The 9 Deadly Sins of Aircraft Performance
Many of our members attended a FAAST seminar about aircraft performance recently. It’s a very important subject, especially to those of us flying in mountainous terrain, in single-engine normally aspirated airplanes! Mr. Stephen Ruks did an excellent job, and agreed to share the powerpoint slides & notes with us to post here. Enjoy reading – and be safe!
The Nine Deadly Sins POWERPOINT:
Slide 2 – References
Pilots Handbook of Aeronautical Knowledge Ch. 7 & 10
Your AFM or POH
AC 00-6A Aviation Wx
AC 61-84B Role of Preflight Preparation
FAA-P-8740-2 Density Altitude Brochure
Slide 3 – Wings Program – What is one of the best things you can do to stay safe?
Chances are 97% greater that you won’t have an accident if enrolled in Wings
Slide 4 – Introduction – Misunderstandings or lack of knowledge about our atmosphere and how it affects aircraft performance is one of the leading causes of fatal aviation accidents. This seminar will remind you of some of the things you already know and maybe show you a few things you didn’t know.
Slide 5 – Altitude types – How many altitude definitions are there?
- Indicated altitude
- True altitude
- Absolute altitude
- Pressure altitude
- Density altitude
Read on your altimeter
Height above MSL
Height above ground level
Indicated altitude set at 29.92
PA corrected for non-standard temperature
- Indicated altitude—read directly from the altimeter (uncorrected) when it is set to the current altimeter setting.
- True altitude—the vertical distance of the aircraft above sea level—the actual altitude. It is often expressed as feet above mean sea level (MSL). Airport, terrain, 7-7 and obstacle elevations on aeronautical charts are true altitudes.
- Absolute altitude—the vertical distance of an aircraft above the terrain, or above ground level (AGL).
- Pressure altitude—the altitude indicated when the altimeter setting window (barometric scale) is adjusted to 29.92 “Hg. This is the altitude above the standard datum plane, which is a theoretical plane where air pressure (corrected to 15 °C) equals 29.92” Hg. Pressure altitude is used to compute density altitude, true altitude, true airspeed (TAS), and other performance data.
- Density altitude—pressure altitude corrected for variations from standard temperature. When conditions are standard, pressure altitude and density altitude are the same. If the temperature is above standard, the density altitude is higher than pressure altitude. If the temperature is below standard, the density altitude is lower than pressure altitude. This is an important altitude because it is directly related to the aircraft’s performance.
Slide 6 – Pressure – How do we describe atmospheric pressure, what is it, and how does it affect us? ^ More importantly, how does it affect our airplanes?
Slide 7 – Pressure – What causes pressure variations?
^ Altitude – Pressure decreases approximately one inch of mercury per 1000 feet increase in altitude.
^ Temperature – The rate of decrease of pressure with altitude in warmer air is less than in colder air. You will have to climb higher in warm air to reach the same pressure altitude as in cold air. Temperature id the biggest factor affecting density altitude
Slide 8 – Pressure Gradient – Given the same change in pressure, the rate of change of pressure is greater in cold air than in warm air.
Slide 9 – Effect of non-standard temperature – Notice that True Altitude varies with variations in temperature along the same pressure line or gradient.
Slide 10 – Typical GA Pitot System – Explain disadvantage of having static port on only one side of aircraft. Briefly discuss effects of icing on pitot-static system. If both ram and drain holes freeze up with ice, airspeed indicator acts as an altimeter – the higher you go the higher the airspeed reads. If static port freezes up, altimeter and VSI remain constant and airspeed is not accurate.
Slide 11 – Altimeter – Give brief description of how an altimeter works.
Slide 12 – Altimeter Setting – Standard Atmosphere:
29.92 inHg or
1013.25 hPa (or mb)
at sea level @ 59º F (or 15º C)
One inch of mercury = 1000 feet
One hectoPascal = 100 millibars
1 inHg = 33.8653 hPa — Use this for conversion between inHg and hPa
Slide 13 – High Density Altitude
Three important factors contribute to high density altitude:
Reduced air density equates to decreased performance
Explain chart below:
Slide 14 – High Density Altitude
Warmer air will hold more water vapor than cooler air which affects engine performance more than aerodynamic performance.
With high humidity, expect longer take-off rolls and lower climb rates. Add 10% to take-off distance.
Slide 15 – The Nine Deadly Sins (and their unforgiveness)
Kurt Anderson, an NTSB Investigator, who has investigated more than 400 accidents, gave a seminar on the results of interviews with many of the pilots who survived accidents and discovered nine things that pilots learned and then forgot, never learned at all, or learned wrong. Those nine things have been dubbed “The Nine Deadly Sins.” Scott Gardiner of Seattle FSDO published, in FAA Aviation News, May-June 2004, the article discussing those nine deadly sins. Now I’m going to present them to you.
