What are the limitations of using a single shaft between the compressor and turbine in a jet engine? Question Background

I'm studying the internal structure and components of a turbojet. I can see there is a shaft between the compressor and the turbine, so that energy is mechanically extracted from the exiting gas and used to help the compressor rotate. You can appreciate it in the next image:

Question

I understand the shaft is there so that the turbine helps the compressor move. But this introduces a serious constraint: both the compressor and the turbine need to rotate at the same speed. Is this true in general? How is this managed when there are several shafts connecting several compressors/turbines? Are reduction boxes needed here or do they, in fact, move at the same speed? Is this a big deal, does it introduce any serious limitation/drawbacks regarding the exiting air power?

Just started studying jets, hope the answers are not very trivial.

How is this managed when there are several shafts connecting several compressors/turbines?

The different shaft will rotate at different speed from each other, but the relative compressor/turbine pairs will rotate at the same speed.

Expanding a little, in the most complex case that I heard of, there are 3 shafts: High Pressure (HP), Intermediate-Pressure (IP) and Low Pressure (LP)

Image source

HP turbine and HP compressor will rotate at the same speed, but this is not necessarily equal (in fact is highly likely that it will be different, otherwise there would not be the need for different shafts) to the speed of the IP compressor/turbine couple. And this will be different also from the speed of the LP turbine/fan couple.

  Are reduction boxes needed here or do they, in fact, move at the same speed?

They generally rotate at the same speed. And generally you don't want gear boxes in the central section of your engine. Apart from the maintenance nightmare that they would become, given the rotational speeds and energies involved, they would be extremely heavy, inefficient and would constitute a critical system quite prone to failure.

An exception are the fans, they have enough space and are in front of all the rest of the engine, allowing the housing of a gearbox within their enclosure. You can find a better discussion about this case in this question; keep in mind that here you have 3 compressor stages with only 2 turbine stages, the gearbox was necessary to have a speed differential between fan and IP compressor without requiring 3 turbine stages and an additional shaft. (thanks to @fooot for pointing the PW1000G out)

  Is this a big deal, does it introduce any serious limitation/drawbacks regarding the exiting air power?

As everything in engineering, there are trade-offs. The ideal performance would be obtained via an infinite set of infinitesimally thin turbines/compressors, each rotating at a slightly different speed, but this is not technologically feasible. The engine designers will then settle for a slightly sub-optimal performance, but it will be actually achievable with 1 or 2 (3 in extreme cases) discrete shafts.


How do you determine whether you can us LNAV/VNAV minima on an RNAV approach? If we look at this approach plate it has

LPV DA
LNAV / VNAV DA
LNAV MDA
CIRCLING

I understand that if you have WAAS you can shoot the approach to LPV DA minimums. If you have only a GPS with a DME providing lateral navigation you can shoot the LNAV MDA. If you are more than 30 degrees off centerline you shoot the circling. So my question is, how can you shoot the LNAV / VNAV DA on a non-precision GPS approach? Does this require a special GPS that has vertical guidance?

The STC documentation provided with the GPS / FMS installation will indicate the type of minima it can go down to.

Typically, GA aircraft like a Garmin 430/530 will only be certified down to LNAV minima.

Business type aircraft with FMS's may be certified to LNAV/VNAV minima.  These installations use a barometric altitude input to calculate a pseudo glidepath to be displayed.


Do I need to copy old entries to my new logbook? i just got a new logbook to replace my cheap paper cover one, just wondering if i need to copy over all the logs(that my instructor wrote) or just leave it and when i need to tally up my hours read two. Is there any rules saying that I cant copy  logs that someone besides me wrote? Should I carry both with me?

My recommendation - don't copy over anything from your old logbook.  Just start your new logbook with your next flight or  other item that needs to be logged. You will simply have more than one logbook.

You don't need to carry your logbook with you while you are flying.  Pilot cert and medical is all that is required.  If you need to produce a record verifying that you are qualified for a particular type airplane, operation, etc., you can produce your record (logbook[s]) at a later time.


Can you report a crash landing on an emergency frequency even if you are not at the airport? You see a crash landing near the airport but you're in a car, no other planes have reported the status of the plane, and Approach can no longer contact the plane. Can you report the crash on an emergency or approach frequency to the tower?

If you are in a car, you probably don't have the full picture. Chances are, if the accident happened "near the airport", emergency services are probably already on the way. This is because the pilot would have been communicating with ATC, and would probably have indicated that he was in trouble before the actual accident. Even if the pilot did not have time to notify ATC, if a plane suddenly "dissapears", search and rescus will be initiated.

In any case, if will do no harm to call the normal emergency services (dial 112 or 911 in some countries). Don't overcomplicate things and try to get in touch with ATC - just dial the emergency number directly, and they will know what to do.


Navigation theory for Private pilot Can someone help me explain what is the magnetic bearing, heading and track, please?

Magnetic Bearing:
This is the direction of the plane's nose relative to magnetic north at the plane's current position. Simply stated, it's the bearing that your plane's heading indicator gives you. It's from 000 to 359 degrees(where 000 is north, 090 is east, 180 is south and so on). ATC directions are also given usually in magnetic bearings. Most aircraft heading indicators give reading in magnetic bearing only. Note that bearing is always stated in 3 digits.

Magnetic heading/heading. Same as magnetic bearing.

Track:
This is where things get a bit complicated. Suppose your plane is flying with its nose pointed at 090 degrees(east). You also have a wind blowing in from 000(north). This 'blows' the plane to the south a bit. Now even though your plane is pointed towards east the direction of its flight path is different! This is where track comes in: it's the direction of the flight path of the plane. In some cases it may be the same as magnetic heading (no winds etc), in others it might be different (crosswinds).


Does stall angle of attack in inverted flight change due to the camber of an asymmetric airfoil? What is the stall angle in an inverted flight configuration? How does it relate to the stall angle in normal flight?

As visible in the image below, in upright flight the lower pressure is on the extrados of the wing. In inverted flight the lower pressure is on the intrados.

(source)

The air stream separation at the stall will occur on a side with different characteristics. One could expect the stall will happen differently.

Equivalent representation, where the gravity is inverted and the wing orientation remains in the same direction. Flying inverted implies flying at a negative angle of attack.

The values taken into account are the airflow direction and the chord line, reflected in the angle of attack value.

As commented, while the two previous images suggest a wing in horizontal flight, the situation can be extrapolated to any stable linear trajectory with a negative AoA.

The horizontal plan or the pitch angle are not necessary to determine the stall angle (though they influence the stall speed).

Short answer: Asymmetric airfoils have different positive and negative stall angles, the largest absolute value of the two depends on factors like nose shape and camber. With positive camber (normal and utility aircraft), the negative stall angle can be the largest (in absolute values) but the maximum negative lift available before stall will be smaller than for the positive stall. Larger Reynolds numbers push the stall further away in both directions

It depends on the airfoil. With symmetric airfoils, the stall angle is the same for positive and negative stalls. Positively cambered airfoils (the sort mostly used) have their negative stall at a smaller absolute value of the lift coefficient compared to their positive stall, but the stall angle can well be at a higher absolute value.

Below you see a polar plot for a supercritical airfoil which I used for this answer. The positive stall angle of attack is 8°, while the negative one is around -10°.

Polar plot of the R2A airfoil at Mach 0.6 (own work)

The stall angle depends on details of the nose contour and the camber: Positive camber means that the zero lift angle is shifted to negative values, so there is some bias to negative values in the polar. However, if the lower part of the nose has very high curvature, it will create a high suction peak which leads to flow separation just aft of the nose already at a small negative angle of attack.

One extreme case would be the Göttingen 417a airfoil. Airfoiltools unfortunately plots only a range of Reynolds numbers suitable for model airplane enthusiasts, but the plot below should get the point across. The positive stall angle is 12° at the highest Reynolds number, while the negative stall angle is only around -8°.

Gö 417 lift over angle of attack. The lowest Reynolds number (blue line) is 50,000 and the highest is 1,000,000 (olive green line). Note that all curves are XFOIL predictions - real-world data might look differently.


Why does this NOTAM make me think GPS is unavailable? First: I'm not a real pilot. I used an on-line flight planning tool, and in the brief was the following "GPS NOTAM":

GPS 11/032 ZSE NAV (CHLK GPS 15-15) GPS (INCLUDING WAAS, GBAS, AND
        ADSB)MAY NOT BE AVAILABLE WITHIN A 476NM RADIUS CENTERED AT
        360822N1173846W (BTY214059) FL400-UNL DECREASING IN AREA WITH A
        DECREASE IN ALTITUDE DEFINED AS:
        432NM RADIUS AT FL250,
        375NM RADIUS AT 10000FT,
        340NM RADIUS AT 4000FT AGL,
        253NM RADIUS AT 50FT AGL.
        DLY 2130-2230
        1511172130-1511182230

I must admit I don't understand most of this, but it seems like GPS is unavailable in what seems to be a huge inverted cone of space. What's going on here?

This outage is due to department of defense GPS interference testing taking place 17-18 Nov 2015.  The interference generated may make the GPS signal unreliable and the outage cone described by NOTAM lets you know that you can't trust the GPS signal during this time.  This interference may be purposeful jamming or live testing of devices that may interfere with GPS to ascertain what level of interference occurs.


Would a gimbaled, always horizontal cockpit be beneficial to fighter jets? Would it be possible to have a spherical cockpit that was gimbaled in jets such as fighters so the pilot was always upright. With all controls in the cockpit wirelessly communicating with the rest of the jet?

Wouldn't this be an advantage as it would be easier to aim and the pilot wouldn't get disorientated and could see anywhere by maneouvering?

