Performance and Limitations Flashcards
Describe the composition of the atmosphere
78 percent nitrogen, 21 percent oxygen, 1 percent other gasses, such as argon and helium.
What do we mean when we talk about “atmospheric pressure”?
It is the force exerted on a given area by the weight of the atmosphere. Essentially, the atoms and molecules in the air that comprise the atmosphere have mass, and therefore gravity pulls on them, meaning they have weight. Since air is a gas, this weight is applied in all directions, causing a force called air pressure when the molecules collide with something.
What happens to air pressure as altitude increases, and why?
The weight of the atmosphere decreases, therefore the pressure decreases as well.
By approximately how much? does the pressure decrease?
-1 inch of mercury (Hg) per 1,000ft of altitude gain.
So if on a given day, sea level pressure is 30.00, what ambient air pressure would you expect to exist at 5,000ft MSL?
25.00, 5 inches fewer.
Obviously the altimeter setting is the pressure in inches of mercury that gets entered into the Khollsman window. But what exactly does your current altimeter setting represent?
It’s the value to which the pressure scale in the Khollsman window is set so that the altimeter
indicates true altitude at field elevation.
So does that mean that the altimeter setting represents the ambient air pressure at field elevation? In other words, does the altimeter setting equate with station pressure?
No, the altimeter setting represents what the baseline, sea level pressure would be in that
particular atmosphere at your location at that particular time. Put differently, the altimeter setting is station pressure (ambient air pressure at field elevation) corrected to mean sea level.
If I stand outside with a barometer and it shows 29.72, is that the altimeter setting?
No (although it would be if you’re standing outside at sea level). From there you would have to adjust/ratio it down to sea level pressure, then it becomes your altimeter setting. So if you could drill a well below you down to sea level, then lower the barometer down to the bottom, whatever the barometer shows down there would be your altimeter setting.
Say you are flying out of an airport located at sea level. You set 29.92 in the Kollsman window on a standard day. What altitude would the altimeter read?
0ft MSL.
Say you are flying out of an airport located at sea level. You set 29.92 in the Kollsman window on a standard day. What altitude would the altimeter read?
If at that point you walk away from the plane, then return the next day when the ambient air pressure is 28.92 (instead of the 29.92 that it was the day before when you set the altimeter), what altitude would the altimeter show now?
It would show approximately 1,000ft because a pressure of 28.92 is found at 1,000ft in the standard atmosphere; the altimeter is showing your pressure altitude.
Say you are flying out of an airport located at sea level. You set 29.92 in the Kollsman window on a standard day. What altitude would the altimeter read?
If at that point you walk away from the plane, then return the next day when the ambient air pressure is 28.92 (instead of the 29.92 that it was the day before when you set the altimeter), what altitude would the altimeter show now?
What would happen to your indicated altitude if at this point you adjusted the altimeter setting to 28.92?
It would show 0.
If there were an airport a mile away that was built up on a 5,000ft MSL cliff, what would you expect that airport’s altimeter setting to be?
Close to the same: 28.92. The field elevation alone shouldn’t influence your altimeter setting (if the airport is located in the same climate area). Airports have barometers that measure the ambient air pressure (station pressure), then that pressure gets ratioed down to what it would be at sea level, and that becomes the altimeter setting.
Say you are flying at a true altitude of 5,000ft MSL with an altimeter setting of 30.00. You fly into an area of low pressure where the altimeter setting should be changed to 29.00, except you don’t change it, it still shows 30.00. Will you be flying above or below the altitude shown on your altimeter? Explain why.
Below. At first, when the altimeter setting is accurate (as in, when we are flying in an atmosphere where the sea level pressure is actually 30.00), the pressure outside the plane at 5,000ft MSL is approximately 25.00 (30.00 - 5 inches, i.e. -1 inch for every 1,000ft of altitude gain). At this point, the accurately-set altimeter senses 25.00 and indicates our true altitude: 5,000ft. From here, as long as the altimeter setting doesn’t change, the altitude will always read 5,000ft when the altimeter senses a pressure of 25.00. Once the low pressure system moves in, the new atmosphere has a sea level pressure (the altimeter setting) of 29.00, meaning that the ambient air pressure of 25.00 that our altimeter is sensing (and we are tracking, because it corresponds to 5,000ft) is now found at a true altitude of 4,000ft (29.00 - 4 inches, i.e. -1 inch for each 1,000ft of altitude). Because our altimeter is essentially just a barometer that tracks pressure levels, it will track the 25.00 pressure down to a true altitude of 4,000ft. The altimeter is still using the old pressure scale where sea level pressure is 30.00, though, and on this scale the ambient air pressure of 25.00 will always correspond to 5,000ft. So 5,000ft MSL is still showing on the face of the instrument, and we are flying a true altitude 1,000ft below the altitude we think.
