How do blimps control altitude? If you have an airship filled with helium, how can you adjust your altitude?  i.e. how do you "go up" or "go down"?

Even staying still would be a challenge: The way I see it, you'll never have exactly the same average density as air and therefore will always be either slightly falling or slightly rising.

Is it the case that they have an extra tank of compressed gas as well as vents on the blimp and they make adjustmenst by increasing/decreasing internal pressure of the balloon?

An airship pilot has three ways of adjusting lift:

Every lighter-than-air vehicle needs some ballast to adjust its weight. This could be sand, but mostly water is used.
A blimp is only partially filled with helium. The envelope is held taut by filling a bag inside with air, called a ballonet. If you press a little more air in this bag, the overall density of the gasses in the envelope goes up, while the volume stays practically constant. This allows for fine adjustment. Forward and aft ballonets even allow for trimming the pitch attitude.
When the airship moves, the air does provide a little dynamic lift or downforce, depending on pitch attitude and elevator deflection. This is a little tricky, because airships are not stable, and a pitch-up command will lead to altitude loss below a critical speed.

The ballonets in this cutaway drawing of a Goodyear blimp are numbered 2 and 4.

On a calm day, the biggest disturbance is actually the sun. If the envelope is exposed to direct sunlight, it will heat up and the gas within will be heated as well, causing the blimp to rise. If the sun is covered by a cloud, the gas cools down and loses some lift. On partially cloudy days it is not easy to keep the same altitude!

Another threat is an unstable atmospheric temperature gradient. Glider pilots love this, because it will give them strong thermals, but the gas inside the airship will behave similarly to that bubble of warmer gas which causes thermals. When the airship rises, the gas inside cools less quickly than the surrounding air, and the airship will gain lift when it goes up. It becomes really bad on the way down: Now the gas warms up less quickly as well, and the airship loses lift as it comes down. The airship pilot needs to drop some ballast or create more dynamic lift, or his craft will slam into the ground.

Long-range airships like the LZ-127 used a mixture of hydrocarbon gases as fuel. This mixture (called Blaugas) was balanced to be just as heavy as air and filled some of the gas cells within the envelope. Consumed fuel would be displaced by air, so no lift adjustment was needed on a long trip.


Why do airplane indicators start at 12 (o'clock), unlike cars that start at 7? A car's speed and RPM indicators usually start at 7, lower left, but airplanes' altitude and airspeed indicators start at noon. What's the origin of this convention?

Most old-style indicators (RPM, cyl. head temperature, outside air temperature, oil pressure, etc.) are geared in such a way as to present their full operating range over a needle swing of 270 degrees. It is also common for the gauge mechanism to be oriented in such a manner as to put the nominal operating point somewhere near the middle of the needle's swing arc, where the gauge mechanism introduces the fewest errors in the reading.
Early (crude!) altimeters were also 270 degree swing gauges (i.e., barometers), but this made it impossible for the pilot to read that gauge to an accuracy better than perhaps 500 feet- which is insufficient for modern use. Later versions of the altimeter had gear trains inside them to magnify the needle movement enough to read the gauge to an accuracy of ~50 feet, but this in turn dictated that the gauge range cover multiple revolutions of the needle around the face of the instrument... like a clock.
Since we are all used to reading clocks that start at the "12:00" position, the multi-turn altimeters were set to start at 12:00 too and geared so one full needle revolution corresponded to one thousand feet on the "minute" needle, and ten thousand feet on the "hour" needle. This worked fine for small planes that rarely flew higher than 10,000 feet.
Registering multiples of 10,000 could be done with a third needle but a three-needle gauge isn't the easiest thing in the world to quickly scan. A better solution is to have a window cut into the gauge face behind which altitudes greater than 10,000 feet are displayed as numbers instead.
As an aside, in older American-made car speedos (1960's-era), 60 mph was most commonly positioned at the 50% scale point ("straight up" on the dial face) to give the driver a quick visual indication of 60, without having to read the numbers. When the US federal maximum speed limit was reduced to 55 during the oil crisis of the mid-1970's, 55 was then positioned at the straight up spot.


What's the typical cost (and its breakdown) for a long-haul commercial flight? I realised that this question is very broad, but I intend it as an example to illustrate the actual costs incurred by a typical long-haul commercial flight.

For the purpose of this question, I would assume the following:

A380 or (modern) B747 in typical 3-class configuration
Full occupancy
12 hours flight time (or more)

Here are the cost position I can think of right now:

Fuel
Aircraft amortisation
In-flight crew salaries
Ground crew salaries (boarding crew, aircraft preparation and cleaning, luggage loading, etc.)
Catering
Various insurances
Airport fees (landing tax, etc.)
Overflight fees
Administrative cost (managing/issuing tickets, etc.)

What am I missing? What would be the absolute (and relative) cost budget for each of these?

Bonus question: in airline operations, is this cost typically handled as "per flight hour" (as in general aviation), "per flight", or in another way?

Fuel

The fuel consumption of the A380 is about 11 metric tonnes per hour. With a fuel price of \$1000 per metric tonne, this results in \$11000 per flight hour.

Amortization

An A380 will cost about \$350M. The aircraft will be used for about 25 to 30 years, but let's assume the amortization period is about 20 years with a residual value of \$50M. With an interest rate of 5%, you would pay about \$24M per year.

That is about \$2740 per hour, or \$0.76 per second, flying or not.
Assuming a usage of  60%, taking into account turn around times at the gate and maintenance downtime,  it boils down to \$3915 per flight hour.

Crew

Captain costs \$160K per year, the F/O \$90000. Let's assume they fly about 60 hours per month, the combined flight deck crew cost is about \$350 per flight hour for the base salary. Another \$80 needs to be added for allowances bringing the total to \$430 per flight hour.

Cabin crew rate would be around $40 per hour, so for 20 crew the cost will be \$800 per flight hour.

Also allowances for overnight stays and hotel costs need to be taken into account

Airport charges

These vary extremely between airports. You can basically break the costs down into costs related to the aircraft, the passengers and various taxes by the government.

Aircraft cost usually are related to the Maximum Take-off Mass (MTOM) of the aircraft and the noise category. A Boeing 747-400, while less heavy than an A380, will in some cases be more expensive due to the higher noise level. Many airports differentiate between peak and off-peak hour landings.

I can't find accurate data for the A380 but an old table that I have for the Boeing 747-400 suggest that total the cost vary between about \$6000 to \$25000.

Overflight charges

Fees are levied by countries whose territory is flown over, mainly to cover ATC cost. The cost is typically based on the distance flown and the MTOW. For a 747-400ER over Europe, the fee is on average 145EUR per 100km. Assuming an average speed of about 900km/h, the hourly cost is about 1300EUR/h. These costs are lower over the high seas and over the US.

Rough total

Based on the above, the total will be between \$22,000 and \$41,000 per hour.


Is there a flexible "cloth-like" material that can be used as "skin" for a non-conventional wing? I am in the very early stages of playing with the idea of an ornithopter derived aircraft. The intention is to be able to scale the resulting aircraft, to passenger (or useful payload) carrying capability.

Is there any material that is simultaneously light - but also strong enough to carry the weight of an aircraft (and it's payload)?

Aircraft fabric would be too heavy for a person carrying homebuilt orni-contraption.  What you are looking for is 3M Mylar/Tedlar.  It has been used as a covering material for a number of ultralights like the Lazair for many years.


WHY does a jet engine work? Maybe this isn't the right forum for this question but anyway:

WHY does a jet engine work?

I know how it works: compress Air -> inject fuel -> ignite -> expand -> thrust

But why does this work? Some energy from the expansion is used to turn the turbine which powers the compressor. Why is this self sustaining?

btw: It seems to be tricky to keep a jet engine running. There are examples that airliner engines sometime stall (Loss of airflow/compression in the engine)

Some energy from the expansion is used to turn the turbine which powers the compressor

And in some engines it turns the turbine which has a large fan attached to the other end. The engine produces more power than needed to run itself.

Inside the engine, everything is a very careful balance of pressures. Basically the job done by valves in a piston engine is done by careful engineering in a turbine, which is one of the reasons they cost so much. The inflow to the combustion chamber balances the outflow to the power turbine to the point where it wants to go that way - get it a little bit out of balance and you have noisy stalls like you see in the referenced video.


Why don't nose landing gears have brakes? Nose brakes can help the aircraft in braking and it can also help in reducing the travel on the runway length. So why don't we use brakes in the nose landing gear?

The assumption is wrong. Some planes indeed have nose wheel brakes.

The reason why so few planes have nose wheel brakes is that

The braking force of a brake cannot exceed the weight carried by the corresponding wheel multiplied with the friction coefficient. A nose wheel carries much less weight than a main wheel, because the main wheels are much closer to the center of gravity. There is more force on the nose wheel during deceleration, but still much less than the main wheels carry.
The nose wheel brake itself, as well as stronger structure, mechanisms and wheels would add more weight to the nose of the aircraft.
Nose wheel braking reduces the stability of the decelerating plane. This could be controlled in various ways at the cost of increased complexity.

For the aircraft designer it is a trade off: is is worth extra cost, weight and complexity for reducing the braking distance by a small amount?

Note that there are planes where even some of the main gears are not equipped with brakes, for instance the A380, where no more brakes are required to fulfill the requirements.


How much g-force is experienced in a 45° turn? A 60° turn in an airplane doubles the g-force; how much g-force is experienced in a 15°, 30° and 45° turn?

Is it 0.5, 1 and 1.5g respectively?

The formula for the load factor $n_z$ in a turn is

$$n_z = \frac{1}{cos\phi}$$

where $\phi$ is the bank angle. This makes the load factor in a 45° turn 1.414 or $\sqrt{2}$.

Since Michael correctly mentions the simplifications in his comment, let me expand the formula above. In case of a climbing turn, the load factor depends on the coordinate system used. If it is measured in airframe-fixed coordinates, the load factor will be split into a z-component ($n_z$, pointing down) and an x-component ($n_x$, pointing fore- or backward, depending on the aircraft's pitch attitude $\Theta$; nose-up is positive):
$$n_x = -\frac{sin\Theta}{cos\phi}$$
$$n_z = \frac{cos\Theta}{cos\phi}$$

For an uncoordinated turn the value needs to be modified further with the relationship between bank angle change for a given sideslip angle, which is aircraft-specific.