With enough engineering (read, money), all sorts of things would be possible, but if you picture the aircraft in a hard, level turn, the g-forces would be pressing "down" relative to a conventional cockpit, but "sideways" for a cockpit that was gimbaled so the pilot was sitting upright relative to the horizon.  And that would probably be a lot harder to take than the usual "down" G-forces.  Resisting 9 G's pulling your hand and forearm straight down can be done with a support for your arm (side-stick controller) or a bit of a ledge at the bottom of the handgrip (side-stick or center stick), but keeping precise control of the aircraft with 9 G's pulling your hand sideways might be a lot harder.

Beyond that, the sideways G force would be pushing your head to the side, which probably ends up banging your helmet -- pretty hard sometimes -- against whatever restraint or canopy it meets.

Something that achieves some of the goals you mention is the "Virtual Reality" helmet of the F-35, where the pilot can essentially look through the airframe and "see" aircraft, targets, and whatnot even in places that he wouldn't have an actual line-of sight.  That allows him to look in any direction, reacting to threats, tracking his target, and so forth.  And without having to gimbal the cockpit.  Although for what the F-35's cost, that might actually be cheaper!


Can Transition Altitude (TA) and Transition Level (TL) be equal? In Europe, is it possible that Transition Altitude and Transition Level are equal, that is, TA=8000 ft and TL=FL080? In that scenario, at 8000 feet, what pressure (QNH or QNE) should be set in the altimeter? Is it possible that Transition Altitude is higher than Transition Level, e.g. TA=10,000 ft and TL= FL090? If the above scenarios are not possible, is there some minimum altitude of the transition layer that must always be assured?

The TA and TL will always be at least 1000ft apart to ensure that aircraft flying at each will always be properly separated.
The TA will be a fixed value to ensure terrain and obstacle clearance, while the TL will be the lowest reasonable value that provides this minimum vertical separation above the TA. However, “reasonable” includes not having any possible numerical ambiguity.
So, if your TA is 8000ft, then the lowest possible TL is FL085. Depending on the QNH, though, the TL may be higher.


What is the acceptable position of the brake wear indication for A320? What is the acceptable position of the brake wear indication (PIN) for an A320 to be safely dispatchable?

There are two wear pins. If the end of either pin touches the machined part of the piston housing the heat pack is worn out and the brake must be replaced. Measurement must be done when the brakes are cold. If one wear pin is missing, check to make sure the other pin is there and looks ok. If both pins are missing the brakes need to be replaced.


What is the difference between a functional check and an operational check? Can anyone explain what is the difference between a functional check and an operational check in aviation?
Basically I faced a problem to prepare the task card for functional and operational check of a FDR (Flight Data Recorder).

From the ATA spec 100 which is the standard for aircraft maintenance documentation, they provide the following definitions:

  (c) Following are definitions of three categories of test that shall
  apply:

  1 Operational Test

  a.  That procedure required to ascertain only that a system or unit is
  operable. These tests should require no special equipment or
  facilities other than that installed on the aircraft and should be
  comparable to the tests performed by the flight crews. It is not
  intended that the operational test of the unit shall meet the
  specifications and tolerances ordinarily established for overhaul, or
  major maintenance periods.

  2 Functional Test

  a.  That procedure required to ascertain that a system or unit is
  functioning in all aspects in accordance with minimum acceptable
  system or unit design specifications. These tests may require
  supplemental ground support equipment and should be more specific and
  detailed than an operational test. It should contain all necessary
  information to perform proficiency tests to maintain system or unit
  reliability at an acceptable level, without reference to additional
  documents.

  3 System Test

  a.  That procedure containing all adjustment specifications and
  tolerances required to maintain system and/or unit performance at
  maximum efficiency and design specifications. It shall be
  self-contained and may duplicate other tests. It is normally used at
  major maintenance periods.

I would expect that these definitions are pretty universal in aviation.


Does a failure to respond to FAA questions on ME disqualify for sport pilot? I had submitted a medical evaluation to pursue a private pilot certification. The FAA had requested further evaluation by specialists. I didn't respond to their questions. Does this disqualify me for a sport pilot license?

Yes I believe so.

While the Sport Pilot License only requires that you are medically qualified to get a driver's license there is a stipulation that if you had been denied an FAA medical previously then you cannot just use your driver's license as your medical.  If you don't respond to their questions then you will most certainly be denied the medical.

At this point I think it is best to answer their questions and finish out the process.  If your medical condition is under control and manageable the FAA will usually eventually grant you the medical however, a lot of paperwork, time and money may be required.

This is why you want to do your homework before the medical and be sure that you can pass it.  If you don't pass the medical then you cannot just simply fall back to the Sport Pilot License.


What is the diffference between landing lights, taxi lights and external lights? A comment on this website along with an answer for the same question added confusion to my mind. Before reading this, I thought taxi lights and landing lights were the same, which seems to be true is some cases. I thought those lights were used for approach, taxiing, and take off and were strong lights designed to light the floor in front of the aircraft in the darkest night. Moreover, the answer linked above speaks of exterior lighting.

Now, I got a serie of questions to clarify the role of those "exterior lighting":

What are the difference between them (landing lights, taxi lights, other exterior lights I never heard of)?
What purpose do they serve and when are they used? (Perhaps it is the same question)
Are all aircrafts equipped with?

There are basically two types of (exterior) lights in an aircraft- the navigational lights, which are required to be switched on from sunset until sunrise and other forms of exterior lights, which can be switched off at the pilot or operated only at some parts of the aircraft operation, like the landing/taxi lights.

A (non-exhaustive) list of aircraft lights:

Navigation lights All aircraft are equipped with a steady light near the leading edge of each wingtip. The starboard light is green while that on the port wing is red. The different colors make it possible for an outside observer, such as the pilot of another aircraft, to determine which direction the plane is flying. These are required to be on during operation (in night).
Navigation or Position lights In addition to the red and green lights, most large planes like airliners are also fitted with other steady white navigation lights in various locations like the trailing edges of each wingtip, horizontal tail and top of the vertical tail. The main purpose is to increase the visibility of aircraft from behind.
Anti-Collision Beacon lights These are are flashing (or strobe) light assemblies installed on the upper and lower fuselage of aircraft,used to improve visibility of the aircraft. These can be switched off at the discretion of the pilot under some conditions.
Strobe lights High-intensity strobe lights that flash a white-colored light are located on each wingtip. These flashing lights are very bright and intended to attract attention during flight. They are sometimes also used on the runway and during taxi to make the plane more conspicuous.
Logo lights These steady white lights on the surface of or at the tips of the horizontal stabilizer are used to illuminate the company's logo painted on the vertical tail. These are usually switched off in airports to improve the visibility of the aircraft.

1) Navigation lights 2) Aft light 3) Anti-collision strobe lights 4) Logo light
"Jet-liner's lights 1 N" by CC BY 3.0 via Commons.

Wing lights Many airliners feature lights along the root of the wing leading edge that can be used to illuminate the wing and engine pylons in flight. These lights may be used to make the plane more visible during takeoff and landing or to inspect the wings for damage in flight. Pilots can also use the wing lights to inspect the wings and slats for any ice accretion that might build up when flying through clouds.
Taxi lights A bright white lamp is located on the nose landing gear strut of most planes. This light is typically turned on whenever the aircraft is in motion on the ground for greater visibility during taxi, takeoff, and landing.
Landing lights Typically the brightest light in the aircraft, these are fitted on most planes (and helicopters) for enhanced visibility during the landing approach. These lights can also be used to illuminate the runway at poorly lit airports. They can be located in the wing root, in the outboard wing, or somewhere along the forward fuselage (the usual location incase of helicopters), with some aircraft having them in more than one location.

Boeing 787 Lights; Source: captalk.net

Runway Turnoff lights Usually located in the leading edge of the wing root, these bright white lamps are intended to provide side and forward lighting during taxi and when turning off the runway. These lights are most useful at poorly lit airports but are usually unnecessary. The lights can also be used in flight if greater visibility is required.
Wheel Well lights Some planes are equipped with additional lights in the nose and main gear wheel wells. These lights are provided primarily to assist ground personnel in making pre-flight inspections of a plane at night.

The image below shows the lights in a Boeing 737.

Image from staticflickr.com


Does the F-16 have a speed limiter? In an aircraft like F-16, how do the pilot prevent going faster than Vne (velocity, not exceed)?

From the block 50 manual it seems to indicate Vne is 800KIAS/2.05M.

I believe even the earliest F-16s could go mach 2, so the newer with their much more powerful engine should easy exceed 2.05 mach.

Is there an automated system that reduce throttle when exceeding?

Section V of the Block 50 F-16 Flight Manual, is the "operating limitations". It says:

  MAXIMUM AIRSPEED OPERATING LIMITATIONS
  Refer to figure 5 3. Maximum operating airspeed is 800 knots from sea level to 30,000 feet MSL. Above 30,000 feet MSL, the aircraft is limited to 2.05 mach.

The introduction to section V states (my emphasis):

  The aircraft and system limitations that must be observed during normal operations are presented in this section..

What does "observe" mean? One might assume that means the pilot can just sit back and watch them automatically occur, or the pilot must "observe" them by knowing and acting apon them. As evidence it is most likely this second interpretation, the list of contents in this section appears to be many things that the pilot needs to do.

It does not seem like a list of things the aircraft does automatically.

But more signifficantly, the contents of the flight manual also includes a section describing the flight law control system (FLCS) limiters:

and:

There is no mention of maximum speed.

So, it looks strongly like the FLCS limits 4 things (AOA/G, roll, yaw, rudder authority), and warns you about some other things (low speed), but does NOT limit maximum speed.