So your altimeter is set for 30.00, your true altitude is 4,000ft, and indicated altitude is 5,000ft. Now let’s say that you adjust your altimeter to what it should be: 29.00. What altitude will the altimeter read now, and why?
4,000ft. By entering 29.00 into the Khollsman window, you recalibrate so that when the altimeter senses 29.00 it will indicate a 0 altitude, sensing 28.00 will indicate 1,000ft, 27.00 will indicate 2,000ft, 26.00 will indicate 3,000ft, and our present ambient air pressure of 25.00 will indicate 4,000ft.
A cold front hits and the temperature drops severely. Are you now flying above or below the altitude indicated on the altimeter? Why?
Below. Cold air is denser than warm air, so the pressure levels become more compact; they get scrunched together, pulled down toward the earth. The pressure altimeter does not compensate for nonstandard temperature (we adjust for non-standard pressure when changing the altimeter setting, but there is no temperature equivalent). As a pilot, you will continue flying your desired indicated altitude, even though this altitude is now closer to the earth because the air is denser causing your pressure level to be lower (refer to figure 8-3 in the PHAK for a visual).
When flying over terrain, what is the most dangerous combination of pressure and temperature? Why?
Low pressure, cold temperature. In this case, your true altitude will be lower than what’s indicated, so you’ll think you have more terrain clearance than in actuality.
What expression can pilots use to help them remember that when pressures/temps are high, they will be flying higher than indicated; when pressures/temps are low, they will be flying lower than indicated?
“High to low, look out below; low to high, clear the sky.”
What is the definition of “pressure altitude”?
The height above the standard datum plane (SDP).
What does Standard Datum Plane mean?
A datum is just a reference point from which other things are measured. In this case, the SDP (just think datum) refers to an elevation and pressure used as a reference point. That elevation is sea level, and the pressure is 29.92. In the standard atmosphere, this pressure decreases 1 inch for every 1,000ft of altitude gain above the datum. So your pressure altitude is just your altitude above sea level when the pressure at sea level is 29.92. In other words, it is the altitude that the altimeter shows when the altimeter setting - that is, the sea level pressure - is 29.92. Put differently again, it is the altitude in the standard atmosphere that corresponds to the pressure the aneroid wafers are sensing inside the altimeter.
What is pressure altitude used for?
To calculate performance and to fly flight levels.
What are 3 methods a pilot can use calculate pressure altitude
1) Use the formula: (29.92 - altimeter setting)1,000 + field elevation; 2) set 29.92 into the Khollsman window and read the altitude shown on the altimeter; 3) use a pressure altitude table or graph, such as Figure 11-3 in the PHAK.
What does air density (aka atmospheric density) mean?
It means, the ratio of the mass (or weight) of the air to the volume it occupies. I.e. higher density means more air molecules in a given space.
What affects the density of the air, and how?
Pressure, temperature, and humidity. Pressure has a direct relationship with air density: since air is a gas, it can compress or expand; higher pressure naturally causes the air to compress, increasing density. Temperature has an indirect relationship with density: when air molecules are heated they become agitated and spread out, making them less dense. Humidity has an indirect relationship with density: water vapor molecules have a smaller mass (weigh less) than dry air molecules, i.e. wet air is less dense than dry air.
In what 3 ways does less dense air (a higher density altitude) contribute to a reduction in performance?
1) Less power, because fewer air molecules are contributing to combustion in the engine(s); 2) less thrust, because the prop is throwing back fewer air molecules; 3) less lift, because fewer air molecules are striking the wings.
What are the 3 definitions of “density altitude”?
1) Pressure altitude corrected for nonstandard temperature; 2) the altitude above sea level in the standard atmosphere at which a given atmospheric density is found; 3) the altitude at which the plane feels it is flying.
What are some ways to compute density altitude?
Use the graph in the PHAK; use an E6B or a flight computer; or use the formula: 120(OAT - Standard Temp) + Pressure Altitude = Density Altitude.
What kind of air density exists at a high density altitude?
Low.
