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76

If the aircraft is cruising at FL350 it is operating in Reduced Vertical Separation Minima (RVSM) airspace (between FL290 and FL410). With an altimetry discrepancy between the two altimeters of more than 245ft, the aircraft is no longer RVSM approved. It shall therefore descent or climb out of the RVSM altitude block. For the Airbus A321 the operating ...


48

In a word, the tropopause. Gas turbine engine efficiency improves with colder & denser air. As an airplane climbs through the troposphere, the density & temperature both drop, and the loss of density is more than offset by the lower temperature. Above the tropopause, however, the density continues to drop while the temperature holds (approximately) ...


44

When flaps are retracted they do nothing, which is the whole point. The byproduct of lift is drag, a larger wing will create more lift, but more drag as well. More drag equals a slower cruising speed, or bigger engines to power past the drag along with higher fuel consumption. Flaps let airplanes cruise faster by getting out of the way.


34

For 747-100 and -200 aircraft at 35,000 ft and above, you can do it, but it's hard to keep the airplane within 100 feet of the assigned altitude, and you typically can't do that (or at least I couldn't) without practice. 200 foot altitude excursions were the norm when I first took control if I had not done it for a while. For the few first officers that ...


34

There are multiple factors that affect an aircraft based on its cruise altitude. The cruise altitude directly affects the aircraft pressurization and aerodynamics. In order to keep the cabin altitude around 6000 to 8000 feet, the fuselage would have to withstand a higher pressure differential. This would require more material, and make the plane heavier. ...


27

There is something called the "Cost Index" - it is basically how you choose between speed and fuel economy. Company dispatchers, after knowing that a flight has been delayed, will calculate this value, and the pilots simply input the updated value into the FMC (Flight Management Computer). That will instruct the autopilot to output more thrust, i.e. fly ...


27

The only reason for your flight to operate at such low altitude is because it is cheaper for them to do so. As you said it is due to weather, other route/altitude may not be available. They can cancel the flight but that is likely to be costly. They may have to find accomodation for you and crew until they can put you to the next flight. Sub-optimal flight ...


27

The autopilot pitches to hold the flight level when it captures the level at the top of the climb, so later on as the aircraft gets lighter and wants to climb further, the A/P will lower the nose as required to hold the flight level (the A/P is able to move the elevator through its servo's link to the elevator controls; it can also work the trim if the servo ...


26

I know there is already an accepted answer, but some key facts are missing. Mainly, the optimum cruise altitude is where thrust and lift requirements for both take-off and cruise balance well. An additional benefit is the cooler air which increases the efficiency of heat engines. With increasing flight altitude, the airliner needs: Bigger engines to ...


26

I used the playback function of Flightradar24 for the 18th at 23:00 UTC, and the amount of traffic above 10,000' (filtering by altitude) seemed very normal compared to other days. I'm baffled as to why they flew so low, but I can address your fuel question in some detail. The difference in fuel consumption is ~693 kg of fuel, and would cost an extra ~$415, ...


25

Your concerns about heavy flaps are well founded. The designers try to get away with as few high-lift devices as they can afford to. But not fewer! If you observe the trend over the years, flaps became more complex with every new airliner generation, starting from simple split flaps in the 1930s to triple-slotted flaps on the Boeing 747 in the late Sixties. ...


24

No real reason other than it keeps people from bugging the flight attendants by asking "how high are we??". It also lets the flight attendants know that the aircraft is at cruise so won't be climbing and making the carts harder for them to push around.


24

Above Transition Altitude (e.g. this is 5000ft in Germany) the altitude is measured in flight levels (FL) - each FL equals 100ft and is measured above an artificial QNH of 1013,25 hPa. If you cruise for example at FL380 that will be 38000ft above the pressure level of 1013,25 hPa. This system assures that 2 aircraft which are 10 FL apart (e.g. FL370 and ...


20

Mainly the options 2 and 1 from your list. What nobody mentioned so far, but is rather important here is that the angle of attack depends on speed1 and weight. While the "speed" does not vary much for most airliners, the difference between empty and fully loaded airliner is rather large. Now if the fuselage was tilted down, it would generate lift directed ...


16

It was because they could get there faster on a "TEC route." IFR flights are subject to congestion management at the ARTCC level, which means they have to wait their turn in line to be allowed into the airspace. That used to be done with holds (and still is in many other countries), but the US will slow down aircraft, reroute them or even delay takeoff to ...


13

Climb to cruise burns fuel. Adding additional drag burns fuel. Adding retractable mechanisms adds weight that burns fuel. More drag, even at higher cruise altitudes, requires larger engines for the same cruise speed. Larger engines burn more fuel (despite increases in modern engine fuel efficiency). Retracting high-lift, high drag devices reduces fuel burn ...