Would the flyby in this video be illegal under 91.119(a)? I was watching this video

             and just curious about the legality. Would it just fall under 91.119(a) operation of an aircraft 'anywhere'

This depends how you look at it but there are definitely some FARs this guy is in breach of. First one that comes to mine is 91.119 on minimum altitudes, it's pretty straightforward (assuming this is an area other than congested area)

Except when necessary for takeoff or landing, no person may operate an aircraft below the following altitudes:
(a) Anywhere. An altitude allowing, if a power unit fails, an emergency landing without undue hazard to persons or property on the surface.
(c) Over other than congested areas. An altitude of 500 feet above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 feet to any person, vessel, vehicle, or structure.

He is pretty clearly within 500 ft of that person...
You can see that there is a case for sparsely populated areas but I would argue that if there is a backyard party going on this is not a sparsely populated area.
Then as mentioned there is the catch all in.13

§91.13   Careless or reckless operation.
(a) Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.

I'd say that's reckless but of course that's up for debate. Then again it's impossible to say where this happened so we can't know what type of airspace this is in and if there were possible issues there. I would go on to say that at an altitude that low I would turn on my landing light to increase awareness and enable me to see the ground better.
If you really want to push it you could even go as far as saying that maneuver is aerobatic flight since it's an abnormal attitude (at the altitude it's being performed) and most likely not seen in normal flight. For the record that attitude looks to be about around 30 degrees which would be a standard steep turn (something I would never attempt that close to the ground).

For the purposes of this section, aerobatic flight means an intentional maneuver involving an abrupt change in an aircraft's attitude, an abnormal attitude, or abnormal acceleration, not necessary for normal flight.

If you think it fits under this definition the pilot is also in breach of 91.303

No person may operate an aircraft in aerobatic flight—
(b) Over an open air assembly of persons;
(e) Below an altitude of 1,500 feet above the surface;

I would consider a backyard party (as described in the video description) as an open air assembly of persons and he is clearly below 1500 ft.
The Counter Point: While the above points make a pretty solid case for the FAR's broken in the video, in theory if the maneuver is pre agreed upon, planned out, and executed as planned it may not be considered reckless. In the end of the day as a pilot you live at the mercy of the FAA and their decisions. In this case I would be most worried about that guy running into the propellor. I'm sure things like this happen more often than not.


Should helicopters fly when they the lack power for vertical climb out of ground effect? Following up on this question: If a helicopter lacks power to climb vertically (i.e., with no translational movement) out of ground effect, is it ever considered safe to fly?  If so, under what conditions?

Or is the hover capability of a helicopter generally considered to be independent of its safe flight envelope?

Up to a certain speed, which will be in the POH, the greatest power requirement is when hovering out of ground effect.  Next is hovering in ground effect and power required in forward flight is the lowest, and decreases up until a point where the increase in parasitic drag starts to increase the power required.

See a typical power required curve.

From the AOPA blog

So it is possible to not have enough power margin to be able to hover OGE but enough to hover IGE.

To perform a normal take-off (ignoring a lot of side effects such as need for pedal), come to a normal hover height, about 2-3 feet, then push the cyclic forward.  Increase collective smoothly to maintain altitude and allow the aicraft to accelerate which it will, since increasing collective increases power and the rotor thrust now has a forward horizontal component. As significant translational lift kicks in at about 10 kts, a reduction in collective is required to avoid climbing early.  You don't want to be climbing with high pitch, and therefore high drag, if the engine quits.  When reaching about 45 kts or so, pull back on the cyclic to adopt a climbing attitude.  The aircraft will settle at about 65 kts.

To perform a "creeping" take-off, increase collective to become light on the skids.  Gentle forward pressure on the cyclic will start a slide on the ground. As translational lift kicks in, apply a little back pressure to lift gently to climb to about 1-2 feet to remain in maximum ground effect and continue to accelerate.  Wait until 45 kts is reached, and off you go.  During this manouver, the collective is left where it is until established in the climb, aside from small adjustments needed to keep the altitude constant during the creep.

I am not sure if it is a certification requirement, but every helicopter I've flown is capable of safely flying when there is not enough power margin to hover OGE, and most helicopters I have flown are on the weak side (R22, B206, B206L, S300, Enstrom 280) so I guess all helicopters can do so.

I checked a few types certs and I can see nothing about creeping take-offs. I am reasonably sure that the only limits that apply are for RPM, MAP, torque, N1, N2, temperatures etc. As long as you don't exceed those and are in accordance with the POH, then it doesn't matter how you get airborne. As you can see from the power required curve, the power required for forward flight is a great deal lower than for hover.

I've copied in your comment:

  I understand that vertical climbing is not hovering, but neither is it
  "flying" in the same sense that translational flight adds lift.
  Positive vertical climb out of ground effect with no translation is, I
  deduce, the hardest thing a helicopter can do. The maximum rate of
  climb in that attitude seems like something you'd have charts for, but
  I don't even know a term or acronym for it.

Because of translational lift, a helicopter requires less power to climb when flying forward than it does to hover, in or out of ground effect.  The greatest power requirement is when hovering OGE and increasing the hover height requires more power still so yes, climbing vertically OGE requires more power than any other regime.  There is no chart, or term for this that I am aware of simply because you should not do it and the POH will detail restrictions, not least, the "dead mans" or avoid curve.

http://flyinghelicoptersinalaska.blogspot.co.uk/2010/04/why-dont-helicopters-go-straight-up-and.html

The only time you should ever climb vertically (a "towering" take-off) is when operating out of a confined area when it would not be possible to get out without clearing the trees, walls or whatever.  Some pros do it all the time, e.g. power line inspection/repairs, logging and so on but they accept a risk of operating outside of the limits and are certified and insured appropriately.

The avoid curve shows the combinations of height and speed in which test pilots (i.e. the best) have demonstrated that a safe landing is possible if the engine quits.  You should not fly in the shaded area so you can see (and that chart is similar for all helicopters) that a towering take-off should never be used unless there is no alternative, and only then, for the shortest possible time.  Coming into a high hover, say 50 feet, or continuing a vertical climb OGE is prohibited by virtue of it being in the shaded area(s) of the chart.

Helicopters are not always intuitive.  Many people are surprised by the fact that climbing needs less power than hovering, and the faster you go (up until that point where the parasitic power increases dramatically), the less power is required.  Imagine a car where less power was required to accelerate and go faster!


What are the best traffic pattern airspeeds in a Cessna 172N? I have problems with my landings, and I would like to know what is the best speed
in the traffic pattern, especially on base and final? I am landing with 20 degrees flaps (in different weather conditions), and to be honest as a student pilot I am confused by the Cessna 172N POH, can anyone help?

This is really something you should discuss with your instructor, but if you want some general guidance then this should work:

Downwind: 70kts, 10° flaps
Base: 65kt, 20° flaps
Final: 60kts, 30° flaps

I say "guidance" because you may need to adjust this but it is in line with the POH values that you quoted in your comment and FWIW it's what I would use 'by default' in an unfamiliar C172. Although personally I wouldn't use 40° of flaps unless it's a short-field landing: the aircraft will come down more quickly with them fully extended.

But I'm not an instructor and I don't want to make assumptions about what he's teaching you and why, so you should definitely ask him this question. Ask him what power setting (RPM) he recommends on each leg, too.


landing with east to west wind on runway 11/29 i was wondering if anyone could help me out with this doubt i have. the kananga airport in dr congo has a single runway labelled 11/29. upon checking on google maps the number 11 is on the left end and the number 29 is on the right. if the wind is blowing east-west which of the two sides would i say i was landing on? i know planes land against the wind so i figured one would land from left to right , meaning one would land on runway 11. but after i read about runway numbering i got more confused because i understood that the number 11 should actually be on the right side rather than the left. can anyone help me understand why the runway  has an 11 on the left (which i understood should be west) and 29 on the right (supposedly east)? and if im correct to assume that with an east to west wind one should land from west to east meaning one would land on runway 11?? thanks a lot

if im correct to assume that with an east to west wind one should land from west to east meaning one would land on runway 11??

That is correct. The convention is that the runway number is the magnetic heading of the runway to the nearest 10 degrees, but with the low-order 0 removed. Thus, to the nearest 10 degrees, the magnetic heading of runway 11 is 110 degrees. That's the approximate heading the aircraft is going to have when they land on that runway. That would be a landing (or takeoff) to the east rather than the west. In other words, it's the heading of an aircraft using the runway as opposed to where the runway end is in a left-right sense when looking north.

Kolwezi Airport has only a 3° west magnetic variation. Thus, in your area magnetic heading and true heading are nearly the same.


How do I remind myself to run in-flight checklists? When flying a basic light aircraft with no "modern" instrument the pilot needs to remember to do various checks on a regular basis (FREDA every x minutes, change fuel tank every Y minutes etc.).

Just flying the plane can be consuming a lot of attention (especially without auto-pilot or if you become unsure of your position) and it's easy to forget to run those checks. I wouldn't want to forget changing fuel tank so I was wondering:

What are the tools that I can use to make sure that all checks are done on time?

In my view, the best tool for reminding yourself of recurring "household chores" is by using a timer, if the noise profile of your aircraft allows it. (Try it! You can probably hear more than you think, even in a loud prop). Either use a kitchen timer (which has a really simple physical interface, although two hands may be required) or your smartphone (a bit more complex interface, typically allows for single hand usage, with the added benefit that you can run multiple independent timers, and that the alarm will not stop sounding until you dismiss it). Modern GA EFIS (for example G1000) typically have timing facilities built in.

If you are unable to use an actual timer, determine the interval of the task (for example every 15 minutes), and use your normal watch. (For example "Check the tank every time the minute-hand passes the 0, 15, 30, 45 marks").


Determining total engine life in relation to SMOH/SFOH When evaluating the value of an aircraft, the total hours since major (or factory) overhaul (SFOH/SMOH) is an important indicator of price since it tells us how many hours the engine has operated since the last overhaul.

I read in another answer that the typical time between overhaul (TBO) interval for a piston engine is 2000 hours.

