Performance and Limitations Flashcards
Factors Affecting Performance
Atmospheric conditions Pilot technique Airplane configuration Airport environment Center of gravity CG loading Weight and Balance
Loading CG Forward
- better controllability
- better stability
- worse maneuverability
- recover easier from stalls
- worse fuel burn
- worse range
- worse TAS
Loading CG Aft
- worse controllability
- worse stability
- better maneuverability
- hard to recover from a stall
- better fuel burn
- better TAS
Air Density relative to Altitude
Air density decreases with altitude
At high elevation airports, an airplane
requires more runway to take off. The aircrafts rate of climb will be less and the
aircrafts approach will have to be faster, to stabilize the effects of lower air
density.
Air Density relative to Temperature
Air density decreases with temperature
Warm air is less dense than cold air because there are fewer air molecules in a given volume of warm air than in the same volume of cooler air. As a result, on a hot day, an airplane will require more runway to take off, will have a poor rate of climb and a faster approach and will experience a longer landing roll.
Describe the combination of high temperature and high elevation and how it effects the performance of an airplane
The combination of high temperature and high elevation produces a situation that aerodynamically
reduces drastically the performance of the airplane.
The horsepower out-put of the engines decrease because its fuel-air mixture is reduced.
The propeller develops less thrust because the blades, as airfoils, are less efficient in the thin air.
The wings develop less lift because the thin air exerts less force on the airfoils.
As a result, the take-off distance is substantially increased, climb performance is
substantially reduced and may, in extreme situations, be non-existent
Humidity
Although it is not a major factor in computing density altitude, high humidity has an effect on engine power. The high level of water vapor in the air reduces the amount of air available for combustion and results in an enriched mixture and reduced power
Density Altitude
Density altitude is pressure altitude corrected for temperature. It is the altitude at which the airplane thinks it is flying based on the density of the surrounding air mass.
We relate aircraft performance to density altitude
Calculating Density Altitude
Density altitude in feet = pressure altitude in feet + (120 x (OAT - ISA temperature))
- Pressure altitude is determined by setting the altimeter to 29.92 and reading the altitude indicated on the altimeter.
- OAT stands for outside air temperature (in degrees Celsius).
- ISA stands for standard temperature (in degrees Celsius).
Keep in mind the standard temperature is 15 degrees C but only at sea level. It decreases about 2 degrees C (or 3.5 degrees F) per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C (or 34 degrees F).
For example, the density altitude at an airport 7000 feet above sea level, with a temperature of 18 degrees Celsius and a pressure altitude of 7000 (assuming standard pressure) would be calculated as follows.
- ) 18 – 1 = 17
- ) 17 x 120 = 2040
- ) 2040 + 7000 = 9040 feet Density Altitude
This means the aircraft will perform as if it were at 9,040 feet.
Calculating Density Altitude (Option 2)
Pressure altitude = (standard pressure - your current pressure setting) x 1,000 + field elevation
That’s a pretty simple formula since two of the variables will always be the same and the other two are easy enough to find. Let’s say our current altimeter setting is 29.45 and the field elevation is 5,000 feet. That means (29.92 - 29.45) x 1,000 + 5,000 = 5,470 feet.
density altitude = pressure altitude + [120 x (OAT - ISA Temp)]
We already have the value for pressure altitude from our last calculation; OAT is degrees Celsius read off our thermometer (let’s say it’s a balmy 35 °C today) and ISA Temp is always 15 °C at sea level. To find ISA standard temperature for a given altitude, here’s a rule of thumb: double the altitude, subtract 15 and place a - sign in front of it. (For example, to find ISA Temp at 10,000 feet, we multiply the altitude by 2 to get 20; we then subtract 15 to get 5; finally, we add a - sign to get -5.)
So, in the example above: density altitude = 5,470 + [120 x (35 - 5)]
Working out the math, our density altitude is 9,070 feet.
Pressure Altitude
Pressure altitude is the attitude displayed on the altimeter when the Kollsman window is set to 29.92 inches of mercury, or 1013.4 millibars.
Pilots cannot use pressure altitude below 18,000 feet, because then the aircraft’s true altitude would change depending on temperature. That is why every time a pilot checks in with a new air traffic controller, the local altimeter calibration is necessary. The altimeter indicates how high the airplane is above sea level by calculating the difference between the pressure in the aneroid wafers and the atmospheric pressure fed into the static port.
How do you calculate pressure altitude?
The pressure altitude can be determined by any of the three following methods:
1. By setting the barometric scale of the altimeter to
29.92 “Hg and reading the indicated altitude
2. By applying a correction factor to the indicated
altitude according to the reported “altimeter setting,”
3. By using a flight computer
OAT
Outside Air Temperature
Calibrated Airspeed (CAS)
Indicated airspeed corrected for instrument and position error.
When flying at sea level under International Standard Atmosphere conditions (15 °C, 1013 hPa, 0% humidity) calibrated airspeed is the same as equivalent airspeed (EAS) and true airspeed (TAS). If there is no wind it is also the same as ground speed (GS). Under any other conditions, CAS may differ from the aircraft’s TAS and GS.
True Airspeed (TAS)
Calibrated Airspeed (CAS) corrected for density altitude
True airspeed is the speed of your aircraft relative to the air it’s flying through.
As you climb, true airspeed is higher than your indicated airspeed. Pressure decreases with higher altitudes, so for any given true airspeed, as you climb, fewer and fewer air molecules will enter the pitot tube. Because of that, indicated airspeed will be less than true airspeed. In fact, for every thousand feet above sea level, true airspeed is about 2% higher than indicated airspeed. So at 10,000 feet, true airspeed is roughly 20% faster than what you read off your airspeed indicator.
Groundspeed (GS)
The movement of your airplane relative to the ground is called groundspeed.
It’s true airspeed corrected for wind.
With a true airspeed of 100 knots and a tailwind of 20 knots, you’d be flying a groundspeed of 120 knots
Loss of RPM During Cruise Flight (Non-altitude engines)
Probable Cause: Carburetor or induction icing or air filter clogging
Corrective Action: Apply carburetor heat. If dirty filter is suspected and non-filtered air is available switch selector to unfiltered position.
High Oil Temperature
Probable Cause/Corrective Action:
Oil congealed in cooler - Reduce Power. Land. Preheat engine
Inadequate engine cooling - Reduce Power, increase airspeed
detonation or preignition - Observe Cylinder Head Temps (CHT) for high reading, reduce manifold pressure, enrich mixture.
forth coming internal engine failure - Land as soon as possible or feather propeller and stop engine
Defective thermostatic oil cooler control - Land ASAP. Consult Maintenance Personnel
Low Oil Temperature
Probable Cause/Corrective Action:
Engine not warmed up to operating temperature - Warm engine in a prescribed manner
High Oil Pressure
Probable Cause/Corrective Action:
Cold Oil - Warm engine in prescribed manner
Possible internal plugging - Reduce power, land ASAP
Low Oil Pressure
Probable Cause/Corrective Action:
Broken Pressure Relief Valve - Land ASAP or feather propeller and stop engine
Insufficient Oil - same as above
Burned out bearings - same as above
Fluctuating Oil Pressure
Low oil supply, loose oil lines, defective pressure relief valve - Land ASAP or feather propeller and stop engine
