May 9, 2012
QUIT DOING THIS OR DIE!!! (MYTHS of aviation, some of which can kill you)
(Note: The topics on the need for left-rudder in propeller airplanes, and not stalling when turning downwind, were first made clear to me by Peter Garrison in various articles he has published, but I joyfully take full credit for the inflammatory nature of my writing, and all of the other topics. Some contribution was also made by Flight Instructor Rob Coppock).
MYTH: IF YOU SET YOUR ALTIMETER PROPERLY, YOUR ALTIMETER WILL INDICATE YOUR ALTITUDE.
WRONG! Have you ever noticed that ATIS gives you the local barometric pressure, BUT NOT THE PRESSURE LAPSE-RATE??? We have a standard altimeter (29.92″) and a standard pressure lapse-rate. Of COURSE, we do NOT see those exact conditions each day we fly! To correct for the ever-changing vagaries of nature, ATIS lists the local pressure (at sea-level, assuming standard lapse-rate) that we enter into our altimeters (which always assume some standardized lapse rate as well). Since both the ATIS and our altimeters assume the same standardized lapse rate, if we dutifully enter that pressure into our altimeters, our altimeters will be perfectly accurate… AT THE AIRPORT ELEVATION! We can easily get altitude accuracy to within ten feet! BUT, as we CLIMB, we climb into air whose pressure lapse-rate is NOT the same everywhere! Because our altimeters are based on pressure, they indicate increasingly erroneous values as we climb! In fact, when I check my GPS altimeter versus air-pressure altimeter, it is normal to see an error of FIFTEEN feet on the airport runway, and ONE-THOUSAND-FIVE-HUNDRED FEET AT 25,000 FT!!! (And no, I did NOT dial 29.92 into my altimeter to fly a flight-level! This was with the most accurate altimeter setting I could get for the area! I simply got a flight-level block from ATC to allow me to do this without crashing into anyone). So, when my altimeter was indicating 24,000 ft in the G-1000 of my Columbia-400, the airplane was actually at 25,500 feet! (I know this from my GPS tachometer, also built into the G-1000).
NOTE: This is NOT an insult against the accuracy of any Garmin instrument… I am certain that my Garmin is calibrated perfectly! It is checked every annual, and matches my standby altimeter PERFECTLY! My airplane is in perfect condition: New, calibrated, tested, and backed-up. But the static-port altimeter is perfectly matched to the GPS altitude on the GROUND, but off by WELL OVER A THOUSAND FEET up at 25,000 ft or so. This is all because while ATIS DOES report the baro pressure, they do NOT report the PRESSURE LAPSE RATE! So, when you think about it, how could you even EXPECT your altimeter to be accurate up high, since pressure lapse-rate will vary, just like the barometric pressure does?!!?!?
What is really fascinating is that this hardly matters: ALL of the airplanes are calibrated to run on the SAME standard lapse rate, so ALL of the airplanes will have the SAME error, so no airplanes will hit each other! As well, since the error goes to zero at the airport elevation where the pressure is measured, your altimeter will always be accurate as you approach the airport, so your instrument landings always work out just fine!
So, high in the sky, our altimeters are WAY off, but we don’t care because we are far from landing and all the planes have the SAME error so they do not smash into each other when asked to hold certain altitudes.
So this does not really matter, right?
Say you are in the clouds or in a foggy dark night (on an IFR flight-plan or NOT!!) Flying at 10,000 ft, using an altimeter from a nearby airport that is near sea-level. Since you are high above the airport, a non-standard lapse rate may cause an altimeter error of perhaps 800 feet. This may seem fine, since you are way above the airport and all the other airplanes in the area have the same error, so you will not hit them…
But what about that mountain peak that juts up in front of you to 9,500 feet?
MYTH: I CAN FULLY DEFLECT THE CONTROLS BELOW MANEUVERING SPEED!
WRONG! BELIEVE THIS AND DIE! The wing structure in light planes is usually certified to take +3.8 G’s, -1.52 G’s (plus a 50% safety factor). Put more load on the wing than that and you should consider yourself dead.
But here is the nice part: Below a certain speed, the wing simply cannot put out a full 3.8 G’s of lift! It will STALL first! This speed is called “Maneuvering Speed”. “Maneuvering Speed” is defined as the maximum speed the plane can be moving and still STALL before the WING BREAKS no matter how much you pull back on the stick.
If you are going slower than this and you pull all the way back on the stick, the wing will STALL WITHOUT PHYSICALLY BREAKING. If you are going faster than this and you pull all the way back on the stick, the wing can put out so much lift that it can be expected to break. So, as a result of the info above, people think they can deflect the stick as much as they like below maneuvering speed and stay alive. WRONG! The maneuvering speed is based on pulling BACK on the stick, NOT PUSHING FORWARDS!