Where do websites like expertflyer.com get their information? I've been looking at internet resources like expertflyer.com and KVSTool which query information like live seat maps, upgrade availability and fare bucket information. To my knowledge, this seemed like proprietary information unique to each individual airline. Is there a public API or database that these pieces of software are using or do they simply scrape the websites of each individual airline? Perhaps this might be a question better suited for Programming SE but I thought I'd start here.

Simply put, ExpertFlyer gets its data from purchasing it from the GDSs (global distribution systems for airlines) these are the same systems that travel agents (online and off) use to book tickets/travel for their customers. This has a cost but has advantages vs free but less reliable means of acquiring flight data.

Other services may screen scrape (take) information from public airline websites, however airlines frown upon that. We have seen active campaigns by airlines to go after services that collect data in this manor.

Please note, this post is not meant to be "promotional" however fully answering the question posed requires explaining how different services may acquire flight data. Also, if the name "ExpertFlyer Voice" didn't indicate to you, I am a representative of ExpertFlyer.com


What is the EASA equivalent of the FAA's 91.517? What is the EASA equivalent of the US 91.517 regulations (on passenger information)?

The EASA regulation regarding passenger briefings begins at AMC1 CAT.OP.MPA.170 Passenger briefing.

Here is a link to the document where the above specific reference can be read:
click here


How, exactly, did US1549 land on the Hudson? A comment by @AsheeshR says:

  The Hudson River Landing was due to a combination of piloting skill and an autopilot system that was engaged right till impact and kept the plane in a controlled descent within a safe flight envelope.

It was an Airbus aircraft, so the autopilot has priority over pilot input, but how exactly did the autopilot play a role in getting them down safely?

What exact procedures did Sully and the crew follow when landing? Was there anything in addition to standard ditching procedures, that might have contributed to them landing in one piece?

The NTSB report is a great resource when looking for information about an incident like this.

There is an Engine Dual Failure Checklist discussed starting in section 1.17.1.2 of the report. This includes steps to attempt restarting the engines, and further steps depending on whether or not the engines can be started, and finally steps to help prepare for a forced landing. Pilots train on engine failures regularly, so they are already familiar with the procedures. The report discusses how their actions compared to what the checklists say. Since the incident occurred at such a low altitude right after takeoff, they didn't have time to complete all of the steps. They did what they had time for, and had to make quick decisions about what their options were.

The report also discusses in section 1.17.3.2 the ditching training the pilots received. There was guidance in the manual, but no specific scenarios included in their simulator training.

The autopilot is great for use in normal situations. However, as soon as anything is going wrong, the pilot should take control of the plane. You do not want to take time to figure out "what's it doing now" in a difficult situation. So in this situation, the autopilot probably disconnected when the engines went out, or was manually disconnected by the pilots.

Section 1.6.3 talks about the flight envelope protections. I believe these protections are what you are referring to when you say that the autopilot has more control. They are designed to protect the plane from stalling or otherwise exiting controlled flight. Because the pilot was able to start the APU to provide electrical power, the plane remained in the "normal law" mode where these protections were available. This limits the control the pilot has in order to protect the plane, in the sense that the plane will override pilot inputs to avoid dangerous situations.

Section 2.7.2 discusses the impact of these protections in this case.

  The airplane’s airspeed in the last 150 feet of the descent was low
  enough to activate the alpha-protection mode of the airplane’s
  fly-by-wire envelope protection features...

  Because of these features, the airplane could not reach the maximum
  AOA attainable in pitch normal law for the airplane weight and
  configuration; however, the airplane did provide maximum performance
  for the weight and configuration at that time...

  The flight envelope protections allowed the captain to pull full aft
  on the sidestick without the risk of stalling the airplane.

An interesting point in the report is that Airbus certified the plane to be able to land on the water intact under certain conditions. Here is how the actual conditions compared to the certified conditions.

Section 1.6.7

The report mentions that it would be extremely difficult to meet all of these requirements in an airplane with no power. The values that are higher, notably the mass and descent rate, are factors in the damage the airplane received.


Why does the KC-46 use two sensors for MCAS when the 737 Max doesn’t? this article states

  The KC-46 has a two-sensor MCAS system, which “compares the two readings,” the Air Force said.

I’m aware that the 737 uses two sensors and always has, but the computer reads them simultaneously based off which pilot is flying the plane. So if the Captain is flying it’s reading data from the left side, correct?

At this point I’m assuming Boeing’s decision to pull data from one sensor was to reduce the workload and make for a earlier introduction?

According to the article, the KC-46 uses the dual FCC/AoA sensor for computing both FCC channels and comparing the command signals. With two disagreeing signals there is no way to determine which one is correct and the usual remedy is to disengage both, warn the pilot, and engage in the appropriate crew procedure. And that requires training.
The B737MAX was promoted as requiring only very limited differences training relative to the NG, so the setup for the KC-46 could not be copied. The original design of MCAS in the B737MAX also used two transducers for activation - two different types of transducers, both of which had to be over a threshold value, as mentioned in this answer. Original design MCAS only had limited authority of 0.8 deg stabiliser. From the crash report of Lion Air 610, page 204:

MCAS is designed to function only during manual flight (autopilot not
engaged), with the aircraft’s flaps up, at an elevated AOA. As the development of the 737-8 (MAX) progressed, the MCAS function was expanded to low Mach numbers and increased to maximum MCAS command limit of 2.5 of stabilizer movement.

The report states that in the failure analysis, Boeing classified the uncommanded MCAS failure condition as Major (which may not occur more than once in 10$^5$ flying hours), instead of Hazardous (1:10$^7$) or Catastrophic (1:10$^9$) allowing for indeed a single transducer input. Again from the report, page 206:

If the probability of an undesirable failure condition is not below the maximum allowable probability for that category of hazard, redesign of the system should be considered. If the uncommanded MCAS failure condition had been assessed as more severe than Major, the decision to rely on single AOA sensor should have been avoided.

Why all of this was done for the B737, while the KC-46 uses two sensors, is a conglomeration of factors. Two sensors is best. Time pressure for plane delivery, airline customer pressure for limiting differences training, the observation that previous versions of B737s with accumulated flying hours of 250 million had experienced no catastrophic failures due to AoA sensor failure (reference again from the crash report), all led to the perhaps justifiable belief that the one sensor was sufficient. If rapid action was taken by the crew to contain the failure - but the failure could not be reognised in time. Again page 206:

During the single and multiple failure analysis from the air data system worst case scenario of “failure of one AOA followed by erroneous AOA”, Boeing concluded that the effect would be hazardous until the flight crew recognized the problem and took appropriate action to mitigate it. Since the training or the guidance for actions taken in such situation were not provided, the effect category should have remained hazardous.
Since the FCC controlling the MCAS is dependent on a single AOA source, the MCAS contribution to cumulative AOA effects should have been assessed.


Why did the north / south runways close at Daugherty Field (KLGB)? The north / south runway at KLGB were recently closed.  Does anyone know the reason for the runway closures?

From a Long Beach Post article:

The two runways in question have not been in operation for a number of years and the study ultimately found that removing them will create a safer and more efficient airport for the City of Long Beach.
The removal will allow for new taxiways and possibly opportunities for economic development.

It seems by checking the summer (June) wind rose for Long Beach—shown below—the longest runway 30-12 has the best wind averages, even when the southerly wind dominates the month. It would be cheaper to use than to maintain two old ailing runways.
(Source) 30-12 runway drawn over the wind rose.


Does ground clutter exist over the ocean? Ground clutter is a radar term that basically means a bunch of noise, or cluttered returns, that occurs when you point the radar down at the ground (like for a radar patrolling aircraft). The terrain reflects radar and you get a big blur.

Today, ground clutter is not a problem because we have great computers that can filter out the static ground returns, and only show the moving returns. However, I want to ask if ground clutter needs to be filtered when over the ocean.

The ocean is pretty flat, but always has at least some small waves. Do small waves produce any ground clutter? In other words, does ground clutter exist over the ocean during calm seas?

Yes, ground clutter does exist over the ocean. It depends on the smoothness of the water surface, radio frequency, antenna characteristics and atmospheric properties whether the radar will actually receive reflections from the sea surface.

A perfectly smooth sea surface will not generate clutter, but the combination of small waves and a strong inversion layer (temperature getting higher with altitude) will cause ground reflections, even for surface based radars looking above the horizon. The inversion layer will cause super refraction - the radio wave bends towards the ground / sea - and will cause a reflection off the waves which is received by the radar.

In an extreme inversion layer a so called atmospheric duct may form in which the radio signal makes multiple bounces of the sea. At each bounce part of the energy is reflected, resulting in multiple ground clutters at regular distance intervals.

Ground clutter is not always perceived as a problem. In some cases its absence may help to identify objects of interest. For example, in weather radar the absence of ground clutter behind a cloud indicates that the cloud has absorbed all the radio energy which is an indication of its severity.

source: Airbus Flight Operations Briefing Notes, Adverse Weather Operations (PDF)


Can lift be increased by increasing the angle of incidence of the wing? Can the lift of a plane be increased by increasing the Angle of Incidence (angle between the chord line of the main wing and the axis of the fuselage)? Meaning, can more lift be generated while the plane is in a horizontal position (i.e. not pitching nose up or down) if the wings are angled leading edge up.

Yes.  Angle of Attack is included in the Lift formula.  Lift generally increases with angle of attack in a linear fashion until AOA reaches stall.  This is why it is not a good idea to fly at too high an AOA.  Better to increase Velocity.  Lift increases with Velocity squared.

The Navy Vought F-8 Crusader had a "variable incidence wing" that was raised to a higher pitch relative to the fuselage for take-off and landing.  Interesting to note the variable incidence wing would also improve forward visibility on landing because the fuselage nose would have a lower pitch to the horizon while the wing was at a given angle of attack.  Trailing edge flaps also raise the wing chord incidence to the fuselage with similar benefits.