- Using sea level VSPEEDS
- Using gross weight VSPEEDS
- Ignoring effect on TAS
- Ignoring effects in mountainous terrain
- Ignoring effects on landing speed and distance
- Ignoring climb gradient
- Ignoring runway environment
- Using incorrect flap settings
- Combining soft-field and obstacle techniques
“Practical Density Altitude” by Scott Gardiner, FAA Aviation News, May-June 2004
Slide 16 – Sin #1 – Using Sea Level VSPEEDS
Slide 17 – Sin #1 – Using Sea Level VSPEEDS
If you mistakenly attempt to climb at your sea level indicated best angle of climb speed, you are probably four to seven knots too slow. You have taken an airplane whose climb performance may be poor at best and made it downright lousy! There is a really good chance the airplane will not climb at all and will simply mush into the obstacle as you will see later on in this presentation. You will also see later that combining techniques can prove to be hazardous. Use best rate of climb for the density altitude you are flying and never best angle of climb except for nearby obstacles.
There are some serious drawbacks to using best angle. A full power climb results in a high pitch angle (so much for see and avoid), poor engine cooling, close to stall speed, and may be uncomfortable for passengers. Avoid best angle of climb.
Slide 18 – Sin #2 – Using Gross Weight VSPEEDS
Vy decreases with decrease in gross weight
If you attempt to climb out of a high density altitude airport at a reduced gross weight while using your sea-level, maximum gross weight best rate of climb speed (indicated), you combine Sin#1 with Sin #2. The result can easily be that you are attempting to climb at a speed that could be 15 knots too fast! Such a mistake can turn minimal climb performance into negative climb performance! This deadly combination is precisely what is leading to our most common density altitude accidents!
Explain the effect of DA on TAS using above chart. See next page . . . .
Slide 19 – Sin #3 – Ignoring effect on TAS
Turn radius increases by the square of the TAS. The formula is TAS2/11.26 tan Θ. Assuming IAS of 150 @ sea level, standard temperature, the turn radius would be ≈ 2000’, or one-third of a mile, But at 8000’ with a temp of 95º your TAS is now 180 and the turn radius increases to ≈ 3000’. But remember! That’s turn radius – you have to double that for a 180º turn! You now need a full mile to turn around and that’s using a 45 bank angle for the maneuver. How much more power do you think you’ll need at 60 degrees of bank – remember the drag goes up 300%!
So you tell yourself, “Just slow down, turn radius will decrease.” But now with this increased AOA how much more drag are you adding. Remember, the slower you fly, the higher the induced drag, and it’s multiplied by 300%. Does your engine have enough power reserve? Very doubtful, that’s why we have the accidents.
IAS = 150
PA = 8000
Temp = 95ºF
TAS = 180
DA = 12,000
Looking at this Lift-Drag Chart, I want to throw an additional factor into it. Consider an airplane in a 30 degree bank. Induced drag increases by 33%. At 45 degrees, it increases by 100%, and at 60 degrees it increases by 300%. This is not shown on a normal L-D chart but imagine how much power your engine must now develop to maintain altitude in a 60 degree bank at a high DA (12,000’) where your engine is struggling to maintain power and altitude.
Slide 20-21 – Sin #4 – Ignoring the effects in mountainous terrain.
Considering all the information I’ve just presented to you, let’s look at the three airplanes flying straight out of the screen. Consider the effects of wind as depicted by the arrows Strong downdrafts for airplane #1 requiring additional power to maintain altitude. Airplane 2 is scooting along normally, and airplane 3 is enjoying the advantage of the updraft.
“Flying in the vicinity of a ridge results in downdrafts for the pilot of Airplane 1. Airplane 2 might escape the downdrafts, but a course reversal either to the right or to the left would leave little maneuvering room between the airplane and the ridge. Airplane 3 takes advantage of free lift from the up slope airflow and retains the advantage of an into-the-wind escape route.” The official explanation is technically correct, but it does not go far enough. Since it is questionable whether or not Airplane 2 can complete a 180-degree turn, we can assume this valley is not very wide. Most pilots choose to fly up the correct side of the valley (Airplane 3 in this case), but push on too far before deciding to reverse direction. As long as things are going well for Airplane 3, the pilot continues bravely on course. It’s only when things get tight that the pilot of Airplane 3 decides to make the 180 but turning around at this point results in a radius of turn that places the airplane somewhere between Airplanes 1 and 2. This is precisely the valley location described in the official explanation as an area of downdrafts! The trap has been sprung. Another aircraft smacks the terrain and often with fatal results.
