipped cockpit should carry a separate carbon monoxide detector. And smokers should quit.


Weather Info Could Lead to Trouble
As a civilian (retired Navy) weather forecaster for the Navy, I was greatly disturbed by portions of “Stone-Cold Flying” [Risk Management, February]. Aside from the digs about the accuracy and reliability of weather forecasting in general (which any professional would take offense to), there were several questionable statements.

Mr. Leis says, “Icing sometimes shows up where none was supposed to be and is frequently absent from where it’s supposed to be.” Icing happens in a constantly changing environment. If the parameters needed for icing to be present are met, then icing is present. Whether ice accretion is encountered by an aircraft is really a different matter.

The recommendation of having the pilot “studying the temperature aloft conditions” causes me to wonder how many pilots might read this and conclude that the environment is no more than a snapshot, with no regard for warm or cold advection.

The paragraphs describing the way moisture is “carried” around a low pressure center is an incomplete analysis of the dynamics involved. Any forecaster who tells you that “both sides of the low are the same – with icing the same on either side” is not a true forecaster.

The dynamics are lengthy, but in extremely simplified terms, the moisture in the eastern section is being advected from the south or southeast. In this section of a low the temperatures are warmer (can hold more moisture) and moisture can easily be carried aloft to greater heights due to the upward vertical motion around the low.

The freezing level is higher in this section; therefore icing can be expected in higher elevations. Due to cold air advection on the west side of the low, the freezing levels are lower, the air cannot hold as much moisture and icing is generally encountered in lower levels, depending on the size and strength of the low pressure center.

Mr. Leis excluded the “possibility” that the low may be going through an occluded phase, which means the moisture at high levels in the atmosphere may be wrapping around the low, causing the probability of severe icing to be present in the northwest quadrant and possibly even the southwest quadrant, near the center of the low. Some of the worst weather conditions can be encountered in the vicinity of an occluded front.

If you take Mr. Leis’ article as gospel, you would think “the icing chances are dramatically reduced, if not eliminated,” as he stated in his article.

The ability to forecast weather today has vastly improved. Is it absolutely accurate? Heavens no! Is anything?

These sciences are not perfect because there are often too many variables to consider. In weather, the symptoms we refer to as conditions are constantly changing and forecasts must be altered accordingly.

-Wesley C. Miller
San Diego, Calif.

Unfortunately, weather reporting for GA fliers has become far removed from true forecasting. The purpose of the article was to present solutions for pilots operating in hazardous winter flying weather. We certainly couldn’t develop scenarios for every possible winter cold or warm front condition that might occur.


Breathing Oxygen on Lears
An associate forwarded me a copy of Aviation Safety’s discussion of the Payne Stewart Lear accident [Critical Moments, January]. In John Lowery’s otherwise excellent article, I did notice a few technical discrepancies pertaining to Lear 35 systems.

“Early model” Lear 35s (35-002 through 35-112) do not have a CABIN ALT annunciator light; they only have the aural warning horn and other features mentioned in the article. The O2 pressure gauge in the cockpit will always show actual bottle pressure regardless of valve position, as it is plumbed between the bottle and the regulator.

When the bottle is turned off, the distribution lines to the cabin are immediately vented to ambient, as evident by a slight hissing sound. The flow indicators in the crew masks will then show red and there will be absolutely zero pressure available at any of the masks, yet the O2 pressure gauge in the cockpit will continue to show whatever pressure is in the bottle.

This info is detailed in the maintenance manual, or better yet, try it out on an actual Learjet.

-Dan W. Vossman
Columbus, Ohio

You’re right. The Lear expert we had review the copy before publication did not pick up those discrepancies, and the training manual we consulted did not tell of the red in the flow indicators.


What’d He Teach?
I read with interest the John Lowery article about Payne Stewart’s Lear 35 crash. I noticed the statement on page 3 that, “...the captain was a former Air Force pilot who reportedly was an instructor in the high altitude physiology (altitude chamber) course.” I am currently an Air Force pilot and physiology instructor, so naturally I wanted to verify Lowery’s speculation. In my research of rated physiologists dating back a quarter century, I can find no record of Michael Kling as a physiology instructor. Do you happen to know Mr. Lowery’s source for this statement?

-Jeremy Horn
Via e-mail

Mr. Lowery was not speculating that he was an instructor, only referring to the fact that other media had quoted Sunjet president James Watkins saying Kling had been an instructor. We, too, have been unable to verify that information independently. Perhaps it was a case of resume padding.