However, what is the best way to determine the actual TBO interval for a given engine model? I'm hoping someone might know of a handy list somewhere rather than having to dredge through service manuals for each model I'm interested in.

2000 is common but that depends on the engine. The official TBO is what ever the TC/manufacture states, here is the list for of the Lycomings published by Lycoming and here is the continental list. These are manufacturer recommended TBO's and are presumably based on data and tests they have run but they are in the end an estimate and usually reflect a relatively cautious value.

The "actual" TBO is a somewhat subjective number that no one can give you. Internet data and general collaboration can help you get a picture of what to expect for a given airframe/engine combination but individual units must be handled on a case by case basis. When assessing any aircraft its good to look for statements like "generally makes it to TBO" or "this aircraft had a lot of engine issues" or "oh man that turbo charger made that a finicky power plant" then come to your own conclusion. Forums, message boards, and old APOA articles can be your friend on these matters. Oil analysis and compression testing is the best objective way to measure engine health and a good pre-buy is important for that. Some people say "if it ain't makin' metal keep on flying it"


Airbags in aircraft - is that just a military thing? The Bell OH-58D (a military helicopter used by the USA) has an airbag to make life better in the event of a crash.

I had never heard of such a thing in an aircraft before and I wondered if this unique to the Bell OH-58 and, if not, whether civilian light aircraft come with such an option ?

Airbag seatbelts on planes are a "new" thing (well, since about 2001 apparently), but they're available on new-production aircraft and can be retrofitted to older General Aviation planes as well.

You can also find them on some airliners (typically in the exit rows where the next row is too far away to assume a proper brace position).

AmSafe (the folks who make a good percentage of the aviation-approved seatbelts) has a nice video (or two) on the subject of aviation airbags.


Are there any airports that do not have an IATA code but have a huge number of traffic? I am collecting a list of all airports in the world for my project, at present I have 46000 airports in my database but only 7000 airports have IATA code. So my question is, if I only take 7000 airports with IATA code and delete the rest will I lose any major airports or airports with huge traffic.

Are there any airports that have no IATA code but do have a huge passenger traffic or provide connections to major airports?

According to this thread, pretty much anything with scheduled air service will have an IATA code to facilitate booking.
From looking at the FAA data, it looks like everything with more than 10,000 enplanements (enplanement=1 passenger boards an aircraft) is considered a "primary" airport, and they all have an IATA code unless the airport is strictly general aviation and unscheduled service.
For these reasons, you probably won't find anything with "huge" passenger traffic but no IATA code. There may be airports with unscheduled service to major hubs, though.


Why do some military aircraft use variable-sweep wings? There are a couple of American military aircraft (the retired F-14 and the B-1 come to mind immediately), that have variable swept wings.  I know that they keep the wings full out (roughly perpendicular to the body) during take off and landing, and they have the wings swept back for high speed flight.

But I've never really understood why? I assume that at lower speeds the wings-out configuration creates more lift.  But, why does sweeping the wings back help at high speed?

For bonus points: how does the performance gain make up for the cost in weight and complexity created by having a variable swept wing?  Or perhaps there isn't much of a gain and that's why most military aircraft don't have variable sweep?

By sweeping wing backwards, you essentially make the air 'see' another airfoil.
Look at these two diagrams:

When the air meets the wing, it travels along $V$. This 'seen' airfoil is a lot thinner than $V1$ (which is perpendicular to the leading edge).  As the aircraft experiences less pressure, the aircraft experiences less drag. However, this also reduces the velocity of the air. This is good if you're looking to delay things such as shockwaves as the air goes supersonic, since spreading the force out more delays this effect and the flow separation which comes with it.

Unfortunately, for all the advantages, It would have a very high landing speed. So the wings are swept forward, the aircraft has a more effective aerofoil for lower speeds, and the minimum airspeed decreases correspondingly. There is now more drag, but also more lift which you need for the slower speed.

Wikipedia puts this nicely in not all too-complex terms:

If we were to begin to slide the wing sideways (spanwise), the sideways motion of the wing relative to the air would be added to the previously perpendicular airflow, resulting in an airflow over the wing at an angle to the leading edge. This angle results in airflow traveling a greater distance from leading edge to trailing edge, and thus the air pressure is distributed over a greater distance (and consequently lessened at any particular point on the surface).
This scenario is identical to the airflow experienced by a swept wing as it travels through the air. The airflow over a swept wing encounters the wing at an angle. That angle can be broken down into two vectors, one perpendicular to the wing, and one parallel to the wing. The flow parallel to the wing has no effect on it, and since the perpendicular vector is shorter (meaning slower) than the actual airflow, it consequently exerts less pressure on the wing. In other words, the wing experiences airflow that is slower - and at lower pressures - than the actual speed of the aircraft. One of the factors that must be taken into account when designing a high-speed wing is compressibility, which is the effect that acts upon a wing as it approaches and passes through the speed of sound. The significant negative effects of compressibility made it a prime issue with aeronautical engineers. Sweep theory helps mitigate the effects of compressibility in transonic and supersonic aircraft because of the reduced pressures. This allows the mach number of an aircraft to be higher than that actually experienced by the wing.

Is there a performance gain? Depends on your definition:
Having fixed swept wings would imply that:

The landing speed might be unacceptably high, and with that, long runways would be required.
You could design a fast and light aircraft.Take the F104 Starfighter for example, which had thin, short wings for the same effect but couldn't move them, and was extremely dangerous at slow speeds.

Having variable wings:

The F14 was intended for carrier operations, where you would want a
slow landing speed. Making an aircraft go that fast would probably have been impossible otherwise. It was also designed for long range operations, adding weight of fuel.
Adds weight and mechanical complexity to the airframe, which increases maintenance cost and fuel burn.

If I understand it correctly, today aircraft today rely more on system such as high-lift devices like for instance slats and flaps.


What is the purpose of an inboard aileron? I've noticed that larger Boeing aircrafts (such as the 777, 767, 747, etc) are equipped with inboard ailerons. What is their purpose and why hasn't Airbus incorporated them on the A330 or the A340?

As far as I know, the inboard ailerons are used for high speed maneuvering.
The rolling moment induced by the ailerons is linearly dependent with the distance to the center of gravity (e.g. the farther out, the stronger the moment, the harder the roll).
As the aileron is an aerodynamic surface, the force induces is equal to:
$$F_{aileron} = c_l(\alpha) \frac{1}{2} \rho V^2 S_{aileron} $$.
The moment induced is equal to this force, multiplied by the distance to the c.o.g.:
$$M_{aileron} = y_{aileron} \cdot c_l(\alpha) \frac{1}{2} \rho V^2 S_{aileron} $$
If you're flying in cruise , and you want to make a smooth turn, you need a relatively small $M_{aileron}$.

However, the $V^2$ term, is already very large. So we want to make the rest of the terms relatively small.

We could do this with outboard ailerons: Use the outboard aileron with a large $y_{aileron}$, combined with a small $c_l(\alpha)$, usually the outboard $S_{aileron}$ is fixed by the low speed control requirements.
However, you can provide $M_{aileron}$ more accurate if you use the inboard ailerons (with a small $y _{aileron}$). The smaller distance makes the moment a bit smaller, and less susceptible to small deviations.

I think the Airbus A330 & A340 use their spoilers to maneuver at high speeds. They simply extend their spoilers -which are usually also reasonably close to the c.o.g.- a tiny bit, this changes the lift distribution, also causing a rolling moment. I did not research this fact, so I'm not 100% sure this is the reason the A330 and A340 do not have inboard ailerons, but I do know some aircraft use this method for high speed maneuvering.


In the US, is it customary to tip the line personnel? The etiquette of interacting with the line personnel wasn't really taught to me during flight training, and I feel that every time I ask for line service, I'm doing something wrong by not tipping. I don't travel with groups or other pilots very often, so I don't see this in practice...

Do line workers in the United States expect tips for service (fuel/parking) from GA pilots?

I've never tipped for aircraft refuelling or other line services, and have never been criticized or looked at the wrong way for not doing so. That said, tipping in the US has nothing to do with the industry in which the person works; it's a personal reward for good service, and it is both appreciated as well as remembered the next time you are there.


What are some of the fastest UAVs? How do the pilots ensure that their controls are being transmitted to actions? I would clarify that the drones I refer to are UAVs that were piloted using a control station. I know many military projects include such capabilities but I want to know how pilots work on such supersonic UAVs and fly at such high speeds. I read that it was an alternative to the high risks of piloting and training a supersonic aircraft pilot, but I'm curious about such projects and how they fly at such high speeds and how they know when they're in control and when do they know that these crafts are out of control.

The question of the "fastest drone" is not entirely answerable, since whatever answer I give, I'm sure the United States military has a secret drone that goes twice the speed that we have no idea about. I will however put forth the Lockheed D-21 as one of the fastest piloted drones. It tops out at 2,500mph, and was used as a high altitude aerial reconnaissance UAV.

On the other hand, one of the workhorses of the US Air Force is the Global Hawk, a surveillance drone that travels at 400mph.

Regardless of which one you are flying, the speed of communication is the same. Radio transmissions travel close to, or at, the speed of light. That's roughly 670,616,629mph. That's enough to go around the earth more than seven times per second. Due to this, you have near instantaneous feedback from the drones sensors, and near instantaneous control. Yes, things happen faster at 2,500mph, but it is still very controllable.

Beyond the fact that lag is a non-existent problem, take a look at the control stations these guys use.

Due to the fact that they are on the ground which has no meaningful weight limitations, they can equip themselves with a ton of equipment, information, and controls that they would not otherwise have on an aircraft. A few items shown on their screens are sensor feedback for avionics, navigational data, and mission data. With all of these, you really have no human-based excuse for a trained pilot to "lose control" of their aircraft, excluding instances where their transmission may be jammed by a hostile actor. In this situation, the drone has an automatic "Return To Base" function that activates after you have lost control of the drone for a set amount of time.