While in flight, most planes simply pitch the entire plane with the elevator to increase Angle of Attack.


What are the options for renting an aircraft in the Caribbean with an FAA certificate? I've been looking for places to rent planes in the Caribbean, and haven't been able to find anything that wasn't charter service. Does anyone know of any pilot resources or specific rental or flight schools in the Caribbean, specifically the southern Caribbean?

You would be much better off renting the plane in the United States. It will be difficult to find a rental anywhere in the Caribbean, except maybe Puerto Rico.

You may want to read Greenspun's Flying in the Caribbean. It's a little dated but has good information.


Why do some airports park the aircraft backwards at the gate? (YouTube) Push-in at Dortmund Airport.

While at most airports, aircraft park at the gate facing the building, there are some airports that park aircraft backwards, e.g., Dortmund Airport (shown above), and London City Airport. After arrival and engine shutdown, aircraft are then pushed back into their parking position.

What is the benefit of parking aircraft backwards?

The airports that feature this parking orientation are of small size, with the apron (or part of it) very close to the runway.
Taking Dortmund Airport as an example:

When an aircraft of a certain size (for example Airbus A319, Boeing 737) reaches one of the parking positions 5-12, it is pushed backwards into its parking position. The engines are switched off. Jets of this size, which are parked forward on their own, would cross the obstacle surfaces, in this case the lateral transition surface, with their stabilizer. This, in turn, would result in an increase in obstacle clearance OCA / H (obstacle clearance altitude / height).
Source: de.wikipedia

In other words, the tall tail fins of certain aircraft when parked nose in would violate the runway's lateral transition surface: an imaginary box around the runway that is meant to be clear of high obstructions, to ensure the safe operation of the runway and any navigational equipment.
For Dortmund Airport, I suspect the aircraft are also "pulled-out" before their engine-start due to the lack of jet blast deflectors (the building, personnel, and vehicles, would be unprotected). For example, the blast from the breakaway (taxi) power of an A320 with CFM56 engines is 20 m/s (72 km/h wind) up to ~60 m behind the plane.
It is also why London City has "caution your blast" holding point signs, so aircraft taxiing onto the runway wouldn't use too much thrust with the apron and its personnel and vehicles close behind.

London City Airport showing tails farthest from the close runway, and highlighted in blue is a jet blast deflector behind one of the parking stands. In this airport the pilots steer into this position without the help of a pushback tug.

Example of how the runway area in Dortmund Airport may affect one apron and not the other (yellow circle).


When is flotation gear legally required for pilots and passengers on a non-commercial flight in a small airplane over water? If I'm renting an airplane to fly myself and some friends over water beyond gliding distance, but within 50 nautical miles from the nearest shore, am I legally required to have any flotation devices whatsoever?

According to 91.205 if you are flying for hire over water you must have life preservers for each passenger. However, does renting an airplane for recreational use trigger this requirement? In other words does renting and aircraft from an FBO qualify as operation for hire for purposes of section 91.205?

Apparently, the answer is never.

After several days of searching, I was unable to find anything in the FARs that requires the pilot of a small aircraft that is NOT being flown "for-hire", to have flotation devices on board, for a flight over water, regardless of the distance from shore.

Section 91.205 requires flotation devices, but only for flights "for hire" over water. So this section doesn't apply to this question.

Then, there is section 91.509(12) which requires flotation and other survival gear for flights 50 nm, or 30 min out from shore, but this entire section is under Subpart F - Large and Turbine-Powered Muti-engine Airplanes and Factional Ownership Program Aircraft. So this entire section doesn't apply to this question either.


Propeller Efficiency calculation different by two methods Please refer to this MIT page which I used for my calculations:
https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node86.html

I calculated my aircraft propeller's prop, thrust, and torque curves, and subsequently the efficiency at a certain operating point (given the input thrust requirements and airspeed), by dividing the useful power (=Tu_0) by the input power (torqueRPS), where u_0 is the flight speed.

Now, in the propeller plane, the airspeed is u_disk according to Actuator Disk Theory, and can be calculated using the formulas and the known info. When I calculate the input propeller power according to another formula on that page [Power in = Thrust * u_disk], I get an answer not equal to the one using the torque*RPS formula. Where did this extra power go? I was expecting them to be the same, and the aerodynamic losses to be factored into the useful power being lower.

In my example, the flight speed is 3 m/s. Due to the conditions of the aircraft and environment, this requires 42N of thrust, which requires 4.5N-m of torque at 1503 RPM. This is all due to the propeller characteristics. Now, the useful work is T * airspeed = 42 x 3 = 126W.

Using the calculations on that page, we can get the accelerated speed at the disk is 11 m/s. So the input power (According to the page) is 42 x 11 = 464 W. This is different from the conventionally calculated input power of Torque * RPS = 4.5 * (1500/60) = 700 W.

Defining "useful" as moving the plane forwards, the rate at which useful work can be done is equal to whatever power which has been imparted to the fluid, i.e thrust x velocity at disc. This is propeller power. The rate at which useful work is done which is equal to whatever power which has gone into actually making the plane move forward, i.e thrust x freestream velocity (since freestream velocity and airspeed are interchangeable depending on the frame of reference). This is propulsive power

Consequently there are two different measures of efficiency, the propulsive efficiency which is a measure of how efficiently the plane uses the propeller power and the propeller efficiency which is a measure of how efficiently the propeller uses shaft power. The equation on the MIT webpage is for propulsive efficiency but you want to know how efficiently your shaft power (2π x torque x RPS) was used, therefore you have to calculate propeller efficiency so you don't use that equation.


If I get my Private Pilots License before college, how far will that set me ahead, if at all? So I am looking into getting my PPL and I was wondering if there was any benefit when it comes to college. I am wanting to go to Purdue, preferably, and major in Professional Flight. If I went ahead in getting my PPL now would that get me ahead in college? If it doesn't I don't think it would be worth spending all that money.

Well, since you want to go to Purdue specifically, a good place to start would be the Purdue Professional Flight website.  I found this there:

FAQ: Students Coming to School with a Private Pilot’s Certificate. Is there an advantage?

Depending on each individual situation, a student coming in with their Private Pilot’s Certificate can be either an advantage or disadvantage. The only real difference is cost. Sometimes it is more cost efficient for a student to get their certificate from Purdue or an outside flight school. It is ultimately up to the student.


How is optimal Glide Path Altitude calculated? Good day,
On a PPL checkride quite a while ago I got asked "if you are 10 miles from the airport how high should you be for a 3 degree glideslope?" Can someone run me down on how to do the math to figure it out quickly?

While @JohnK’s answer is more accurate, the usual rule of thumb is 3nm/1000ft to determine when to start your descent. This “pilot math” is much easier and faster to do in your head, and the error is within the variation due to wind anyway.
In your case, the question is reversed, so 1000ft/3nm, or 333ft/1nm.


What are the exact dimensions of a B-2 Spirit? I want to make a 3D model of a B-2 Spirit bomber. However I'd like to know the excact dimensions of it so that I can make it at least somewhat accurate.
I'd like to know the lenghts of the following red lines(each straight line separately):

I doubt you will find an official source for these. Luckily, we have google maps.

Using this example from Whiteman AFB (https://www.google.com/maps/@38.7250446,-93.5602627,145m/data=!3m1!1e3?hl=en) and distance measuring tool we get the following lengths:

1 - ~4.65 m (~15.24 ft)

2 - ~3.69 m (~12.10 ft)

3 - ~7.83 m (~25.69 ft)

4 - ~10.71 m (~35.13 ft)

5 - ~4.58 m (~15.04 ft)

6 - ~31.52 m (~103.40 ft)

The rest you can get from the published dimensions (height, length, wingspan), as an example.


Will helicopter control inputs be reversed because of precession if the fuselage starts a rapid spin? My question stems from a loss of tail rotor effectiveness. If a helicopter with a counterclockwise rotating blade like the R22 loses the tail rotor and the fuselage starts a rapid right turn and thus gains momentum, will the control inputs be reversed or will it create a double gyro from the main rotor system  and cause control inputs to be normal?

Short answer: no.

What are your flight control inputs?

Collective Pitch.
Cyclic adjustments to pitch and roll
Yaw / anti torque from the pedals, which control the tail rotor.

Let's take them in order.

Collective.  If the control inputs were reversed for collective pitch, you'd not be able to get the torque off by lowering collective (which is the usual response to a loss of tail rotor and the attendant upset it causes by loss of the anti torque function of the tail rotor being lost).  If the helicopter design reverses what the collective does, in terms of pitch increase or decrease, dependent upon the state of your tail rotor, nobody in their right mind will fly it.

Once again for emphasis: if you lose the tail rotor, you reduce collective pitch to reduce the torque, and thus the requirement for anti-torque (which is why the tail rotor is there in the first place).  Check the flight manual for your model of helicopter for further details on that in-flight malfunction / emergency.
Cyclic.  The control linkage between the cyclic and the means of tilting the rotor do not change if your tail rotor fails.  They apply their changes to the "tilt of the disc" as long as they are connected when you move the cyclic.

What happens as the fuselage beings to rotate (particularly if the rate of rotation increases) is that the pilot is at risk of being disoriented by the rotation and making control inputs that make the situation worse.  (See above: first, reduce pitch, and find a place to put the aircraft down per the flight manual).  The rotation of the fuselage does not change control linkages, it changes the OODA loop for the pilot:  Orient/Observe/Decide/Act is a continuous process (loop) when flying such that you make a control input to make the aircraft do something based on your perceiving a need to change.

The problem with a control input made when you involved with the uncommanded rotation of the fuselage is that the pilot's aim point keeps changing.  That isn't a problem of precession, that's a problem of "getting behind the aircraft."  It's a cognitive, not a mechanical, problem to overcome so that this doesn't end in tears.
Yaw Pedals.  Per your problem statement, they don't work, so reversing control inputs isn't an issue.  Your feet are along for the ride.  Your brain and your hands get you to the ground safely, or not.  The way to deal with yaw is via the torque reaction to main rotor pitch: with power on and no tail rotor, changing collective pitch will tend to make the nose move one way or the other.  (Your flight condition: In cruise flight, in a hover, or in one of the many between states that a helicopter can be in will influence that).