When would density altitude be the same as indicated altitude?
When temperature and pressure are standard, i.e. in the standard atmosphere.
What happens to density altitude as temperature increases
Density altitude increases.
If a low pressure system moves in, will this increase or decrease the density altitude?
Increase.
What kind of performance would you expect while flying at a high density altitude?
Poor.
The air is colder at higher altitudes . . . so shouldn’t the air be more dense, and therefore shouldn’t performance increase during climbs?
No, because at the same time, air pressure drops rapidly, and the density-reducing effect of the rapidly diminishing air pressure is dominant.
How would you expect your takeoff roll and climb-out to be affected if taking off out of an airport with a high density altitude? Why?
Increased takeoff roll distance and a reduced rate-of-climb. Before lift-off, the plane must reach a faster groundspeed, requiring more runway; plus, the reduced power and thrust add a need for still more runway. As for rate-of-climb, the performance reduction caused by the thinner air will obviously translate to a lower climb rate (ft/min). Also, angle of climb will be reduced, because the higher density altitude translates to a faster true airspeed, and therefore the plane covers more distance horizontally over the ground for any vertical increase. In other words, the plane will climb at a shallower angle.
When approaching to land at a high density altitude airport, what would you change about your approach speed and landing configuration, if anything?
Nothing.
How would you expect your landing roll to be affected by a high density altitude, and why?
Landing roll would be longer, because the plane touches down at a faster groundspeed, so overcoming that momentum with the brakes takes longer. Also, the effects of any mistakes during the landing (like sideloading) would be more serious, again, due to touching down at a faster groundspeed.
Would you expect takeoff performance to be degraded more by a low pressure system, a warm front, or high humidity? Why?
The warm front, because although pressure has the greatest effect on performance vertically in the atmosphere (because it drops so rapidly as altitude increases relative to the temperature drop), temperature has the greatest effect on density horizontally in the atmosphere (temperature often changes by as much as 40 degrees F over the course of even one day, whereas pressure generally stays within a window of about .1 or .2 inches). This is easily provable with the performance charts. As for humidity, although it does degrade performance, in general it’s effects are negligible in comparison with those of pressure and temperature.
How does density altitude affect true airspeed (TAS) for any given indicated airspeed (IAS)?
The higher the density altitude, the higher the TAS in relation to IAS.
By approximately what percent of your IAS does your TAS increase for every 1,000ft MSL of altitude gain?
2%.
At 10,000ft MSL, approximately what would you expect your TAS to be if your IAS were 100kts?
120
Say you’re setting up for turns-around-a-point and you see that your groundspeed is 5kts faster than your indicated airspeed. Does that mean you have a tail wind?
No, because it doesn’t factor in TAS. For instance, if you’re at 5,000ft MSL with an IAS for
100kts and your groundspeed is 105kts, that probably means you have about a 5kt headwind. This is because the TAS would be about 110kts at 5,000ft MSL, so if the GS is 5kts less, that would suggest a 5kt headwind.
As density altitude increases what happens to the IAS at which the plane stalls?
Stays the same.
As density altitude increases what happens to the TAS at which the plane stalls?
Increases.
How and why does CG affect the following performance aspects of a properly trimmed airplane: Takeoff
a forward CG makes the airplane nose-heavy, which can make raising the nose more difficult during rotation. Pilots can alleviate this by applying a bit of nose-up trim prior to departure. An aft CG can make rotation too easy, potentially leading to a tail-strike during rotation (especially during the takeoff-roll portion of a soft field takeoff), or even a power on stall after lift off.
How and why does CG affect the following performance aspects of a properly trimmed airplane: Stall speed:
Stall speed: a forward CG increases stall speed. This is because the plane is more nose-heavy, which requires the horizontal tail surface to produce more negative lift in order to level out the plane’s pitch attitude. This added negative lift is a down force pushing down on the tail, making the plane heavier, increasing wing loading. To support this extra weight, the wings have to create more lift, and they do this by operating at higher angles-of-attack (AOA). This new, higher, AOA is closer to the critical AOA. So from here, as the plane slows and AOA slowly increases in order to maintain altitude, the plane will stall sooner, i.e. at a higher airspeed. An aft CG has the opposite effect.
How and why does CG affect the following performance aspects of a properly trimmed airplane: Cruise speed
Cruise speed: as explained in the previous answer, a forward CG results in the tail producing more negative lift - a down force on the plane that the wings have to support by increasing their AOAs in order to produce more lift. So, both the tail and the wings are now producing more lift, and therefore more induced drag. This causes a slower cruise speed. An aft CG has the opposite effect.