What happened with the YF-16 prototypes? What happened with the original two prototypes of the YF-16 that were ordered during the Lightweight Fighter Program? Were they converted into combat-capable F-16's?

Some of them are in museums, YF-16 was a technology exploration and didn’t commit to production.

YF-16

72-1567 – Virginia Air and Space Center, Hampton, Virginia
72-1568 – Rome ADC

YF-16A (Full-Scale Development)

75-0745 – Used as a traveling exhibit, on loan from the National Museum of the United States Air Force, Wright-Patterson AFB, Ohio
75-0746 – Pylon-mounted gate guard, McEntire Air National Guard Base, South Carolina
75-0748 – Cadet Area Quadrangle, U.S. Air Force Academy, Colorado
75-0750 – Experimental Aircraft Display Hangar, National Museum of the United States Air Force, Wright-Patterson AFB, Ohio

YF-16B (FSD)

75-0751 – under restoration at the Air Force Flight Center Museum, Edwards AFB, California.
75-0752 – Frontiers of Flight Museum, Dallas, Texas


What is the drag optimized wing form for given lift and bending moment? I am trying to build a passive hydrofoil board.

Something like this:

I want to use wood and i am coming to the conclusion that i want to optimize drag for a given lift and bending moment.

Doing my research i found out that an elliptical wing isnt the best approach.
But a linear tapered wing should work better.
Is there a rough formula for this optimization?

I already came across two articles:
"Lift Distributions for Minimum Induced Drag with Generalized Bending Moment Constraints" and
"THE SPANWISE DISTRIBUTION OF LIFT FOR MINIMUM
INDUCED DRAG OF WINGS HAVING A GIVEN LIFT AND
A GIVEN BENDING MOMENT"

I only need a rough formula/shape

You have already found the most relevant articles. In 1933 Ludwig Prandtl published "Über Tragflügel kleinsten induzierten Widerstandes" in the Zeitschrift für Flugtechnik und Motorluftschiffahrt. 17 years later R.T. Jones came to the same conclusions in NACA-TN-2249. The 2013 article "Lift Distributions for Minimum Induced Drag with Generalized Bending Moment Constraints" by David J. Pate and Brian J. German goes one step further and optimises drag at a lower and structural mass at a higher lift coefficient.

But that is not all that relevant to your case. You want a wing which supports your weight plus the small surfboard which sticks out of the water. At those small loads, structural mass is almost negligible. Scaling laws mean that structure becomes more important as aircraft grow in size and mass. You know that elephants have much more massive legs relative to their body size than antelopes (or even ants, for an even more drastic comparison), since body mass scales with the cube of linear dimension while structural strength scales only with the square of linear dimension. This means that wing spar weight will be proportionally higher for larger aircraft.

As a consequence, insects have more elliptic wings than albatrosses, and model aircraft have optimum wings which are much more elliptic than the optimum wing of an airliner. The optimum shifts from an elliptic load distribution at very small scales to an almost triangular distribution at large scales.

What is more relevant is the angle of attack range in which this wing is meant to operate. The pumping motion seems to lead to quite large angle of attack variations, so you cannot optimise that wing for just one flow condition. Instead, it must be robust enough to produce tolerably low drag over the whole operating range.

Drag has two components: Pressure drag and friction drag. Pressure drag has one part which stems from lift creation, which in turn depends on the mass of fluid affected by the wing. That part drops with speed and wing span, so you want to be fast with super wide wings in order to minimise that kind of drag. However, friction drag increases with speed, so there is an optimum which varies with wing planform.

But there is another issue. Wide wings are very sensitive to angle of attack variations while more narrow, stubby wings will tolerate a wider angle of attack range. Also, the increased chord length of more stubby wings produces less friction drag per area than the narrow but wide wing.

Next to consider is the relative thickness of the wing's cross section (called an airfoil) and its camber. A thicker airfoil will create more static lift (buoyancy) but will create less dynamic lift. Make the airfoil too thin and its useable angle of attack range will become very small. I would select a thickness between 12% and 15% of chord length. Camber should reflect the operating lift coefficient: For highly loaded wings, use more camber. The tail surface, however, should be uncambered.

Another important factor is the incidence of wing and tail. The tail should have less for positive static stability, but you need to include the wing's camber into that calculation.

All those questions can only be answered when we know the flow speed, the mass on the board and the size of both surfaces. But you have to start somewhere. For that, it might be best to start with a proven design, collect the operating parameters and then go back to the drawing board with a sound basis for optimization.

For a first shot, start with a trapezoidal wing planform in which the tips have 70% of the chord of the root. Aspect ratio should be 5 or 6. You may add highly swept tips and 3D-shaping as you desire, but don't overdo it.


Can someone explain to me why these planes just don't turn about their center of gravity? Look at this one:

The thrust force from the engine should generate some clockwise moment, right? Why won't it turn and just crash then? I guess the wings generate some sort of anti-clockwise moment? Can someone explain this?

The thrust force from the engine should generate some clockwise moment, right?

If we take your photo as a reference (and thus we intend "clockwise in the picture plane"), yes, it is. It is called pitch moment, because it would change the pitch of the aircraft (in your case, a A-90_Orlyonok), and in the case shown in your picture is negative, because it would make the aircraft pitch down.

  Why wont it turn and just crash then? I guess the wings generate some sort of anti clock wise moment?

Because the wings and the horizontal stabilizer produce a total net pitch moment that counteracts the one generated by the engine.

They do so by having the center of lift in a different location w.r.t. the center of gravity.

The "normal" configuration for airliners would have an opposite pitch moment due to the engines, since in that case the engines are usually below the wings (and, being usually low wings, below the center of gravity). It is well illustrated in this image taken from the FAA:


Why are number of landings logged? I have flight logs for a WW II B-24 navigator who flew with a combat crew in China Burma India.  There is a column for number of landings.  The entries range from 1 to 5 or 6, mostly 1 or 2.  The time can be as long as 8:00 to 10:00 or more.  Why is the number of landings recorded?  Is that still done today?

Yes, it's still done today, at least in military aviation.

It's to ensure that the undercarriage does not excede the number of landings it has to endure before being checked/overhauled/replaced. Landing an aircraft can typically be the most stressful regime of flight in terms of loading of certain bits of the aircraft, so it is pertinent to log how many times it happens per flight.


Why does the TAF provide forecasts at irregular intervals? A TAF is published 4 times a day at 6 hour intervals. However if you look at the forecasts within a TAF one might be at a 4 hour interval and the next at an 8. Just look at this example:

TAF KPDK 071124Z 0712/0812 32009KT P6SM SKC
FM071600 33011G18KT P6SM FEW045
FM080000 32008KT P6SM SKC

Why not publish at consistent intervals?

I assume you're referring to the FM (FROM) groups. The TAF itself is published at regular intervals but sometimes the conditions are expected to change significantly in between those times; obviously, the weather doesn't operate on a neat schedule. So the TAF will show the expected time of those changes.

According to the National Weather Service:

  FROM Group: ie. (FM1600)

  The FM group is used when a rapid change, usually occuring in less
  than one hour, in prevailing conditions is expected. Typically, a
  rapid change of prevailing conditions to more or less a completely new
  set of prevailing conditions is associated with a synoptic feature
  passing through the terminal area (cold or warm frontal passage).
  Appended to the FM indicator is the four-digit hour and minute the
  change is expected to begin and continues until the next change group
  or until the end of the current forecast.

  A FM group will mark the beginning of a new line in a TAF report. Each
  FM group contains all the required elements -- wind, visibility,
  weather, and sky condition. Weather will be omitted in FM groups when
  it is not significant to aviation. FM groups will not include the
  contraction NSW.


Can a plane be IFR certified, but not allowed in IMC? The Evektor Harmony LSA & RV-12 SLSA have all the necessary equipment to be certified for IFR flight, except the only engines allowed for purchase are the Rotax 912 ULS & iS. These engines are not allowed in IMC.

Question: Can these planes be certified for IFR, despite the IMC limitation?

if no, can they be used for training for one's instrument rating and for the instrument rating checkride?
if yes, then if one were to be flown into IMC, would be this be a legal violation or a warranty violation?

Question: Can these planes be certified for IFR, despite the IMC limitation?

No

Refer to  FAA Order 8700.1:

  B.  Type Certificate Data. Appropriate type certificate data will
  indicate whether the aircraft meets the requirements for IFR
  operations.

  (1) Section 91.9(a)prohibits aircraft operations without compliance
  with the operating limitations for that aircraft prescribed by the
  certificating authority.

if no, can they be used for training for one's instrument rating and for the instrument rating checkride?

Yes, as per FAA Order 8700.1:

  9.  USE OF AIRCRAFT NOT APPROVED FOR IFR OPERATIONS UNDER ITS TYPE CERTIFICATE FOR INSTRUMENT TRAINING AND/OR AIRMAN CERTIFICATION
  TESTING.  The following paragraphs are intended to clarify the use of
  an aircraft not approved for IFR operations under its type certificate
  for instrument flight training and/or airman certification testing.

  A.  IFR Training in Visual Meteorological Conditions (VMC). Instrument
  flight training may be conducted during VMC in any aircraft that meets
  the equipment requirements of part 91, 91.109 , 91.205 , and, for an
  airplane operated in controlled airspace under the IFR system, 91.411
  and 91.413 . An aircraft may be operated on an IFR flight plan under
  IFR in VMC, provided the PIC is properly certificated to operate the
  aircraft under IFR. However, if the aircraft is not approved for IFR
  operations under its type certificate, or if the appropriate
  instruments and equipment are not installed or are not operative,
  operations in Instrument Meteorological Conditions (IMC) are
  prohibited. The PIC of such an aircraft must cancel the IFR flight
  plan in use and avoid flight into IMC.