The problem with the uncommanded rotation of the fuselage isn't some change in precession, it's the disorientation of the pilot whose reference line keeps changing as the helicopter rotates.  The whole point of being the pilot is to control the aircraft.  The pilot's objective in this case is to reduce the disorienting rotation by reducing collective pitch and flying the aircraft (lacking your anti-torque inputs) as you normally would.  You won't get increased precession (vis a vis the cyclic inputs) due to the anti torque control being lost.  Your rotor system has enough inertia if you keep it at flight rpm where at least that isn't your problem.  Your problem is in lag, and making control inputs that are out of synch with the (constantly changing) orientation of the helicopter.

Depending upon aircraft type and the situation, you will likely need to secure the engine(s) before final landing so that you don't create a rotating situation right before touchdown if you need to add collective while you still don't have the anti torque from your tail rotor.

Check your model's flight manual for the detailed procedures to handle a loss of tail rotor.


Is it even remotely feasible to turnback a single engine aircraft with an engine failure? Note before reading further that I'm asking this question purely on theoretical grounds, though you are more than welcome to talk about practical aspects as well.

Now, my question. It's common knowledge amongst many pilots that turnbacks are one of the least safe methods of attempting to survive an engine failure in a single engine aircraft. However, is it actually possible at all to turnback an aircraft to return to the airport perimeter or the runway? How much of it is simply pilot error in executing a possible manoeuvre, as opposed to pilots thinking that they can pull something off that is just impossible, no matter what?

For the purposes of this question, we shall assume the following:

there is only one runway, so a reciprocal turn is required (we shall assume 210 degrees)
the aircraft has a flaps up stall speed at MTOW of 75 knots
the aircraft has a best glide ratio of 1:12 at 100 knots

Alternatively, if you have any other information that is more akin to a real aircraft, you can use that in lieu of this information.

Sources or an explanation as to why this it is or isn't impossible would be appreciated in your answers.

Nobody mentioned the theory so far, so I'll try to fill in.

Disclaimer: The below calculations and estimates are based on what I know about the physics involved. I don't know whether they are actually mentioned in pilot operating handbook of any aircraft.

To successfully turn back, you after climbing at best climb rate, when the engine quits you push down, execute the turn at suitably slightly higher speed to maintain the angle of attack and glide back.

The rate of climb is thrust/weight minus lift/drag, the glide rate is just lift/drag.

Glide rate in the turn is higher by factor depending on the bank angle (coordinated turn; uncoordinated turn is always worse). Calculating best glide rate turn leads to bank angle should be around 45°, speed should be about 19% higher to maintain best glide angle of attack and about 41% higher rate of descent than in straight glide. According to the article linked in the other answer, flying at just above stall speed (which is also 19% higher than in straight flight) should be better due to tighter circle despite worse lift/drag ratio.

The radius, and thus length of the turn of course depends on speed. At 110 knots (true speed, not indicated) the turn will take about 1.4 nm, at 160 knots it will already be about 3 nm. At 60 knots it will only be 0.42 nm. Since you need to maintain indicated speed, the true speed will slightly increase with altitude.

So for the turn back to be possible the plane needs to have sufficient power to climb faster than it will descend in the glide and you have to be high enough to allow for the extra gliding distance (which due to higher rate of descent corresponds to even longer straight glide).

All or most planes will have sufficient thrust at sea level to climb faster than they descend in glide. But since thrust decreases with altitude and temperature, if your departure point is high or warm enough, it may no longer be the case.

Even if the thrust is sufficient, the distance between climb and glide profile starts at zero on lift-off and will only become sufficient to cover the extra miles in the turn above some altitude. The altitude depends on many factors mentioned above.

It's not reasonable to try it without knowing what altitude is needed for it at current conditions.


How do anhedral and dihedral affect the efficiency of a wing? Higher pressure air on the underside of a wing generally rolls outboard around the wingtip and over the top of the wing reducing lift and therefore increasing induced drag.
My intuition would tell me a wing with a dihedral would increase this effect as the higher pressure airflow on the bottom of the wing would flow outboard easier than anhedral. Is the opposite true with anhedral? Would the airflow flow inboard?
Are there any other factors that could contribute to efficiency?

The lateral flow effect you are talking about affects both top and bottom so would roughly cancel out. Pressure gradient induced lateral flow is outbound on bottom but inbound on top. The wing is at a fairly low angle of attack in cruise flight; even near stall AoA is only about 18-20 degrees and retains fairly normal flow up to that point.
With very large dihedral or anhedral the isolated airfoil section would produce lift just as efficiently as a flat wing but the overall aircraft is less efficient because it wastes the lift produced. At 45 degrees from level the wing would need to produce lift equal to 1.4 times the aircraft weight. Same as being in a 45 degree bank but with no turn as the wing vector forces are fighting each other. At low angles the wasted lift is not significant compared to the effect on handling and stability. At 5degrees dihedral or anhedral the lift is 1.0038 times the gross weight, or 0.38% less efficient.


Is the standby altimeter required for day VFR flight in a C172 Nav III? I am very confused about whether the standby altimeter is required for day VFR flight: the KOEL (Kinds of Operations Equipment List) says “0” for day VFR but “1” for day IFR, while the equipment list has it as “R” (required).

You don't need the standby altimeter for VFR (day or night) but you do need it for IFR.

The Comprehensive Equipment List (section 6 in the POH) lists all the equipment required by the type certificate:

  Required items or equipment for FAA certification (14 CFR Part 23
  or Part 91)

But, even if a piece of equipment is required by the type certificate that doesn't mean it's required for a specific flight. The Kinds of Operations Equipment List (section 2 in the POH) lists the equipment that's required for airworthiness in day and night VFR and IFR:

  identifies the equipment required to be operational for airplane airworthiness
  in the listed kind of operations.

14 CFR 91.7 says that the aircraft must be "in an airworthy condition" to fly, and that's what the KOEL defines.

The reason that the standby altimeter is marked as required in the Comprehensive Equipment List is that the C172S was type certified under the old part 23 rules. 23.1311 required IFR-certified aircraft that use electronic displays - like the G1000 in the NAV III - to have a standby altimeter, either mechanical or electrically isolated from the main one (emphasis mine):

  For certification for Instrument Flight Rules (IFR) operations, have
  an independent magnetic direction indicator and either an independent
  secondary mechanical altimeter, airspeed indicator, and attitude
  instrument or an electronic display parameters for the altitude,
  airspeed, and attitude that are independent from the airplane's
  primary electrical power system

In summary:

The standby altimeter is marked as required in the Comprehensive Equipment List because it's an IFR certification requirement
Operationally, the standby altimeter is only required for IFR flight, per the KOEL


Why is the circulation around a centrifugal impeller blade tip the opposite to a wing tip? Why does the circulation around a centrifugal pump blade tip go from low pressure to high pressure as an effect of the slip caused by the non-uniform velocity distribution (the opposite of a wing tip). The reason stated in my book says that below the blade there is low pressure leading to high velocity and vice versa for the part above the blade. In an airplane, there is vortex at wing ends where air moves from below(high pressure) to above(low pressure) as given in the diagram.

I think what's confusing you is that the text book is using the term circulation to describe speed relative to the average airflow, not actual circulation around the blade.

There's still a black arrow pointing outwards on the high pressure side of the blade - no air is travelling inwards.

However if you look at the difference between actual speed and the average speed, it's negative on the high pressure side and positive on the low pressure side.

I don't fully understand the theory, but I think you're supposed to imagine the surface of the aerofoil (or an infinitely thin layer of air next to it) to be circulating, and causing the velocity gradients.

Wikipedia says:

Circulation is often used in computational fluid dynamics as an intermediate variable to calculate forces on an airfoil or other body.


Why does the speed tape on the Lockheed U-2 go the opposite way? I found this clip online that shows the speed tape on the Lockheed U-2 going what I perceive is the opposite direction to the 'normal' convention. On most aircraft, the higher values are above and so the tape scrolls downwards, but in this case it is mirrored.

Observe indicator on left for the first few seconds in the following clip:

Here's a photo as well from a french forum, actual source unknown:

Why do this? On every aircraft I can think of it is the opposite.

Is it the fact that you pitch down, speed increases? Then you got the speed/altitude indicators moving in different directions, so I don't see how that would make it any more clear, especially when it appears it's the only aircraft doing this.

In essence, you are correct.
It was a conscious decision to do it that way:
- when the nose pitches up, you're "pitching" toward the slower airspeed on the tape...
- and you're pitching toward the higher altitude on the altitude tape.

My question:  why do most aircraft have the higher speed near the top of the tape?  Is it "just because the altitude tape does it that way"?  Or is there a better reason.

Having flown the U-2 with the glass cockpit for over 10 years, it is intuitive and is "normal" for me to see that.


Why are passenger required to put their phones in Airplane Mode during taxi to gate? Why do airlines require that phones remain in airplane mode during the final taxi to the gate after landing? As far as I can tell, it does not affect the airplane or anything else negatively or at all. Is this true? If so, then what is the point in keeping the phone in airplane mode until the cabin doors open? Is it just for the sake of convention?

Thanks a lot in advance for any responses!

In general, they don't. U.S. airlines generally allow you to use your phones as soon as the plane lands.
Southwest:

Effective today, Southwest Customers with cell phones onboard may begin using them after the flight attendant gives permission to do so upon landing at the destination and taxi to the airport gate.

Source: Southwest Press Release from Sept 2, 2003
Delta:

Use of cellular devices prior to takeoff is not allowed per Delta policy so that flight attendants may properly prepare the cabin for departure and conduct safety demonstrations uninterrupted. Cellular devices may be used upon landing as instructed by the cabin crew.