Note what I said above: The “Maneuvering Speed” is defined as how fast you can go and not be able to put out more than 3.8 G’s of lift… but the while the plane is certified for POSITIVE 3.8 G’s, it is only certified for a NEGATIVE G-load of 1.52 G’s!!!!! In other words, you can fail the wing in the NEGATIVE direction by pushing FORWARDS on the stick well BELOW the maneuvering speed!!!
Nobody know this. They just think they can fully-deflect the stick below Maneuvering Speed and live but THAT IS ONLY TRUE FOR PULLING THE STICK BACK AND BEING GUARANTEED 3.8 G’s OF POSITIVE G-LOAD STRUCTURE, NOT PUSHING FORWARDS, WHERE THE WING CAN FAIL AT A MEASLY 1.52 G’s!
Also, for airliners, certification requirements require that the rudder can be fully-deflected below maneuvering speed, BUT ONLY IF THE PLANE IS NOT IN A SIDESLIP OF ANY SORT! WHAT A LOAD CRAP! In a wonderfully-timed accident shortly after Sept 11 2001 that everybody thought might be terrorism, an Airbus pilot stomped the rudder in wake turbulence while the plane was in a considerable sideslip. The COMBINED loads of the sideslip and rudder deflection took the vertical stab to it’s critical load! A very simple numerical analysis based on the black box confirms this. The airplane lost it’s vertical stab in flight and you know the rest.
Also, if you are at your maximum allowable G-limit (say 3.8) and you put in some AILERON CONTROL, you are actually asking for MORE LIFT FROM ONE WING THAN THE ALLOWABLE LIMIT! SO COMBINED ELEVATOR AND AILERON CAN BREAK THE PLANE, EVEN IF THE ELEVATOR IS POSITIVE-ONLY!
SO, WHEN YOU THINK THAT YOU CAN DO AS YOU PLEASE WITH THE CONTROLS BELOW MANEUVERING SPEED, YOU ARE WRONG!!!!! You instructor was wrong. The flight training manual was wrong. Your examiner was wrong. The FAA was wrong. I am right. Heed me. (and also look at a V-N diagram and the aircraft certification limits to prove it to yourself).
MYTH: AT POSITIVE ANGLE OF ATTACK, THE DESCENDING PROP BLADE IS AT A HIGHER ANGLE OF ATTACK, SO IT PUTS OUT MORE LIFT, PULLING THE PLANE LEFT.
NOT THE WHOLE PICTURE!!! You only THINK this because you stand on the ramp beside your airplane on the ground, looking at the prop hub, imagining the nose pointing up and therefore the right-hand blade seeming to take a greater “bite” out of the air. Here is where your visualization is WRONG: If the nose is pointed up with respect to the air stream, then that right-hand prop blade is actually NOT DESCENDING STRAIGHT DOWN LIKE YOU ARE VISUALIZING, BUT IS INSTEAD MOVING FORWARDS A BIT AS IT DESCENDS, SO THE ANGLE OF ATTACK IS NOT INCREASED LIKE YOU THINK ON THE DESCENDING BLADE!! So why DOES the plane pull left in a climb even though the descending blade is NOT at a higher angle of attack? Look to helicopters for the answer. Their ADVANCING blade (the main rotor blade that is coming FORWARD) wants to put out a LOT more lift since it is moving at it’s rotational speed PLUS the speed of the aircraft. The RETREATING rotor is traveling a lot SLOWER!! It is traveling at it’s rotation speed MINUS the speed of the craft. So there tries to be a lot more lift on the right side of the rotor system, and the pilot has to enter a lot of correction in blade pitch to fly straight! A climbing airplane is the same: The DESCENDING blade is actually ADVANCING INTO THE AIR A LITTLE BIT (THUS MOVING FASTER) SINCE THE PLANE IS TILTED UP! The CLIMBING blade is retreating away from the onrushing air a bit for the same reason! Thus, the DESCENDING blade puts out more lift from it’s HIGHER SPEED when the airplane is at high angle of attack. The descending balde is moving FASTER through the air.
MYTH: I MIGHT STALL IF I TURN DOWNWIND
WRONG! The myth is that if you are flying into a headwind (of say 20 knots, and you are indicating 100 knots airspeed, thus covering ground at 80 knots) then if you turn 180 degrees, the headwind will become a tailwind and you will find yourself with only 60 knots of indicated airspeed after the turn! (80 knots groundspeed with a 20 knot tailwinds leaves you with only 60 knots) WRONG!! MYTH! Of course you have a headwind before the turn, and a tailwind after, but this has no aerodynamic impact on your whatsoever. You can forget about the ground when doing (non ground-effect) aerodynamics. It means nothing. It is nothing but dirt underneath you that is not touching you, and that you tend to use as a reference since most places you go are conveniently located on it. You might as well use the moon as your reference, or the sun, or maybe the center of the Milky Way Galaxy. These are nothing but reference points, and have no impact on the nature of the flying machine. The aircraft is borne by the wind, and reference to GROUND SPEED has no impact at all on aerodynamics.
So, you might be saying: Wait a minute! If you have 100 knots of airspeed for the whole turn, then you must have come into the turn with 80 knots of ground speed, and come out of it with 120 knots of ground speed! You just got 40 knots of speed for free! IMPOSSIBLE!