For example, you can obtain an Instrument--Rotorcraft in an R44, even though its not legal in IMC.

  Although the R44 IFR Trainer was designed for instrument flight
  training, it is not approved for actual IFR operations. While student
  pilots are flying on instruments, the instructor must act as the
  safety pilot and operate only under VFR conditions.

if yes, then if one were to be flown into IMC, would be this be a legal violation or a warranty violation?

This would be a legal violation of 91.9(a)

  91.9(a) Except as provided in paragraph (d) of this section, no person may operate a civil aircraft without complying with the operating
  limitations specified in the approved Airplane or Rotorcraft Flight
  Manual, markings, and placards, or as otherwise prescribed by the
  certificating authority of the country of registry.


How can aircraft determine true north for inertial navigation systems? Aircraft use GPS and inertial navigation systems in combination. Sometimes INS needs alignment (i.e. to be told where it is AND what orientation it has). How can an aircraft be be aligned to true north for proper INS alignment? Are markings at the parking location good enough?

How can an aircraft be be aligned to true north for proper INS
alignment? Are markings at the parking location good enough?

A small 0.5° error in assumed heading translates into 10 NM error in computed position on the distance Madrid-Montreal (3,000 NM). As many other errors must be taken into account, north must be known to a higher degree to achieve this accuracy, and a better accuracy is also expected This cannot be done using simple markings.
But true north is related to Earth spin axis, which can be accurately sensed by an INS using a gyroscope.

Inertial systems belong to two families: platform and strapdown:

In platform INS (PINS) accelerometers and gyros are mounted on a gimballed platform kept level and oriented to north by torque motors driven by gyros outputs.

In strapdown INS (SINS) these sensors are firmly fixed on the aircraft.

PINS are mostly using momentum wheels. Recent gyros are SINS and not inertia-based, they usually measure length differences created by Sagnac effect (fiber optic, ring laser, acoustic...):

Ring laser gyros in AN/WSN-7 US Navy navigation system (source)
Regardless of the type, alignment is similar and consists in sensing Earth spin axis and gravity, and is twofold: Leveling, then gyrocompassing.
Alignment principle
Accelerometers and gyroscopes arrangement in an inertial system, and final orientation:

Leveling
Axes which in the end will point to north and east are in a first step leveled by tilting the assembly. Leveling solely involves accelerometers and gravity.
Leveling is real for PINS, it gives the platform its working orientation, found by zeroing north and east accelerometer outputs while the unit is stationary. After alignment, accelerometers are able to directly sense accelerations along north, east and up directions.
Leveling is virtual (software) for SINS. It consists in evaluating the initial orientation to build a transformation matrix. This matrix is used for projecting the values issued by tilted sensors onto a virtual level platform. The matrix is then constantly updated with orientation changes detected by gyros.
Gyrocompassing
Likewise leveling, gyrocompassing is real or virtual according to the type of inertial system. Gyrocompassing goal is to determine true north using gyros.
After leveling, for a stationary aircraft, north and east gyros (designated X and Y below) sense only Earth rotation, in quantities depending on their orientation relative to Earth spin axis:

On the left hand picture, X gyro fully senses Earth rotation effects as it is aligned with spin axis, Y gyro doesn't detect any rotation.
On the middle picture, the system has been rotated by 90°, the result is reversed.
The right hand picture depicts the general case: Both gyros detect Earth rotation rate, but in quantities depending on the heading. X detects a rate proportional to the cosine of the heading, Y a rate proportional to the sine.

Finding north is done by rotating (virtual rotation for SINS) the leveled assembly about the vertical axis until the east gyro output is null, meaning its axis points to east or west. This ambiguity is removed by sensing rotation direction on the other axis which now points north or south.
Earth rate detection depends on (cosine of the) latitude angle:

Earth rotation rate sensing by an inertial system versus latitude
Merging what we exposed above, the sensed rates Rx and Ry are given by:
$$  \begin {cases}
Rx & = & \omega \times \cos \alpha \cos \phi \\
Ry & = & \omega \times \sin \alpha \cos \phi
\end {cases}$$

$\omega$ is the constant for Earth spin rate, $\approx 15$°/hour (360° in a sidereal day),
$\phi$ is the latitude,
$\alpha$ is the heading of the aircraft (or of the reference axis).

With two linear equations for two unknowns, finding latitude and heading is straight forward, the only condition is Rx and Ry are known. That is where the shoe pinches. Gyrocompassing is actually difficult and slow.
Actual application of the principle is challenging
While the principle is simple, it's difficult to measure Rx and Ry. The reason is $\omega$ is tiny (0.004°/s), measures must be repeated during a significant time. Five minutes is sufficient at the equator, but twenty minutes are not unusual at 45° latitude.
The pilot can provide the current approximated latitude to allow this process to converge more quickly.

Boeing 737, FMS page used to enter the initial position estimate to speed up gyrocompassing

Sensor fusion, alignment in motion and loss of alignment
Alignment may be facilitated, and subsequent measures can be improved by associating additional sources of data, and merging sources after filtering excessive divergence, this is known as sensor fusion and IRS/GNSS hybridization. These sources include GNSS (position and Doppler velocity), magnetometers, air data, even Doppler velocities of known targets but that's more for underwater rovs to replace GNSS.
The different sources allow to maintain the natural cumulated drift associated with the integration process, which is central to inertial positioning, below some threshold, e.g. thresholds fixed by required navigation performance procedures.
In some conditions the inertial system can even be realigned in flight from GNSS position and Doppler velocity (e.g. Northrop Grumman LCR-110). Such in-flight alignment should be common in the future.
Level and north orientation are kept after alignment by adjusting the orientation and compensating detected aircraft orientation changes. The sidereal alignment is therefore not maintained.
Most inertial systems cannot be realigned in motion, but they can be re-leveled, and while the north direction and the position cannot be recovered, the system is still be able to provide aircraft attitude (mode ATT).

The two IRS switches in the normal (navigation) position (source)


What is an aviation handler? I was reading this page about Eurocontrol and flight plans. The author refers several times to a "local handler" who apparently assists in dealing with ATC.

What is a handler, who do they work for and what do they do?

Handlers are private companies who provide services (for a fee) to pilots and passengers when they fly in and out of an airport.

Depending on the airport, sometimes handlers are required by the local authorities to be used, and other times there are not even any handlers available (a lot of US airports do not have them, for instance).  Often though, they are optional and there to assist crews that are unfamiliar with the procedures at a new airport or who want their assistance as a convenience.

They typically assist with coordinating things like fuel, lavatory service, flight plans, local permits, passenger transportation, catering, coffee/ice/newspapers, etc.  Many times they pay for everything (in the local currency) and provide one consolidated bill for the client to pay.

Note that handlers can be employed by local FBO's (Fixed Based Operators), or they can be part of a larger international company that provides handling services all over the world.  In either case, they are generally coordinating all of the different parties and services needed in order to operate an aircraft in and out of their airport.


What is the highest possible thrust generated by 15cm ducted fan? I recently developed a sudden interest in flying. I'm wondering whether a pair of small ducted fans could lift a person off the ground.

Lets say the ducted fan is 15cm in diameter. What would be the most thrust a fan like that could produce?

A 15 cm diameter fan could maybe lift 4 kg of mass with a tip speed of 0.6M.
By increasing the power and the blade chord length (in other words, increasing the solidity of the rotor), this may be increased to maybe 6 kg or more, but a single fan would probably never lift more than 10 kg mass.
The thrust of a rotor is $C_T \cdot \rho \cdot \text{Area} \cdot \text{tip speed}^2$.

$\rho$ = air density ($1.225~\mathrm{kg~m^{-3}}$ at sea level).
$\text{Area}$ = area of rotor disc ($\mathrm{m^2}$)
$\text{tip speed}$ = the linear speed at the tip of the rotor = radial speed * radius.
$C_T$ = coefficient of thrust. (is usually in the order of 0.02 to 0.05 for small rotors)

Assuming $C_T$ of $0.03$, the calculation shows $40~\mathrm{N}$ ($=4~\mathrm{kg}$) thrust.
Disc area is one of the most important parameters, even a 30 cm disc would generate 4 times the thrust. 60 cm would make almost 65 kg.
Apparently, a 50 cm diameter disc can possibly generate 45 kg thrust on its own. So two of them could carry and maybe even lift-off a person equipped with this backpack (the person + the system must be less than 90 kg).


Do FAA regulations keep aircraft away from 18,000 feet MSL to avoid conflict with FL190? In the US, is there any FAA Regulation that keeps all aircraft away from 18,000 feet MSL… Since theoretically, FL190 could be lower than 18,000 feet MSL, and create all kinds of confusion with keeping safe aircraft separation?

It's not the altitude that is adjusted, but the flight level. Aircraft are allowed to fly at 17,500 feet MSL. Per the rules provided in other answers, this would be VFR flight to the north or east. an IFR aircraft could be flying at FL180, and this would provide 500 feet of separation.

However, 14 CFR 91.121 determines the lowest usable flight level. If the altimeter setting is 29.92 or higher, FL180 provides at least 500 feet separation from the next VFR altitude, and at least 1000 feet from the next IFR altitude. However, if the altimeter setting is lower, then the lowest usable flight level is higher than FL180.

Current altimeter setting   Lowest usable flight level
29.92 (or higher)           180
29.91 through 29.42         185
29.41 through 28.92         190
28.91 through 28.42         195
28.41 through 27.92         200
27.91 through 27.42         205
27.41 through 26.92         210


Material alternatives for aircraft leading edge Are there any material alternatives for the aluminium used on the aircraft leading edge? By alternatives, I mean composite alternatives.