Source: Everything You Need to Know About Using Devices on U.S. Airlines

Perhaps of interest, the rule against using cell phone during flight in the U.S. is not an FAA Federal Aviation Regulation. It is instead an FCC (Federal Communications Commission) regulation designed to prevent interference to the cell network as a result of thousands of phones flying around at 600 mph hitting hundreds of cell stations at once. In general, the FCC acknowledges that this rule is outdated and that this is no longer an issue for the current cell network. As such, the FCC has issued a proposal to remove this regulation. However, some airlines (including Delta and Southwest) have publicly stated that they will continue to not allow cell phone use for voice calls during flight even if it is legal, as the majority of their customers have indicated that they don't want to listen to their neighbors chatting on a cell phone for hours on end.

Delta will not allow cellular calls or internet-based voice communications onboard Delta or Delta Connection flights. Our customer research and direct feedback tell us that our frequent flyers believe voice calls in the cabin would be a disruption to the travel experience.
In fact, a clear majority of customers who responded to a 2012 survey said they felt the ability to make voice calls onboard would detract from – not enhance – their experience. Delta employees, particularly our in-flight crews, have told us definitively that they are not in favor of voice calls onboard.
-Delta CEO Richard Anderson


How does an alpha (AoA) vane work? The alpha vane (also called AoA vane) is an external probe used to measure the angle of attack. I have been trying to understand how exactly it works, but I can't find any clear explanation or simulation.

Is the vane static or dynamic i.e. does it rotate along its central axis?

Given that it has a significant surface area, I think that it would either:

Rotate because of the force/drag exerted by the airflow, and give an angle of attack proportional or equal to its angle of rotation
Measure the force being exerted on it via a force sensor embedded in the surface

Is either of these correct?

In short, how does an alpha vane work?

From what I have studied at my instrumentation course, it rotates and reads the angle between the current position and the reference position.

You then have several (3-4) of them around the fuselage because a roll maneuver (for example) will alter the measurements asymmetrically and in this way you can compensate for this effect.


Is there a tool to learn to pronounce aviation jargon accurately? So I've been reading technical aviation books and I come across a lot words I want to learn to say correctly but unlike simple words which I just look up in an audio Oxford dictionary I can't think of where online I can find audio pronunciations of aviation words. Any suggestions?

With the exception of abbreviations (like FSDO, pronounced fizz dough) there's not much "jargon" in aviation that's not a standard word in whatever language you're speaking. Words like "dihedral", "anhedral", "aileron", etc. should certainly show up in a dictionary.

The abbreviations are "tribal knowledge" the same way they are in any other field (e.g. SCSI, SATA, IMAP in the tech field). I'm not aware of any authoritative compendium of pronunciations.


How did Northrop Grumman propose to make the Global Hawk nuclear powered? Reading further after receiving this answer, I saw the section on nuclear power in the Wikipedia article on the Northrop Grumman RQ-4 Global Hawk:

Nuclear power
Sandia National Laboratories and Northrop Grumman have studied the possibility of equipping the Global Hawk with a nuclear power plant with US Air Force Research Laboratory funding. A nuclear powered Global Hawk would have a very large range and endurance, and more power for on-board systems. Apart from the technical feasibility, the major drawback however is the fact that drones sometimes crash. So whether this type of propulsion will ever be used on Global Hawk (or any aircraft) is questionable.64, 65

Those references do not give may details on the plan.
Radioisotope thermoelectric generators or RTGs can certainly be compact enough, and generate hundreds of watts or more, but I don't know if they could produce enough power to drive an electric engine.
Was the idea to use electric propulsion, or to somehow generate thrust through direct heating of the air? Or something else?
Nuclear powered aircraft are usually envisioned as quite large, so the proposal to add nuclear power to a Global Hawk may have been substantially different.

I didn't find any direct evidence that Northrop Grumman actually proposed a nuclear-powered Global Hawk.

Citation 64 (2 April 2012) in the question doesn't reference a nuclear-powered Global Hawk, only "drones." The pictured Global Hawk was apparently just an illustration.

Citation 65 (August 2003?) in the question does reference a nuclear-powered Global Hawk:

  The US Air Force Research Laboratory has funded at least two
  feasibility studies into nuclear powered versions of Global Hawk., [sic]
  which could extend the vehicles flight time from hours to months.

But this August 2003 page (above) appears to be copied from an earlier February 2003 article in New Scientist (below):

  The US Air Force Research Laboratory (AFRL) has funded at least two
  feasibility studies on nuclear-powered versions of the
  Northrop-Grumman Global Hawk UAV (pictured). The latest study,
  revealed earlier in February at an aerospace technology conference in
  Albuquerque, New Mexico, concluded that a nuclear engine could extend
  the UAV’s flight time from hours to months.

This is the only reference to a nuclear-powered Global Hawk I've found. I'm not sure this is a reliable source, however, for two reasons. First, this article was published in 2003, just five years after the Global Hawk started flying. That's not a lot of time for AFRL to conduct two feasibility studies for nuclear-powered Global Hawks, though not impossible. I've been unable to find these studies. It's possible these studies were for nuclear-powered ISR drones in general, not the Global Hawk specifically. Second, the New Scientist article references a problematic study by Carl Collins on using a metastable nuclear isomer of Hafnium (Hafnium-178m2) as a gamma/heat source by using xrays to induce the transition/gamma release. The claim has since been discredited (though this may not have been apparent at the time of publication). I'm not saying the author is unreliable (he seems like a decent STEM writer for the MIT Tech Review and Scientific American), but he doesn't seem to have a background in defense or aviation. So there's a possibility he conflated "Global Hawk" with "drone," which wouldn't be unusual of even good journalists then or now.

Northrop's nuclear-powered UAV work seems to predate Global Hawk:

  Northrop Grumman is known to have patented a drone equipped with a
  helium-cooled nuclear reactor as long ago as 1986, and has previously
  worked on nuclear projects with the US air force research laboratory.
  Designs for nuclear-powered aircraft are known to go back as far as
  the 1950s. The Guardian

Feasibility of nuclear-powered UAVs

jCisco's arstechnica article gives a surprisingly good overview of nuclear-powered aviation (there's a couple photos I've never seen before). They touch on ANP, the nuclear Bounder hoax, Pluto/SLAM, and Putin's nuclear-powered cruise missiles and torpedoes.

Some discussion of nuclear propulsion: Why don't aircraft use nuclear propulsion?. I'll repeat just one comment I made:

  @vsz the water boiler, turbines etc, have to be there and are heavy You
  can actually dispense with the steam turbines entirely. The HTRE
  series were basically modded J47s that sent air through a reactor
  instead of a combustion chamber. HTRE-3 pic. The whole engine+reactor
  assembly is actually lighter than a conventional engine+fuel,
  especially at long range. Rather, it's the shielding that's quite
  heavy. See SLAM/Project Pluto, the Mach 4 nuclear powered ramjet –
  Hephaestus Aetnaean Nov 28 '17 at 17:20

Northrop Grumman (maker of the Global Hawk) did sponsor Sandia to study the feasibility of using 'non-hydrocarbon' fuels for an "ultrapersistent" (read: nuclear) UAV. (Note that even this report doesn't mention the Global Hawk.) 2012 Report summary. Quotes:

  Upon completion of the CRADA, Sandia provided a final out-brief to
  NGIS UMS [Northrop Grumman Integrated Systems Unmanned Systems].
  Accomplishments: The effort concentrated on propulsion and power
  technologies that went well beyond existing hydrocarbon technologies.
  It contrasted and compared eight heat sources technologies, three
  power conversion, two dual cycle propulsion system configurations
  [indirect cycle?], and a single electrical power generation scheme [to
  power the ever-hungrier avionics mentioned earlier]. Overall
  performance, specific power parameters, technical complexities,
  security, safety, and other operational features were successfully
  investigated. Large and medium sized UAV systems [probably Global Hawk
  or X-47B and larger] were envisioned and operational flight profiles
  were developed for each concept. Heat source creation and support
  challenges for domestic and expeditionary operations were considered.
  Fundamental cost driver analysis was also performed. System
  development plans were drafted in order to determine where the
  technological and programmatic critical paths lay. NGIS UMS and SNL
  felt that the technical goals for the project were accomplished. NGIS
  UMS was quite pleased with the results of analysis and design although
  it was disappointing to all that the political realities would not
  allow use of the results.

Taking this assessment at face value, nuclear-powered UAVs sound technically feasible.

The powerplant is well understood, suggesting a mature/pre-existing concept (eg a "conventional" nuclear fission reactor) rather than an exotic new concept:

  Basic application of the advanced propulsion and power approach is
  well understood

New power/propulsion concepts would probably fall outside the scope of this study, which sounds fairly small and limited to what the sponsor could potentially accomplish in the short/medium term.

Note: "Conventional" is used in the same sense as the OP (I hope): "conventional reactor based on fissionable material." I do not use it here to refer to a specific reactor generation, fuel type, fuel phase, or coolant type.


What adds more weight to a twinjet? Tail-engines or wing-engines? Let's say we have a large commercial jet, with exactly 2 engines, capable of holding at least 200 passengers.

It could be a traditional jet like the Boeing 777, where both engines are in the wing. Or it could be like the MD-90 with both engines at the tail.

My question is, which configuration adds more weight? With engines on wings, we must add pylons and strengthen the wing to resist bending, especially when on the ground. With engines at the tail, we must add weight to strengthen the T-tail. There might be some extra fuselage weight at the back for holding up the engines, but I'm not sure.

There may be other weight additions that I don't know about. Ultimately I want to know which config requires more weight.

Note: I'm not asking which is best. Best would require more than one factor. This question is only asking about one factor, weight.

Tail engines add more weight. Placing the engines in pods underneath the wing relieves the bending moment in flight, allowing for lighter wing root construction. Placing them at the tail requires local beefy frame construction.

Note that the upwards bending moment in flight is the limit design factor:

In flight all of the weight of the aeroplane times the ultimate load factor of 3.5, and this is the bending moment relieved by the weight of engines mounted at the wing.
On the ground, the undercarriage supports the weight of the aeroplane at load factor 1, with a smaller moment arm. Way smaller than the ultimate bending load.
At touchdown, the spring/dampers are dimensioned such that the bending moment at maximum sink rate is below the ultimate design bending moment.