13

When airplane mass, altitude, N1 (engine spool speed) and Mach number change together, it is very hard to come to a meaningful conclusion what is better for fuel flow. However, you see that the grey boxes tend to show the smallest fuel flow and they sit at the upper altitudes, so altitude seems to help. Going above 90% N1, however, seems to increase fuel ...


12

Ultimately the answer to your question is number (1) from your options above - That's just how the aerodynamics happen to work out. To generate a certain amount of lift requires that the wing have a specific angle of attack at a given speed. As the wings are more-or-less permanently attached to the fuselage setting the angle of attack requires us to pitch ...


12

It's probably not a good idea to hand fly jets at cruise altitudes in many cases (US, EU, and others). The reason is that from FL290 to FL410 (29,000 to 41,000 feet standard pressure altitude), a lot of airspace is under RVSM rules (Reduced vertical separation minima). This means that aircraft are only separated by 1000 feet vertically. One of the ...


12

Please let me assume that you wonder about control surface effectiveness rather than their efficiency. Both are closely related, but I prefer to address their effectiveness - doing what the pilot demands them to do. The aerodynamic forces are proportional to the dynamic pressure $q$ of the flow, which is density times velocity squared, as in $$q = \frac{\...


12

I looked up the flight on Flightradar24.com. The descent starts at 16:10 UTC and ends at 16:29 UTC, going from 38.000 feet to 24.000 feet. So, the aircraft descended 14.000 feet in 19 minutes. 14.000 divided by 19 makes roughly 750 feet per minute, which is a relatively normal descent rate for airliners. The cause for this is probably an ATC instruction. ...


12

White lights are usually off in the cockpit in night flights to protect pilots' night vision. Red lights are sometimes used as these do not impact night vision. One of the challenges you'll run into is knowing when it's night, as the start of night varies by latitude and the exact time. Phones generally set their time from the mobile network, which may be ...


11

As has been stated, ATC will assign you an altitude, taking into account your requested altitude, traffic conditions, and of course the FAR. Peter's formulas will apply from a design standpoint. The aircraft designer will use something like that to calculate typical values, which will be included in the aircraft's operating handbook. The table below is ...


11

The questioner seems to have noted that the basic wing with flaps retracted provides a high ratio of L/D (or Cl/Cd). Where L denotes lift, Cl denotes lift coefficient, D denotes drag, etc. We can certainly scale up the basic unflapped wing to provide as low a landing speed as we wish, although landing will be tricky due to the flat glide path. Flaps help ...


11

If you are asking about logging night VFR -time, you are not concerned with lighting conditions. Night time is logged according to regulation based definition of night (at least EASA), tables for night time are usually available from local aviation administration websites. Your logger simply needs to know your position, date and time, and refer to the ...


10

First, let's assume that all lift is created by the wing, and that the plane is flying in ideal conditions, where fuel flow per distance flown is minimal. This point is given by $$c_L = \sqrt{\frac{1-n_v}{3+n_v}\cdot c_{D0} \cdot \pi \cdot AR \cdot \epsilon}$$ Please see this post for more background on this formula. Nomenclature: $c_L \:\:\:$ lift ...


10

Aerodynamic damping is proportional to density, so all maneuvers require less control input at altitude than near the ground. On the downside, excursions from the trimmed condition produce greater amplitudes before stability pulls the aircraft back. This explains why hand-flying needs more attention at high altitude. Background: Damping is the reaction of a ...


10

Generally, climbing as high as you can would be close to the optimum strategy. In the end, the altitude is given by the optimum lift coefficient in cruise, which can be approximated as $$c_L = \sqrt{\frac{1-n_v}{n_v+3}\cdot \pi \cdot AR \cdot \epsilon \cdot c_{D0}}$$ Now it will be important to get the thrust over speed dependency right, which is expressed ...


10

To combine the other two answers: Below the "transition altitude" (which is 18,000 feet in the US, 5,000 feet in Germany), the pilot has to pay attention to the current altimeter setting, and adjust it accordingly. However, they don't have to listen to ATIS/AWOS/etc., most controllers will give you the current altimeter setting when you check in with them, ...


9

Autopilot flying straight and level Upon reaching the VMO/MMO the A/P will be inhibited from trimming down any further, which will result in the airplane climbing at the VMO/MMO. Overspeed protection limits the speed to which the airplane can be trimmed. At VMO/MMO, the trim reference speed is limited by inhibiting trim in the nose down direction. Hand ...


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