If we're talking about transport category airplanes, the options depend to some extent on the wing.  Wings with hard leading edges (no slats) and supercritical airfoils with poor stall characteristics are not very tolerant of what is called "runback ice"; ice that melts or is prevented from freezing by a warm but not hot leading edge and can run back and re-freeze just aft of the leading edge, forming a ridge, creating a dangerous risk of premature stall in the approach configuration.
These wings may require "evaporative" anti icing, which heats the leading edge to above the boiling point, so that supercooled drops that strike immediately turn to steam and can't flow back onto the wing plank as water.
Such a leading edge will run at about 110 degC when anti-ice is on and you will burn yourself pretty nicely if you touch one.  The leading edges can be aluminum since the aluminum's heat treat state isn't affected until over 200C. So a composite material would have to have a transition temperature above 200C to have some margin to be tolerant of overheat cases.
If your wing has slats, runback icing isn't a concern in the approach configuration with slats extended, so the slats themselves may only need to be heated to about 50-60 degC, and the composite options become wider.
If you're not finding any high temperature composites on the web, try searching just for high temperature Epoxy laminating resins.  There are all sorts of resins that can handle temperatures almost as high as aluminum and some that are higher.
In any case, the biggest reason composites aren't used for heat-anti-iced leading edges is poor conductivity. If you heat the leading edge with bleed air blown inside (typically) or with electrical elements bonded on the back of the skin (I think that's how Boeing does in in the 787), you need the heat to transfer to the outer surface immediately with almost no temperature loss (so that a sensor mounted on the inside surface is reading very close to the actual temperature on the outer surface) and you generally want a smooth surface, so aluminum is the ideal material.
Of course you could go with rubber boots and make the leading edge out of whatever you want, if you're willing to live with the inferior performance at ice shedding. Airplanes typically use boots because their engines don't make enough surplus bleed to heat a leading edge. Just about all turboprops and some small corporate jets like the earlier Citations are like that - and some are able to heat the leading edges but have to use boots on the tail, or, make the tail big enough to handle the ice so you don't have to anti-ice it at all (the CRJs do this).


Can you identify this aircraft at Chilliwack(YCW)? This aircraft is sitting at YCW, with no visible registration numbers.  Can you identify it?

It's a De Havilland DH104 Dove. It did have a registration of N4913V but it was cancelled in 2012.

The Dove is an airliner that can seat 8 passengers, and 542 were produced from 1946 to 1967.


Where exactly can I find the FAA's UAS pilot training course (ALC-451)? I'm trying to get my commercial drone operators certificate. I already have a private pilot certificate, so from the FAA website, all I need to do is complete the "Part 107 small Unmanned Aircraft Systems (sUAS) ALC-451" course on faasafety.gov, then file some paperwork. But I can't find the course. Is there a direct link to it?

Even when faasafety.gov is working without issue, the course can be difficult to find with the site's search feature. The Part 107 Small Unmanned Aircraft Systems (small UAS) course is presently available from this direct link: Part 107 Small UAS Course.

Note that you will have to be logged in to do the course. If you follow that link and are not logged in to faasafety.gov, log in and then go to that link again.

Note also that faasafety.gov currently seems to be experienceing occasional technical issues, either due to maintenance or traffic volume. The link may not work reliably at present.


Where is the modification of an aircraft from floats to wheels and vice versa logged? How does a maintenance professional, or pilot if allowed, log the change when the airplane is switched back and forth between land and sea? Do they cite the change in the maintenance logbook as performed via STC #, or performed via maintenance manual section XYZ?

If the aircraft was type certified in either configuration, then no STC is necessary. Technically there should be an entry in the logbook for a landing gear configuration change, because it's considered maintenance and all maintenance is supposed to be recorded. Unless the aircraft is registered as "experimental" then the pilot is not allowed to perform anything more than Elementary Maintenance as defined by the regulations.

In Canada you don't need to reference the section of the AMM you performed the work to, they just make it mandatory to make reference to any work performed in accordance with another standard, such as an STC or a component maintenance manual. I've worked in four other countries and they had the same rule regarding the entry of maintenance, but I couldn't say for sure if it was the same way everywhere.


What is the meaning of "Bulk Strip"? What does the following mean and what ramifications does it have when purchasing an aircraft?

  term R/H-1958 HOURS SMOH@26/02/1988   - BULK STRIP 27/03/1998  ENGINE
  1472 SMOH

Does it mean the engine will be at its lide use and has to be replaced with another or am I correct that they replaced some parts to extend the time based on condition and will require a full overhaul 528 hors?

A bulk strip may be an Australian term, e.g. this airworthiness bulletin and this engine shop. I'd never heard of it before, but looking at those sites it seems to be something like a limited overhaul. The AB and the engine shop both list the steps and they're more or less the same: disassemble the engine, inspect all parts, replace any parts that need replacement, reassemble and test the engine.

I'm not an expert on maintenance by any means, but it looks to me like the difference between a bulk strip and a full overhaul is that the bulk strip is precautionary or diagnostic, as suggested by the machine shop site:

  Not all engines need a full overhaul. You may have had a prop-strike,
  or your LAME may have identified metal in the filter or a crack in the
  crankcase. The pilot may have noticed unusual running of the engine.

  We can ‘bulk-strip’ your engine for a full diagnosis and provide you
  with a report of your engine’s health:

Looking at it another way, an overhaul will replace a lot of parts whether they need it or not; a bulk strip may replace parts if they're worn or broken.


What does this A320 FCTM quote about cold weather and non-precision approaches mean? Can someone explain this paragraph from the A320 FCTM about minimum temperatures for non-precision approaches? I can't understand it.

  COLD WEATHER OPERATIONS For all Non Precision Approaches, there is a minimum OAT. Below this temperature, the error on the barometric
  altitude is no longer acceptable, and altitude should be corrected in
  temperature. As it is not authorized to make these altitude
  corrections to the final approach segment of the FM Flight Plan
  (F-PLN) through the MCDU, it is not possible to use FINAL APP when OAT
  is below this minimum OAT. The flight crew must then use selected
  vertical guidance. This minimum OAT is indicated on the approach chart
  or must be defined by the operator based on the terrain profile (plus
  adequate margin).

I am not sure which part you don't understand, but here is my attempt to explain in more laymen terms.

Short version: When it is too cold, a certain mode for flying non-precision approaches using the Airbus flight computer cannot be used. This is because the altimeter reading is inaccurate when it is too cold, but the computer is not designed to compensate that.

Long version: For all non-precision approaches (i.e. approaches where ground equipment is not providing glideslope signal), there is a minimum Outside Air Temperature.

Since the reading of the barometric altimeter is affected by temperature, when it is too cold outside, the reading can no longer be considered accurate enough for the safety of flight. Corrections should be made (using a mechanical flight computer / lookup table / other tools) to convert the indicated altitude on the instrument to the reading it would have indicated under normal temperature.

Since it is not authorized to make these corrections in the autopilot computer, it is not possible to use the "FINAL APP" mode to fly the approach when the outside air temperature is below this minimum.


Why are airships not more popular? There is this question but I am hoping for more general answers to why airships are not more commonplace.

The previous question does bring up the cost of production (mainly because of hi-tech fibers used in body construction) and the high capital costs for constructing hangars for these airships.

However, civilization did operate fleets of these before and I'm just not certain why they have not 'come back' in a meaningful way. For instance, here are just a few uses that I can think of off-hand:

For tourism! Instead of a water-bourne cruise tourists can literally sail all over (I'd pay for that!)...
As the previous question implied, they can be used to carry cargo vast distances (although I am unsure if there is any real economic advantage here).

I know we use them in a small way for various things (military, sports, etc) but why are they not more commonplace?

I imagine the answer is mere economics but at the same time I feel there should be a market if for nothing else than tourism.

No one has really touched on the possibility of a cruise ship style airship. We have mentioned brief aerial tours but I'm talking about a semi-luxury week long trip in one of these things over very remote areas such as Alaska or the Siberia or even a desert... I realise there would not be room for all the luxuries of a cruise ship (pool, casino, etc) but I image about the same luxuries as a small yacht...

I imagine there are a lot of issues with long trips but can airships make unimproved 'port calls'? IE can they just set down for a few hours to let the passengers stretch their legs?

Airships are much more susceptible to bad weather. With an airplane you can pretty much count on it taking off on time and arriving within a couple of minutes of the scheduled time. This allows operators to employ their aircraft with a reliability which an airship will never be capable of. This means that the investment made into an airliner will lead to a predictable stream of revenue and (hopefully) profit, whereas the airship might need to wait several days until it can take off again, or headwind  might delay it by more than a day on a long trip. In this time, it will accumulate interest and fixed cost, but no additional revenue.

Scheduled air services were indeed first done with airships, but as soon as dependable aircraft were available, only a few niches like transatlantic travel were operated with airships. As soon as airplane performance covered those niches, airships became obsolete.

Today the most credible use of airships would be transportation of heavy equipment into remote areas. Tourism could be a second use, but the number of airships needed to cover this market is small. Airships do have some benefits:

no need to build roads or runways
carrying of outsize cargo is no problem
if the airship is big enough, payload can be immense
destinations can be anywhere, not only on a coast or a big river (where regular ships cover this niche more than adequately)

The few cases where the airship could be employed gainfully would not cover the expense of developing and building one. Development cost alone would be similar to the development cost of a new airliner!

But if we look back in history, even at a time when airships were available, big transportation tasks were done by aircraft. Papua has impenetrable jungles, and to transport dredging equipment and personnel to the Morobe goldfield from 1932 on, aircraft were used rather than airships. The huge dredges were dismantled for air transport and flown into Bulolo, where they were re-assembled and put to work. With a Zeppelin, less dismantling would have been needed, but just the complication of keeping the airship balanced when the load is released made this option less attractive.