Torenbeek 1 mentions the following on this subject:

  Empty weight.
  The following factors have to be considered:

  A wing structure weight saving is possible with wing-mounted engines due to the mass relief effect on the bending moment on the inner wing.
  Engines placed too far outboard increase the landing impact loads and necessitate a large vertical tailplane.
  Engines at the rear of the fuselage require local "beef-up" and lead to loss of useful space in the tail, resulting in added structure weight and a larger fuselage for the same payload.
  Differences in weïght of the tail surfaces depend on various factors which do not permit a general conclusion.

  Summing up, we may say without too much emphasis that the empty weight of a CaraveIle-type layout will typically be 2 to 4% more than that of a comparable design with the engines on the wing.

As mentioned in this answer, at the time of B737 design Boeing had two competing designs from different teams, and the wing mount won. For an Order Of Magnitude check we can compare the wing and fuselage weights as a percentage of OEW for the DC9 and the B737-100. The weight of the wing & fuselage groups as per the table in this answer, the OEW from Wikipedia:

So indeed if we look at the weight only, the underwing configuration wins. The tail configuration compensates with aerodynamic pluses, mainly from the clean wing.

Note that these considerations were of value at the time of DC9 construction, with the narrow engines of the time. A modern 200 pax plane with high bypass turbofans has no good reasons anymore to opt for the tail mount.

1 E. Torenbeek, Synthesis of Subsonic Airplane Design


Taxiing with one engine: Is engine #1 always used or do they switch? I recently travelled with an A320 and an A321 from FRA (Frankfurt) to TXL (Berlin) and back again. On this and on previous flights I already noticed a whining sound, much louder than other sounds, while we were still at the gate or soon after leaving the gate and starting taxiing. I cannot remember exactly. My research showed that this was the PTU's sound (aka “barking dog”), typical for an A320 and its sisters.

One of my references was this post. The author said:

  [The PTU can be heard]
      At the gate, especially if we taxi in on one engine.
      During the taxi, the left engine is running and pressurizing
      the green system.

This led me to this question:

When taxiing with one only engine, do pilots always use the left-most one (#1)?
Or do they randomly choose one of the two (or four)? If they always use the port/port outer engine, doesn't it wear out sooner? Or are the engines swapped at regular intervals (like car tyres  are swapped from front to rear)?

Airbus aircraft have 3 hydraulic systems (Green, Blue, Yellow). Green is pressurized by left engine, Yellow by right engine, and each one can be pressurized by the other using the PTU. In addition the Yellow circuit has an electric pump.

The Blue circuit is a backup pressurized on demand by several means (electric pump and RAT).

Each circuit powers different aircraft components:

Source: A320 FCOM

The choice of which engine(s) to use is based on the need to have pressure:

For brakes
For nose wheel steering system.

Normal brakes and nose wheel steering are pressurized by the Green system. The brake accumulator must be pressurized, this can be done by running the an electric pump without starting an engine..

Engines are numbered 1 for left, 2 for right on twin-engined aircraft and from left to right (1/2 and then 3/4) for four-engined aircraft.

From this document:

Which Engine to Use?

  A320 Family: ENG 1
  A330: ENG 1
  A340: Outer engines (ENG 1 + 4)

Why?

A320 Family:

  ENG 1 pressurizes GREEN HYD (NWS + NORM BRAKE)
  YELLOW HYD pressurized via ELEC PUMP
  PTU is not needed

A330:

  ENG 1 pressurizes GREEN + BLUE HYD
  HYD BLUE ensures ACCU PRESS
  ELEC PUMP are not needed

A340 (outer engines when possible -- exception would be a narrow taxiway and FOD risk):

  ENG 1 + 4 pressurize GREEN HYD (NWS + NORM BRAKE)
  Check ACCU PRESS normal before ENG start
  ELEC PUMP are not needed


Why are gate numbers also marked on the outside of a terminal? As per this question.
What are these numbers visible from the runway at Narita airport?
A UK document is linked in the accepted answer  but it doesn't list big number marking on the outside of the terminal. Surely such numbering is of little value to pilots- too coarse of location information. So who use such a gate number and how?

Of course it's helpful to the pilots, and the ground crew, too. Even if you know the airport layout in detail, it's easier to go to the gate signposted as C27, say, than to go to the seventh gate on the left. Now remember that everybody who has ever flown a plane into that airport has flown there for a first time, and everybody who's ever worked ground crew there had a first week.

Just imagine the corresponding situation of a car park with somewhere between a few tens and a couple of hundred bays, where it's very important that you park in your assigned bay. Would each bay's number be written on it? Of course it would.


How to calculate the distance between two waypoints on an approach plate? Can you please show me how to calculate the distance between the 2 waypoints A and C as shown on the picture? I would really appreciate it. I am having difficulties to calculate the distance between 2 waypoints.

Point A to B = 3.2 miles, point B to C = 3.0 miles.

Total is 3.2 + 3.0 = 6.2 miles.

Click this link to see a complete legend from Jeppesen Charts showing symbols, definitions, etc. The picture below pertains specifically to your question.

(Source: Jeppesen)


Is using a DERATE for Takeoff actually reducing the power of the engine? When one chooses a DERATE for takeoff (TO-1 or TO-2 in the case of a B737), does the system actually reduce the available thrust of the engines, or is it computing a N1 that corresponds to the reduced (de-rated) thrust?
Referring to a B737NG for example, does a setting of "TO 89%" produce the same thrust as a TO-1 or TO-2 with the same N1 of 89%?

I cant' give you a 737 specific answer, but how DERATE or Flex Thrust is applied generally depends on whether the engine is FADEC or not.
If a non-FADEC, it's just an FMS computed bug target setting on the N1 indicators and you move the thrust levers as required to match N1 to the bug. So it's still the pilot actually setting thrust to a lower computer-provided target than normal (if the airplane has an autothrottle system, it's the autopilot doing that job on behalf of the pilot through thrust lever servos).
If FADEC, where you (or an autothrottle system) just move the thrust levers until they click into a detent of some sort for takeoff and let the FADEC computers directly control the engines to achieve a computed N1 setting, it's the computers controlling the engines in accordance with your thrust rating selection when you ask for TO thrust.  So in that case it's the pilot telling the computers what he/she wants, and the computers do the actual work.


What causes the distinct sounds of a jet engine? I live a few miles from a major airport (BOS), under the path that a lot of airplanes use to depart. The planes are somewhat low over my house -- high enough that it's not deafening, but low enough that I can make out the livery. Judging from a couple planes I just looked at on flightaware.com, I'd guess somewhere in the 5,000 - 8,000 foot range.

I can often make out three fairly distinct sounds from the planes:

a mid-range roar, the sort that one would typically associate with a jet flying overhead
a sort of whistling
sometimes a buzz

The buzz isn't always there, but when it is, it's quite loud; significantly more noticeable than the other two.

What causes each of these sounds? I know there are different components of an engine (I'm guessing the main ones for this question would be the fan, compressors, combustion chamber, and turbines, and possibly the nozzle). I'm guessing each contributes to some, but not all, of the sounds.

If it matters, my house is around where the planes turn (they often take off heading more or less NW, and then turn west or SW where I can see them). I think the buzzing (when I can hear it) is loudest when the plane is at about a 45 degree angle to me (so I'm seeing its front-left as it's climbing), but I wouldn't swear to it.

There are several dimensions to the answer on this topic.  First, consider that different flight profiles (TO, approach power, final approach power, etc) will have different engine acoustic signatures.  The sound output from a jet or turbofan is not linear with power, rather it changes with power level, and resulting flows and RPMs. Second, consider that the polar acoustic radiation is different for the aircraft. Nose down vs nose up, banked, etc. and consider that these radiation patterns are changing with respect to power. These are the primary noise factors.

Secondary noise factors include APU operation, and airframe noise. The airframe noise is greatest with airspeed which impacts boundary turbulence energy, and then with gear extension, and flap/slat/spoiler extension and slips.

To this complexity add airspeed, and aircraft configuration.

Different engines have decidedly different spectral content, and some are quite characteristic.

The bulk of the engine noise is exhaust turbulence, and quite specifically the differential of the exhaust velocity and the ambient flow.  This is why turbofan engines are much more quiet, because they have a more gradual gradient between the ambient flow, and the jet flow.

Whines are often forward directed radiation and are largely acoustics from the intake, and most specifically from fans on turbofan engines.

There is a plethora of literature on A320 engine whine and solutions with different engines.

There are also some specific airframe noises.  For example, the same aircraft, the A320 has some open vents on the lower wings, and the addition of vortex generators has been shown to reduce this noise substantially (up to 11db, which would be perceived as half the noise, and is a more than 99% acoustic energy reduction.  Some A320 aircraft can be heard from 30 miles away.

To address your questions, the roar is mostly engine noise.  The whine is also engine noise, but can sometimes, on some aircraft be port or acoustic flutter noise.  The buzz noise is normally airframe or to a much lesser degree airframe and engine interactions.

Apart from civil aviation there has been a lot of work done, and thousands of papers written on acoustic signatures of aircraft. There are various sensors equipped with correlation software which will classify aircraft in the air, from the ground. Aerospace contractors work hard to reduce the signatures of every aircraft from large transport to fighters. Civil aviation responds to different threats, which are the public complaining about noise associated with airports near cities.


Do nacelles around aircraft engines help in reducing the engine sound? I know that the nacelles around the aircraft engines are actually a housing and are generally lightweight and have may components like inlet cowl, fan cowl, etc., but do they help in reducing the engine sound as well or are they just for housing?

Even if they help in reducing the sound, then how much % of sound has been reduced by them?

Is there any quantitative analysis available for that which can show that without housing engines were creating sound at X dB, and after nacelle and proper housing of all other components, the sound has been decreased to Y dB?

Take a modern turbofan: The tips of the fan run at approximately Mach 1.5. We know from unducted props that supersonic tips make them very noisy. The same turbofan without the shroud would be extremely noisy, too. Acoustic liners on the inside of the fan shroud have major contribution in suppressing this noise source.

But there is more: The nacelle also makes it possible to put an intake in front of the fan which helps to equalize flow conditions over the cross section of the intake and to accelerate or slow the flow of air to just the right amount for the engine. Without the intake, the engine would suffer from frequent compressor stalls and surges, so the nacelle in effect widens the operating conditions and avoids those noisy incidents, too.