No, you did NOT get 40 knots of speed for free. You simply aimed your plane in a different direction where it’s true reference-system (the moving air) HAPPENS to be going in the same direction as the reference-system you keep using (the ground). Just think of the air as being still and the earth moving underneath it at 20 mph. Now imagine the airplane making a 180-degree turn through the nonmoving air. Nothing special, right? But your speed over the EARTH was different before and after the turn, because of the movement of the EARTH happening to be in the direction as the plane or not. Now you should see that the downwind turn has no impact on speed at all since you are moving with the AIR. The Earth can rotate underneath you however it likes… it makes no difference to you. You only THINK it does because the millions of years of evolution that caused our brains to turn out the way they did has us thinking about things in reference to the GROUND because that is how we evolved to move: With our LEGS over the GROUND. It is not natural for you to quit thinking about the EARTH reference and use the AIR as a reference instead, so we (sometimes) INCORRECTLY think of the ground as a reference… even when flying!
Someone emailed me in response to this, respectfully pointing out that if flying North into a 20 kt headwind and then turning to the South, it still seems dangerous to him since how could the airplane pick up that extra 20 knots in the downwind turn to the South to hold his precious airspeed? The answer: IT DID NOT HAVE TO PICK UP 20 KNOTS OF SOUTHERLY SPEED AS IT TURNED… THAT 20 KNOTS OF SOUTHERLY SPEED WAS THERE BEFORE THE TURN EVEN BEGAN, IN THE FORM OF A 20-KNOT LOWER GROUNDSPEED AS HE FLEW NORTH! Again, it is nothing but a change in your reference system.
MYTH: THE OPERATING HANDBOOK TELLS ME WHAT THE PLANE CAN DO
WRONG! The Pilots Operating Handbook is based on a factory test-pilot tweaking a brand new plane in perfect condition to it’s absolute limits. You are flying a beat-up 20-year-old airplane that no longer makes rated power and no longer has the original airfoil shapes with its various dents and wrinkles and you have bugs all over the leading edge of the wings while the factory pilot did not. Now you may be thinking that you are as good as any factory test pilot and your engine is fine and that dead bugs make no difference but in fact you often have not practiced the same maneuver over and over in exactly the same conditions until you have squeezed every foot out of the takeoff or landing distance at a given altitude, temperature, and weight, like the test pilot did.
=>You are just arriving somewhere with passengers with 20 other things on your mind and are trying to stay in one piece. That is ENTIRELY DIFFERENT from a test-pilot flying the same routine over and over with sandbags in the plane imitating passengers, with nothing else to do but squeeze every foot of distance out of the plane.
=>The power reduction from your weaker old engine is significant (we all know that out of a fleet of rented Cessna 172’s, they all fly differently and have different performance).
=>As far as the dead bugs go, SIMPLY GETTING WET is enough to increase the drag on some airfoils by 50%.
When there is so much variability among airplanes, and test-pilots having nothing else on their minds but to optimize their flight technique while they practice the same maneuver 50 times in the same plane in identical conditions, you would be a fool to think that the Pilots Operating Handbook will actually tell you what your plane can do. There are plenty of cases to document this, but I was once landing a Cessna 172 at high altitude on a windy day with 4 people on board and when the wind proved too strong to land and I tried to go around the plane could barely manage it… it was all I could do to wallow along at minimum controllable airspeed, full flaps, barely able to stay in the air in level flight. This is not by any book. Another person tells me of a Cessna 152 that took OVER TWICE the distance to take off one day because the grass on his field was DAMP and a little LONG and he took off in the other direction from normal with the slightest upslope. Bottom line: Do NOT base your aircraft performance on the POH except to say “OK, this is the limit that I will never be able to break, but I might get about 75% of this performance if I fly the plane right”.
PS: For those of you that will email me saying: “Austin, that is great you are shedding some light on this, but you don’t have to be so acerbic”, here is my reply:
“You are right, I don’t have to be so acerbic, but I am feeling grumpy now so I am getting a perverted pleasure from being acerbic anyway”.
NOTE: SINCE I HAVE POSTED THIS EMAIL, I HAVE GOTTEN A NUMBER OF REPLIES FROM PEOPLE, AND I HAVE BEEN JOYOUSLY SURPRISED AND ENCOURAGED BY EVERY ONE OF THEM! EVERY SINGLE PERSON ALREADY KNEW ABOUT THESE THINGS, OR WAS HAPPY TO REPORT THAT THEY ARE GLAD TO HAVE BEEN EDUCATED… SOME PEOPLE EVEN SENT ME EMAILS CORRECTLY CORRECTING SOME ERRORS THAT I MADE!!!! (WHICH HAVE SINCE BEEN CORRECTED IN THE DRAFT YOU SEE ABOVE) THUS EDUCATING ME FURTHER!!! EXCELLENT!! EXCELLENT!! EXCELLENT!!