Slide 22 – Sin #5 – Ignoring Effects on Landing Speed and Distance
Often times the discussion of density altitude is limited to takeoff distance and maybe even climb performance. But you have to think of what the airplane is doing when approaching to land as well. Remember the TAS is higher than IAS at density altitudes above sea level so that has to be considered. Imagine a worse case scenario of a runway at 8000’ elevation that downslopes to the edge of a cliff and is only 2500’ long and you have a 10 knot tailwind. So down you go to the landing. You get about halfway down the runway where you finally touch down and you realize that you may not be able to stop so smart pilot that you are you add power for a go-around forgetting that you placed the mixture in full rich for the landing. Are you going to have enough power for the go-around and be able to clear all obstacles in the process? Do you see how these hazards add up and why it is so important to plan ahead? Consider all the variables before operating in high density airports.
Slide 23 – Sin #6 – Ignoring Climb Gradient
When departing airports, be aware of your climb gradient. We are all familiar with aircraft rate of climb — it’s figured in terms of feet per minute. Climb gradient is figured in terms of feet per mile. Consider two airplanes, each climbing at 500 feet per minute. But one is climbing at 60 knots, and the other is climbing at 90 knots. Each will climb 500 feet in one minute. But the first will cover one mile during that minute, and the second will cover a mile and a half during the same minute. The first airplane is climbing 500 feet per mile, and the second is climbing only 333 feet per mile.
Remember that climb gradients are calculated in feet/nm not feet/min. And be sure to use TAS (or GS) for calculations not IAS.
Consider the ILS approach to Ketchikan, Alaska. Minimums are 288’ but the required climb gradient for the missed approach requires 335’/NM to miss a 4000’ mountain directly in front of you! Localizer circling minimums are 2440’.
Slide 24 – Sin #7 – Ignoring Runway Environment
Watch the video clip of the A36 takeoff and think about what might have gone wrong.
OAT = 97F, Field Elev =1293’ Altimeter 29.89”
Density Altitude = 4125’, Rwy 31 4000’ paved
1996 A36, Turbonormalized IO-550,
GTOW = 4095, CG = 86.15”
Different piloting techniques may have resulted in a much nicer outcome.
Now think of departing at our 8000’ elevation runway with the temperature at 95 degrees (DA 12,000’). How well is that going to go?
Slide 25 – Sin #8 – Using Incorrect Flap Settings
In the A36 incident, use of flaps may have made the situation even worse!
Use of the recommended flap setting works just fine when operating at near sea level altitudes but with non-turbocharged engines, there is a density altitude above which the use of takeoff flaps actually increases ground roll. Be sure to check the POH to ascertain if the use if flaps is recommended at high DA.
Page 26 of the A36’s POH Supplement stated, “However, when operating at the increased weights authorized when operations are conducted in the NORMAL CATEGORY expect the following:
- Increased Takeoff Distance of up to 30%.
- Decreased Rate-of-Climb of up to 13%.
- Increased Stall Speed of up to 7%.
- Increased Landing Distance of up to 15%.
- Increase Takeoff and Approach Speeds 2 kts.
- Increase VX and VY Speeds 2 kts.”
Soft field techniques should be saved for soft fields. Just because the runway is of dirt or grass, it is not necessarily soft! Mud in which you leave 3” tracks, beach sand, 6” of snow, or 3” of sleet is a soft field.
Slide 26 – Sin #9 – Combining Soft-Field & Obstacle Techniques
Normally flaps are only used on soft field takeoffs but when used for obstacle clearance, the increase in drag drastically hinders climb performance.
There are numerous instructors out there who routinely combine obstacle takeoff techniques with soft-field takeoff techniques to save time during training. But in actual density altitude situations, a pilot should not combine the two in a normally aspirated, piston-engine airplane.
If we are trying to clear a 100’ tree, we are talking about obstacle clearance takeoffs
not minimum ground-run takeoffs.
For obstacle clearance takeoffs, follow the advice of your airplane manufacturer, which for the vast majority of non-turbocharged airplanes means flaps up and climb at best angle of climb speed for the density altitude.
Always follow manufacturer’s recommendation.
Slide 27 – Summary
Beware, better yet, be knowledgeable, of the Nine Deadly Sins of Density Altitude
Remember the effect of DA on TAS and the TAS effect on airplane performance and humidity on engine performance.
Engine performance degrades with altitude