Another important source of noise is the nozzle: Here the hot, fast gas from the engine is ejected into the surrounding flow, and the mixing of both creates most of the engine noise. Note that noise increases with the sixth to eighth power of jet velocity, so the most dramatic noise reduction is possible with higher bypass ratio engines.

Diagram from the Stanford web page linked above. The jet velocity is in ft/s.

By serrating the trailing edge of the nozzle, this mixing process can be slowed down somewhat and the noise level can be reduced.

Without the nacelle the engine would create much less thrust and more drag, and bigger, more powerful engines would be needed. Bigger engines would create more noise, too, so this is also an indirect way how nacelles help to keep noise down.

A last noise reduction contribution to mention here is the capacity of the nacelle to contain the shrapnel from a failed fan blade. Fan blade failure is all but silent, but if the blade fragments would escape at high speed, the noise of what follows next would also be considerable.


What is a "KALP114A" ATC regulation? My flight (in Spain) was delayed for 6h and after I submitted the claim the airline worker answered me that it was delayed due to KALP114A regulations imposed by ATC, but I didn't find any info, that such regulation even exists.

  the regulation KALP114A, which caused delay, was introduced directly on the flight by Air Traffic Controllers

The flight was on the 14th of April, 2019, from Spain to Germany.

Within Europe, whenever there is need to reduce the number of flights through a certain airspace, the Network Manager (Eurocontrol) puts in a so called "regulation". This results in flights planned through the regulated airspace being delayed or re-routed.

Regulations are given an ID; those starting with KALP are from the Karlsruhe Upper Area Control (EDUU), southern sectors (Alps) above Flight Level 315.

Since your flight was over three weeks ago, I can't check the exact "regulation Id" in the public database, but it is likely that it was related to staffing issues at Karlsruhe upper area control centre.

An example of a current regulation starting with "KALP" is below:

Karlsruhe UAC has severe staffing problems and is causing many traffic delays this year.

A delay of 6 hours due to a regulation sounds extreme to me. I don't know what the origin and destination of the flight was, but typically a re-routing avoids the regulated airspace with only a short delay.


How does WiFi work on an aircraft? I've noticed that when a WiFi system is set up on an aircraft, a small bulbous structure is added. What does it contain and how does this WiFi system work?

The small bulb houses the antenna that provides the satellite communications for the internet link.

Inside the aircraft several wireless access points provide a WIFI signal to the passenger's equipment. The wireless access points are connected to a central modem which in turn is connected to the transceiver unit. The transceiver uses the rooftop antenna to communicate to a geostationary satellite.

source: dailywireless

source: Flightglobal

The antenna is a small dish or phased array that is aimed at the communication satellite using servo motors and/or electronic beam steering. The antenna control system receives information about the position and attitude of the aircraft to calculate the relative position to the geostationary satellites that provides the internet communications signal.

There are also systems available on the market that use a ground based network to communicate with the internet. These systems cannot provide global coverage since the aircraft needs to be in line of radio sight of a ground based antenna.


Does avionics type matter when training for PPL? Soon I want to start training for my PPL (VFR) and I'm currently looking at two flight schools in my area. At one of the two their whole training fleet is equipped with G1000 avionics, while the other also has aircraft with "old school" steam gauges.

My reasoning is that it's better to first learn it the old way, and then upgrade to glass, since it seems much easier than the other way around.

Is this reasoning sound, or does it actually not matter that much when training for PPL? To further clarify: I want to get my PPL for recreational flying, not for commercial use.

The answer is going to be along the lines of "it depends" but in all honesty thats not what you should be basing your decision on when it comes to training schools! Meet some of the crew, see who has better facilities, and which you get a better feel for.

In any case, your PPL training will simply teach you to fly an airplane, and all the associated things such as navigation and airmanship - the type of avionics the aircraft has installed is pretty much irrelevant.

When you get your license the commonest small aircraft at most airfields have traditional avionics, so you'll want to be familiar with them - if you do happen to train with a glass cockpit at most you might want an hour or two with an instructor in a typical small airplane.

On a personal note, and maybe im a bit old fashioned, but I think you should understand how a "gadget" works before using it. I like to see a physical VSI, AH and altimeter. I like turning the little Knobs and manipulating proper buttons in an airplane. One thing I can say for sure is the first time I sat in front of a glass cockpit I knew what everything was, what those displays were telling me. Would the same happen if you'd only ever flown a glass cockpit and got into a 1970's C152? Possibly - I don't know.

Good luck with your training.


What is this unusual leading edge flap on a B737-200? On this video of Air North's gravel runway-equipped B737-200, there's what appears to be a small leading-edge flap (Krueger flap?) inboard of the engine.  Refer to the screen shot below.  What is this?

Addendum

Notice in the video (and screen shot) that this device shows itself while no other flaps are extended.

These are two leading edge Krueger flap surfaces at each side of the wing, you can see details about them according IPC and AMM documents I found over the internet:

Regarding the Addendum, and why it seems that there is no other flaps deployed, I found a similar topic at Airliner.net Forum. As it is mentioned there, when is no hydraulic pressure, they can be extended by their weight:

  ...it has to do with the shut off valves and the actuators relaxing with no hydraulic pressure on them.

and also:

  ...Pressurize the system and they all move to their present commanded position

and another sidenote:

  ...Someone pulled that Krueger Flap to the fully extended position. When they droop because of no pressure on the system there in the mid positions.

and as final note, this issue is also mentioned in the AMM:


How is the airspeed-Mach number transition handled in modern airliners? During the climb phase, how is the transition from constant speed (IAS) to constant Mach number handled? Does the pilot flying have to manually "push the button" on the control panel, or is the transition automatically done by the aircraft? How does the pilot / aircraft know when the transition should happen (i.e., from flight plan?)

I guess it's the same during descent?

Does the pilot flying have to manually "push the button" on the control panel?

It's automatic, with a manual changeover button available to the pilots as shown below (label no. 2).

How does the aircraft know when the transition should happen?

When the target climb indicated airspeed (IAS) equals the target climb Mach number, the changeover takes place, usually around FL260. Likewise for the descent. The target speeds are managed by the FMS and can be changed.

Image shows the FMS CDU.

  (3) Displays computed values or manually entered values for the selected mode.


Are there rules about using the "Fasten seatbelt" sign outside of take-off/landing/turbulence? Related to this question regarding flight attendants continuing to stand during turbulence.

My understanding is that it is the law for passengers to be seated with their belts fastened during taxi/take-off/landing along with the flight attendants.

At other times during the flight, the fasten seatbelt sign is also illuminated at times of turbulence/expected turbulence (as per the question linked above, and usually accompanied by a warning from the captain that they're expecting turbulence) - I assume this is subject to the same rules that insist passengers are actually seated during these periods.

On several recent flights on budget airlines (Ryanair/EasyJet), I've begun to notice that the crew are using the fasten seatbelt sign in order to ensure passengers are seated whilst they're serving food/drinks etc. This was confirmed when I was sat at the front of the plane and over heard the cabin crew discussing this.

Clearly, there is a good rationale for keeping passengers seated whilst hot drinks are being served as the aisles on budget planes are too narrow to easily squeeze past, but on Ryanair/EasyJet flights there are also significant times when cabin crew are simply trying to flog cheap perfume and scratch cards.

During these recent flights where the fasten seatbelt sign was illuminated unnecessarily for long periods of time, passengers begun to simply ignore the signs in order to make use of the toilets (especially people with young children etc).

Are there rules that govern when pilots/crew may illuminate the fasten seatbelt signs, and are the passengers breaking any laws by ignoring them?

You dont have a jurisdiction tagged (although it sounds like this is an EASA matter) your answer may vary depending on where you are asking for. I can answer for the US/FAA.

Here in the states you are technically in breach of an FAA FAR by not remaining seated. As per FAA FAR 121.317

  Sec. 121.317

  Passenger information requirements, smoking prohibitions, and additional seat belt requirements.

  ....

  (f) Each passenger required by Sec. 121.311(b) to occupy a seat or
  berth shall fasten his or her safety belt about him or her and keep it
  fastened while the "Fasten Seat Belt" sign is lighted.

  .....

  (k) Each passenger shall comply with instructions given him or her by a crewmember regarding compliance with paragraphs (f), (g), (h), and (l) of this section.

I cant find a regulation on using it in times its not otherwise required by FAA FAR 121.311 but this discussion seems to elude to the fact its at the "Captains Discretion".


When is a steep takeoff appropriate? There are plenty of videos of aircraft performing "steep" takeoffs, such as this one. When, if ever, is this kind of takeoff appropriate. What is the procedure?

The short answer is, when the surrounding environment requires it. The procedure here is perhaps most akin to a short field take off over a 50 ft obstacle but a few things are going on. The aircraft, as soon as it leaves the ground is climbing at Vx which allows for the maximum angle of climb.

  Vx is the speed for best angle of climb while Vy is the speed for best
  rate of climb.

In your linked video its mostly for show. In practice short field over a 50ft obstacle takeoffs are used for, well, short fields... or when you need to get off the ground fast so as to get over some obstacle at the end of the runway. Generally speaking most commercial airports have long runways that are more or less obstacle free (with some notable exceptions) so short field maneuvers are not what they are in general aviation. There are lots of small fields all over the country that have trees (also known as 50 foot obstacles) power lines, buildings, church steeples or any manner of obstacle at the end of the runway. On top of that many of these strips are on the shorter side (under 3000 ft.) so you simply have less runway to work with and your margins can be far smaller. In this case you want to get off the ground as soon as possibly and climb as fast as possible.

It is the pilots responsibility to operate the aircraft safely and these kinds of maneuvers are well within operating limitations. In small planes they happen more often than you would think, especially if operating out of a short strip. and even us lowly private pilots are trained in short field 50ft obstacle clearance maneuvers. As for the big guys if the airport environment calls for it, it may be performed but I would think it is less common as most commercial airliners can pitch up far more than people in the back would be happy with. Someone with more commercial experience may be able to elaborate on how often they happen in practice there.