For the history buffs: To transport very long items like the dredge shafts, a single-engined Junkers Ju-52 (Ju-52/1m) was used. It had a detachable rear fuselage with  automatic couplings for the control rods. Spherical collar screws ensured precise positioning of the two fuselage sections.


Why do my ears pop if the cabin is pressurized? Simple question:

If a commercial airline cabin is pressurized, why do my ears pop as we ascend and descend? I was always told this was due to a change in pressure, but if the cabin and my body are at the same pressure, there should be no force causing my ears to pop.

Here is an illustration of how cabin pressure changes during a typical commercial flight:
image source: http://aerosavvy.com/aircraft-pressurization/

Note how when the pressure outside the aircraft is changing, the pressure inside the aircraft is also changing, but not as much.

During the flight shown here, you would experience about the same pressure changes that you would in an unpressurized Piper Cub that only went to 7000 feet.


What is the bank angle that should be sustained to move in a uniform circular motion? I want to deduce the rate of turn of targets that move in a uniform circular motion. The radius of the different circles are known in advance. I have searched for my question and found the following formulas to help me:
$$R=\frac{V^2}{11.26\tan\theta}$$

$$\omega=\frac{1,091\tan\theta}{V}$$

The variables used are:

$V$ = true airspeed in knots
$R$ = turning radius in feet
$\theta$ = bank angle in degrees
$\omega$ = rate of turn in degrees per second

As we see from the previous equations that we can get the velocity from the first equation and then substitute in the second one to get the rate of turn. But the problem in the bank angle, how can one put a reasonable values for this angle on different radius. Of course the targets that rotates in circles with small radius will different from that moves in a large radius each one will have its own reasonable bank angle and hence its own speed that it will fly with and hence its own rate of rotation.

Note:

all that because i am trying to sample the plane movement over different circles each T (secs). So i want the rate of rotation to be reasonable to be able to get enough samples to represent that movement. Also i have read that the civilian planes is different from the military ones, so it seems to be complicated topic. Any help is appreciated, thanks in advance.

I hope you also appreciate help that uses the metric system. You should. It makes the calculation much simpler.

So your question is: What bank angle $\varphi$ do I get when flying a circle of a known radius $R$?

All by itself, the question cannot be answered. You need to specify a speed, either the flight speed $v$ or the angular velocity $\omega$, in order to find the proper bank angle. Then it is easy:
$$tan\varphi = \frac{v^2}{R\cdot g}\;\; \text{or}\;\;\varphi = arctan\left(\frac{v^2}{R\cdot g}\right)$$
$$tan\varphi = \frac{\omega\cdot v}{g}\;\; \text{or}\;\;\varphi = arctan\left(\frac{\omega\cdot v}{g}\right)$$
where $g$ is the gravitational acceleration and all parameters should be inserted in their SI units. $\varphi$ will be in radians!

If you don't know either speed, try some values until you get a reasonable angle, which would be not more than 30° for civilian and 75° and more for military aircraft. You can find an upper bound for that angle if you know the maximum sustained load factor of the respective plane because the roll angle and the load factor $n_z$ in a turn are directly related:
$$tan\varphi = \sqrt{n_z^2 - 1}\;\; \text{or}\;\;\varphi = arctan\left(\sqrt{n_z^2 - 1}\right)$$


Why do the old A320s have a lower tailwind limitation compared to the newer enhanced ones? In my company on the older A320s particularly the ones which are in the MSN 2000 range their tailwind limitation for takeoff and landing is 10 knots. But our newer enhanced ones have a 15 knots limitation. Any reason for this? Does Airbus do any modifications to the aircraft so that it's tailwind limit increases?

Short answer: the increase from 10 to 15 knots requires extra certification / flight testing, which is an extra item the airline will pay for.

Over at pprune:

  Boeing offers an increased tailwind limit of up to 15 knots for a fee.

I suspected that but had no proof, so that got the ball rolling.

From page 35 of an Airbus presentation presented to the civil aviation authority of PRC confirmed that being an option:

  Other configurations outside China include options as:

  Water/waste freezing protection for extended flight conditions
  15 knots tailwind certification at landing
  15 knots tailwind certification at take off

And finally from the FAA's Flight Test Guide For Certification Of Transport Category Airplanes, there is a section for certifying takeoff and landing in tailwind greater than 10 knots.

It requires the demonstration of controllability, engine performance, autoflight systems, landing gear vibration, terrain awareness warning systems to not be affected by the higher sink-rank, etc.

Note that for a takeoff in tailwind >10 knots, even the application of takeoff thrust is different from when <10 knots. The advisory circular also calls for a disclaimer of sorts to be added in the AFM:

  The AFM should contain a statement that the limitation for tailwinds greater than 10 knots reflects the capability of the airplane as evaluated in terms of airworthiness but does not constitute approval for operation in tailwinds exceeding 10 knots.

Of course if the airline would also pay even more for the brake cooling fans option, they can reduce the longer cool down period after a tailwind landing, for example.

Some performance enhancements require extra certification and testing, but not necessarily extra equipment.

An airline can request their fleet to be certified for a lighter or higher MTOW depending on their operation. A light MTOW on the certification is cheaper to obtain, and it will allow the airline to save on ramp fees, route charges, etc. But on the other hand they can't carry as much payload/fuel. Other high MTOW options may require extra strengthening built into the airframe and more powerful engines.


What is the advantage of increasing an aircraft's horizontal tail surface area? Most aircraft have horizontal tails. What is the advantage of increasing the horizontal tail surface area?

Increasing horizontal stabilizer area increases pitch stability and the permissible range for centre of gravity at the cost of some aerodynamic efficiency.

Stability in pitch means that the aircraft tends to maintain constant angle of attack (which roughly corresponds to maintaining constant speed with pitch). An aircraft is stable in pitch if the aft surface flies at lower angle of attack than the front surface (this applies to both conventional tails and canards).

Making the horizontal stabilizer larger does the following:

The absolute change in lift force on the horizontal stabilizer when angle of attack changes is larger, so the aircraft returns to the trimmed angle of attack faster (increased stability).
As centre of gravity moves aft, the difference in angle of attack between the surfaces decreases to keep the plane balanced, reducing pitch stability. If the horizontal stabilizer is larger, the angle of attack changes more slowly for the same change in lift, so the centre of gravity may be permitted further aft before the aircraft becomes not stable enough.
As centre of gravity moves forward, the difference of angle of attack between the surfaces increases until angle of attack of the horizontal stabilizer exceeds its stall angle of attack and control is lost. If the horizontal stabilizer is larger, it can provide more (downward!) lift before it stalls, so the centre of gravity can be permitted further forward before loosing control authority.


Why must IFR be cancelled? Watching flying videos and listening to LiveATC, you will sometimes hear a pilot cancel IFR approach.

Why does the ATC care if you are on IFR or VFR? So long as you get on the ground safely, it seems like it wouldn't matter if you were looking at instruments or out the window.

Surely the "radar service" is still running for other aircraft, so it seems it's merely a formality to indicate how a craft was navigating in the event there's an accident. Is there another reason I'm not thinking about?

It's all about making sure everybody knows what's going on. While you are on an instrument approach and thus flying IFR, ATC is responsible for spacing you, giving instructions on heading and speed in order to fit you in with other inbound aircraft. In that situation they expect you to confirm and follow those instructions, and if you don't it makes for a lot of headaches.

If you're on a visual approach, such as when you've cancelled your IFR flight plan after descending below sparse clouds with the runway now in plain sight, then the tower knows not to give you spacing instructions; they instead space IFR traffic around you, while other VFR traffic just has to maintain their separation minima from you. Knowing that you're on a visual approach means ATC won't waste their breath trying to get you to adjust your airspeed, altitude or course to follow a traffic pattern; they'll just give those instructions to other aircraft in the pattern to maintain a steady traffic flow.

... unless you're in Class B space. If you are flying into a Bravo airport, you can still request a visual approach, but approach control has to OK that change (especially for airliners), and ATC is still required to provide separation services to you whether you're VFR or IFR, so they can still give you instructions regarding your airspeed to make sure you fit into the IFR traffic pattern. This may, at times, require an uncomfortably fast landing for GA pilots being spaced in between commercial airliners at a busy time. Some Class Bs like Phoenix or DFW will have a runway more or less reserved for smaller aircraft so there's less disturbance to commercial airliners with higher approach speeds, but not all airports have that luxury; if you want to land your small single at O'Hare or Dulles, you'll probably find yourself spaced in-between airliners and forced to very precisely time your approach so you can touch down and clear the runway in time for the plane behind you to take off. If traffic is heavy, you may not get in if you greet them with "Hello O'Hare Approach, Piper 354 Alpha...". As soon as the O'Hare controllers hear your GA small-prop callsign, if the pattern's too busy they'll deny you Bravo entry and recommend you head for a smaller airport like Chicago Exec or Gary.


What are the differences between intertank valve and crossfeed valve? Regarding the fuel system in an aircraft, I read that there are two valves: the intertank valve and the crossfeed valve.

They are both motor driven butterfly type valve which function is to allow fuel from the left and right hoppers to combine.

What are the differences between them?

In layman's terms, at least on a Gulfstream G450/G550, the cross-feed system allows the engines to be fed from either the left tank, the right tank, or both tanks (one to each engine.) the cross-feed pipes feed into the fuel hoppers, not directly into the other tank. While you can use the cross-feed system to fix a fuel imbalance, doing so would be via using the fuel (in the engines) not actually transferring it from tank to tank.

The intertank system is a valve and pipe system between the tanks themselves, allowing a fuel imbalance to be fixed by initiating/allowing the transfer of fuel from tank to tank directly.

My sources were:
G450/G550 Fuel System
G450/G550 Fuel Unbalanced