How and why does CG affect the following performance aspects of a properly trimmed airplane: Fuel burn:
Fuel burn: the extra drag caused by a forward CG (as discussed in the previous answer) means that the engine has to work harder - i.e. requires more power/throttle/combustion - to maintain a given airspeed. So more fuel gets burned. An aft CG has the opposite effect.
How and why does CG affect the following performance aspects of a properly trimmed airplane: Stability:
Stability: a forward CG makes the plane more longitudinally stable. The plane’s longitudinal stability comes from the way it is designed with the CG located in front of wings/center of lift. To prevent the plane from naturally trying to nose down and descend all the time, the horizontal tail surface exists in order to produce negative lift, thereby raising the nose. Now, if there’s a pitch disturbance that causes the properly-trimmed airplane to nose down, the subsequent increase in airflow over the tail will cause an increase in negative lift, causing the nose to rise back up to its original, undisturbed position. Conversely, if the plane pitches up, less airflow over the tail (less negative lift) will cause the tail to rise and the nose to drop back down. The more forward the CG, the more nose-heavy the plane becomes, and therefore the more down force the tail produces to counter it. In other words, as the CG moves forward, the aircraft becomes more nose heavy and tail heavy, and it becomes harder to displace both the nose and the tail. Thus the plane becomes more longitudinally stable.
How and why does CG affect the following performance aspects of a properly trimmed airplane: Controllability:
Controllability: stability and controllability have an inverse relationship: as stability increases, the control forces required to overcome that stability become greater, meaning controllability decreases. An unstable aircraft - one with an aft CG – requires lighter control inputs to control the airplane (i.e. is more responsive to pushing/pulling the yoke), and therefore is more controllable.
How and why does CG affect the following performance aspects of a properly trimmed airplane: Stall recovery:
Stall recovery: after stalling, an aircraft loaded with a forward CG will have a nose that naturally tends to drop, aiding in stall recovery. An aft CG will have the opposite effect. On a slightly different note, it is also easier to cause a stall when loaded with an aft CG, because an increased AOA (one that could potentially exceed the critical AOA) will be achieved with less elevator control force.
How and why does CG affect the following performance aspects of a properly trimmed airplane: Spin recovery
Spin recovery: recovering from a spin requires recovering from a stall, and a more forward CG helps to lower the nose of the airplane below the critical AOA. A forward CG also results in a longer arm between the CG and the rudder, which translates to more rudder authority. This added rudder authority aids in stopping the spinning rotation. An aft CG, on the other hand, could lead to a flat spin, making recovery extremely difficult or even impossible.
How and why does CG affect the following performance aspects of a properly trimmed airplane: Landing:
Landing: a forward CG makes raising the nose during the roundout and flare more difficult, possibly causing a 3-point/flat landing that could lead to porpoising. An aft CG can make the plane more prone to floating and ballooning in the flare, possibly leading to a tail strike.
How does a heavy weight affect the following: Takeoff:
Takeoff: takeoff distance increases as weight increases. A heavier plane has more inertia that needs to be overcome in order to accelerate the plane to its lift off speed. Imagine putting your car’s engine into a semi-truck; it’s going to take a lot longer to accelerate that semi-truck to a certain speed than it would your car.
How does a heavy weight affect the following: Stall speed:
Stall speed: stall speed increases as weight increases. A heavier plane needs to produce more lift to counteract the extra weight, thus requiring a higher AOA (assuming a constant airspeed). To demonstrate this, imagine your plane cruising at 100kts. If the plane is loaded within limits, it will never stall at this speed. Now imagine suddenly adding 1,000lbs. You will have to increase AOA significantly in order to support this extra weight. Now add another 1,000lbs. You’ll have to increase AOA even more. At some point (provided the wings don’t snap . . .), if you keep adding weight and increasing AOA, the wings will exceed their critical AOAs and stall…at 100kts. This is much higher than the normal stall speed, clearly showing that stall speed increases along with weight. A light weight has the opposite effect.
How does a heavy weight affect the following: Cruise speed:
Cruise speed: cruise speed decreases as weight increases. Heavier planes operate at higher AOAs, meaning more induced drag (the byproduct of the extra lift), causing slower cruise speeds for any given power setting. A light weight has the opposite effect.