There are a few things to keep in mind as well, military planes simply have a much higher thrust to weight ratio than commercial airliners and can in some cases sustain vertical climb outs. Military jets also often have afterburners for an added boost. The commercial planes you see in the linked video are more than likely lightly loaded carrying only a partial fuel load. Camera angles can also make it look as though the plane is climbing much steeper than it really is.


Does temperature affect takeoff performance? A few days ago, a Copa flight from Porto Alegre to Panama was delayed from noon until 12:30 AM. The company explained that the high temperature (about 40 degrees Celsius) would require a longer runway for takeoff since the air was thinner, and that resulted in non-operational conditions for the wings and engines with the available runway length.

One source (in portuguese): http://www.radioguaiba.com.br/Noticias/?Noticia=515816

Is this true? Can a high temperature really prevent takeoff?

Yes, temperature effects airplane performance both the engine itself and the aerodynamics involved.

What Is Density Altitude?

  Density altitude is pressure altitude corrected for nonstandard
  temperature. As temperature and altitude increase, air density
  decreases. In a sense, it's the altitude at which the airplane "feels"
  its flying.

How Will High Density Altitude Affect Flight?

  On a hot and humid day, the aircraft will accelerate more slowly down
  the runway, will need to move faster to attain the same lift, and will
  climb more slowly. The less dense the air, the less lift, the more
  lackluster the climb, and the longer the distance needed for takeoff
  and landing. Fewer air molecules in a given volume of air also result
  in reduced propeller efficiency and therefore reduced net thrust. All
  of these factors can lead to an accident if the poor performance has
  not been anticipated.

Tips for Flying in High Density Altitude Areas

One way of looking at it is that higher temperatures make the airplane fly the way it would at a higher altitude where the air is less dense. An airplane cannot climb to infinity, it has what is called a service ceiling [and an absolute ceiling] that the aircraft can no longer climb any higher. This also because of a decrease in air density, but for a different reason. Raising the temperature of the air also decreases this air density. You can use an Air Density Calculator or a Density Altitude Chart:

to determine how the aircraft will fly. It is especially important at airports with short runways or high actual altitudes [Denver, Colorado for example] where the aircraft already may need to be near maximum performance.

If the density altitude is 4,000ft than that means that airplane will fly like it was at 4,000ft at standard atmospheric temperature. Standard atmospheric temperature [at sea level] is 15 degrees Celsius or 59 degrees Fahrenheit. An airplane departing at a sea level airport, at 100F would feel like it is already flying at over 2600ft. An airplane departing at a 5,000ft airport would feel like it is already flying at over 8700ft. If clearing the trees at the end of the runway was already a concern at this particular airport it may not happen anymore.


How does the B777 Side brace locking mechanism work? The B767 has jury strut actuators and their working is pretty straight forward. They oppose the collapsible geometry of jury strut and lock them in the over centre locking position.

But for B777 this doesn't seem to be the case, I sure can see a small locking piston, but I quite don't understand its working, it seems different from the 767s simple strut locking mechanism

Would be great if you could provide explanation with some picture references.

The scissors struts on that gear physically lock in alignment instead of over centering.  The inboard strut extends into the outer one a short distance, which is forked to accommodate it.  The little actuator with the lever and springs is the downlock mechanism.  The downlock will have some sort of pawl or plunger interface hidden inside that forked interface, roughly where the downlock safety pin is, that engages to lock the inner strut and outer strut together so they are like a one-piece strut.
The actuator pulls in to disengage the mechanical lock, and the springs provide a counter pressure against the actuator to provide a passive "engage" force the rest of the time.  The springs aren't maintaining an over-center condition, only applying pressure to a mechanical lock.
In this video you can see that the downlock release actuator appears to also pull the links out of alignment once the release lever it operates is fully released, and if you watch full screen you can make out the locking mechanism as the struts fold, although the precise details of that mechanism would require access to a 777 maintenance manual.


What risks come from passengers taking baggage during emergency evacuation? In recent news, the pilot of an aircraft that caught fire was chastising passengers for taking their luggage.

What are the major concerns behind his criticism?

I like to put this in context:

All passengers and crew survived.
There were no injuries.
Crew reported that the evacuation was very orderly and efficient.

So what could be the reason for the pilot speaking out so critically? So critical, in fact, that he argues that overhead-compartment doors should lock between takeoff and landing. My guesses include:

another passenger cohort could have been less orderly, and pushing with luggage resulted in injuries or people becoming trapped.
sliding down inflatable exit slides with luggage could cause the slides to puncture and trap other passengers.
the inflatable exit slides are not rates to carry the weight of an adult + luggage (seems unlikely unless the adult is massively obese).

Would appreciate hearing a list that is not just educated guessing.

As soon as there's a fire on board, the absolute top priority is getting everybody out as soon as possible. Aircraft are designed in such a way that, even after a crash landing (or other serious malfunction), the passengers will have a minute or two to evacuate before conditions in the cabin become toxic (fire, smoke, etc); this is done by using flame-retardant material and burn-through protection, but in an intense fire these won't protect the cabin for long, and once they fail no-one will be able to survive inside the aircraft for much longer than that. Smoke inhalation and oxygen deprivation kill just as effectively as being burned.

Therefore everyone has to be able to evacuate from the plane within 90 seconds, even assuming the plane is packed to the gills with passengers, and half the exits are inoperative. Given that the aircraft may be damaged after  a rough/crash landing, some of the doors may be unusable (due to fire or smoke just outside them), it may be dark in the cabin, and people may be injured and/or panicking, it's vital that nothing slows down the evacuation. To get an idea of what a 90-second evacuation of a fully-loaded aircraft looks like, see the video of the A380 evacuation test - and these passengers were calm, uninjured, and prepared. If passengers take hand luggage with them this is slowed down:

The time it takes to get it out of the overhead bins (with everyone else crushing against you to get out).
Carrying the luggage down the aisle (without tripping, hurting yourself or anyone else - again, look at the A380 video, and imagine trying to do that with a suitcase). If you lose your luggage, everyone behind you will have to climb over it to get to the exit.
Maneuvering your suitcase around the corner to the door, not getting it stuck on the door frame, with people behind you literally shoving you out the door.
Plunging down a very steep slide the height of a two-storey building (or more, in case of the A380's upper deck) at high speed, with your suitcase flailing around. At best it whacks you on the head, at worst it punctures and deflates the slide, which given the crush of passengers behind you will probably lead a few people to plummet from the aircraft doors straight to the ground.
Assuming you've made it to the ground, there is someone else plunging down the slide right behind you. If your suitcase slows you down even a little, they will plow right into you, and soon you'll have a crush of people at the foot of the slide.

In this specific case, the fire was fortunately not serious enough to defeat the cabin fire protection before the last passengers had evacuated - but as @Andy said, there are unfortunately other examples where the passengers were not so lucky. To illustrate the risk of fire after landing: Air France Flight 358 ran off the runway at landing, and despite having little fuel on board, the resulting fire completely destroyed the aircraft; luckily, everyone evacuated in time. Once there's been a major malfunction, there is no way to tell how much time you have left to get out - fire can spread very very quickly - and so you have to get out as quickly as possible. Taking luggage with you endangers your life and that of your fellow passengers.


What is an out-of-trim condition and how is it detected? I am reviewing autopilot requirements which mention that whenever an aircraft's altitude changes as a result of air disturbance or an out of trim condition, the inertial sensors will detect this change and an error signal is developed and fed to the relevant control channel, amplified, and applied to the servo actuator so that flight control surface can  be moved.
My question is what is out-of-trim condition? Is it something related to setting trim for constant speed?

The trim system on an aircraft relieves the control pressures so that the aircraft maintains an attitude without pressure on the control system.

When hand-flying this is very noticeable in that you constantly have to be pushing, pulling, or turning (in the event you have aileron trim) to maintain an attitude. Notice how I don't say "straight and level", you can trim an aircraft to maintain a climb/descent or other attitude as long as the trim tabs have enough authority.

Autopilots detect an out-of-trim condition the same way you do. They sense that there is pressure on the controls to maintain the desired attitude, and if this is over some threshold it will display a warning light for an out-of-trim condition. Some autopilots have a trim-up or trim-down light to help the pilot determine which way to trim the aircraft.

Here is a diagram from a S-Tec System 50 Autopilot found in many GA aircraft:

The "out of trim" condition lights are two yellow indicators on the right side of the auto-pilot instrument.

Why is this important? Mostly for the longevity of the equipment. The autopilot works by either physically moving the cables or actuating the hydraulic system. It uses power to run motors which cause the motors to heat up and use electrical power. By trimming the aircraft, you can reduce these forces required by the autopilot so that it isn't working so hard to control the aircraft.

Another good reason is that you don't want the aircraft to do something unexpected when you hit the autopilot "off" or "disconnect" switches. This is especially dangerous in IMC when you don't have a visual reference to anything. There are also conditions which can cause the autopilot to disconnect automatically, and it takes a few precious seconds to figure out what is going on before the pilot gets positive control.


What is the FAA definition of "takeoff" and "landing"? This question is in the context of commercial flights with passengers.

What is the definition of "takeoff" and "landing" phases? Does it include taxi? Is it from lift-off for takeoff? and touchdown for landing?

A link to the official document would be much appreciated.

According to the FAA FAA Airplane Flying Handbook, there are three components that make a takeoff:

  Takeoff roll (ground roll) is the portion of the takeoff
  procedure during which the airplane is accelerated
  from a standstill to an airspeed that provides sufficient
  lift for it to become airborne.
  Lift-off is when the wings are lifting the weight of the
  airplane off the surface. In most airplanes, this is the
  result of the pilot rotating the nose up to increase the
  angle of attack (AOA).
  The initial climb begins when the airplane leaves the
  surface and a climb pitch attitude has been established.
  Normally, it is considered complete when the airplane
  has reached a safe maneuvering altitude or an en route
  climb has been established.

So to have a take-off you must have these three components.  That would include a touch and go as well takeoffs from a full stop.

As far as an official definition of landings I didn't see anything official in the FAA documentation but I would say that the airplane touching the ground would count as a landing.