Are zero visibility landings a possibility for the future of commercial aviation? Airports usually close or slow down activities in fog conditions, e.g. this seems to be a significant problem in winter at Delhi airport.

Large commercial aircraft would be able to land with no visibility (more accurately with no Decision Altitude/Height). However, if using the ground ILS, the runway must offer a Category IIIc landing system. While Cat IIIa and Cat IIIb are common at large airports, there are no Cat IIIc runways in the world of commercial aviation.

The reason is landed aircraft wouldn't be able to safely leave the runway and taxi to the gate because taxiways are not equipped with features appropriate to no-visibility conditions.

On the other hand driverless cars now are experimented on public roads without specific equipment, including in fog, and driverless vehicles push back aircraft prior to taxiing:

Driverless pushback at Heathrow (source Youtube and manufacturer site, can be autonomous with ground markings)

In the long term airports with autonomous tugs on the whole taxi phase may be interesting as they allow savings by providing power supply while taxing, which in turn can decrease fuel needs and mechanical wear.

Is there a possibility such zero visibility landings could happen in the near future, because desirable for airlines and airports?

Practically there are no zero visibility landings.  There is reduced visibility of varying levels.  Reduced visibility landings are only allowed when the appropriate ILS equipment is operational.
The practical issues are actually not the landing itself but separation from aircraft landing afterwards.  On the ground the runway lights are bright enough and close enough for slow navigation.  However the aircraft takes longer to clear the runway and this means increased separation between landing aircraft is required.   It is this that causes delays as aircraft waiting to land are pushed further and further back.

A ground tug to facilitate speedy removal of the aircraft from the runway makes sense in theory, but would be hazardous and the acquisition time wold be non trivial.  While an estimated stopping location could be calculated for an aircraft finishing its roll out after landing it would be subject to a very large variation and the tug would have a large distance to cover to get to the aircraft.  It would then have to connect and start the tow.  The question is whether this wold be quicker than the current arrangement and what the mass penalty would be in terms of changes to the aircraft structure to enable it to use the tug.

In response to comments I would like to clarify that my point about the acquisition time of the aircraft by the tug is concerned with the non trivial and varying distance the tug would need to cover to reach the aircraft.  Any tug configured to pull an aircraft weighing in excess of 100tons would need to be large, heavy, and have high torque to help it safely move the aircraft.  These are all things which will reduce its top speed and so increase the time it takes to get to the aircraft from some optimal starting location.

Multiple tugs:
Multiple tugs would increase the probability that the aircraft would stop within optimal distance of one of them but would also increase the collision risk and add complexity to the local airflow over the runway.
Starting locations:
To facilitate quick response and removal of the aircraft from the runway the tugs would need to be located close to hand.  However they cannot be on the runway and would be required to be back from the edge by a significant distance.  When you consider the finite number of taxiways and the sheer variation in aircraft roll outs this presents a very wide range in possible locations the tug would need to get to with a consequential increase in response times.
Aircraft Rollouts:
When an aircraft lands it typically uses a small amount of remaining velocity to keep it rolling and move off the runway.  This would almost always be quicker than waiting for a tug.
Hybrid tug system:
A hybrid system in which the aircraft uses residual speed to clear the runway and meet a tug has some advantages in terms of optimal tug positioning however it reduces the advantages in fuel consumption and increases the amount of vehicles independently moving around the airfield.


How does an aircraft taxi? Does an aircraft use its engines to taxi or is it always pulled by some other vehicle?

I have seen a small lorry pulling it sometimes; is it really pulling the huge thing, or does the aircraft use its own power somehow?

Aircraft are not always moved with tugs (the small vehicle you refer to). Much of the time, they operate under their own power.

Airliners are generally pushed back from the gate using a tug, as that's the simplest and most efficient method. They then taxi to the runway (and all the way up to the gate, after landing) using their own engines to provide thrust. This is true of both jets and propeller aircraft.

Some work is being done on self-contained electric-powered ground propulsion; one major player in this area is a company called WheelTug. They use high-torque motors and the aircraft APU to allow the aircraft to move itself without starting its engines or requiring a tug to be connected and disconnected.


Out of sight planes during final approach Does it ever happen that planes descending on a descent path (glide slope) block each other so that one (or more) plane is not visible from the tower (at the airport) or localizer? I want to know if that can cause technical issues. For example, how does a plane track a localizer if it's being blocked by the plane(s) in the front?

The 2nd plane's pilot should notice that he is not receiving the localizer on his instruments and break off the approach.


How low was the Hong Kong Kai Tak landing approach over the city? Hong Kong's Kai Tak approach was famous for its unique approach, including the "checkerboard" and its low altitude when passing over Kownloon.  The latter has been memorialized in innumerable photos and film clips.

At what altitude did landing aircraft typically pass over the city on this approach?

As the wikipedia article for Kai Tak, Runway 13 approach states:

  Typically the plane would enter the final right turn at a height of about 650 feet (200 m) and exit it at a height of 140 feet (43 m) to line up with the runway.

Given that some of those buildings underneath the flightpath were 6-storey and therefore ~60-70ft tall, the aircraft were less than 100ft above the highest rooftops. (This I can personally vouch for, since my sister did for a while live in one of those 6-storey apartment buildings and I have stood on the rooftop watching the planes pass over!)


Which single pilot jet has the largest cabin and longest range? I'm looking to rent a jet for a trip round the world with my friends but also since I have my license and MEIR, I'll be able to fly myself and don't want to hire another pilot.

Which single pilot jet has the largest cabin and longest range?

According to Wikipedia's list of light jets, the longest ranges (for single pilot certified jets) are:

SyberJet SJ30: 2205 NM
Pilatus PC-24: 2035 NM
Embraer Phenom 300: 1936 NM
Cessna Citation CJ4: 1927 NM
Cessna Citation CJ3+: 1825 NM

Sorting by max. number of passengers, we get:

Pilatus PC-24: 8-11
Embraer Phenom 300: 7-10
Cessna Citation: 8-9
SyberJet SJ30: 5-6

Considering you are already rated for the Phenom 300, this is likely your best option. Anything larger will always require a minimum crew of 2.


Do Airbus pilots have to practice flying in alternate law? I am sure when getting their type rating for Airbus FBW aircraft, pilots learn how to fly in alternate and direct law. After that, how much opportunity do they get to practice this skill?
On Air France 447 and Indonesia AirAsia flight 8501, the A/P switched off unexpectedly and the control law degraded to alternate 2. It appears in both situations that the PF had trouble stabilizing the aircraft roll attitude. It's difficult to determine whether this contributed to the pitch up and eventual stall but the original pitch up command may have been inadvertent while struggling to level the wings.
Under normal law, which pilots are accustomed to flying, the FAC calculates the amount of aileron deflection required to turn the amount of lateral sidestick deflection into a rate of roll with a maximum of 15° per second. In alternate 2 law the computer cannot make these calculations so it becomes direct stick-to-aileron control.
This can mean that the aircraft responds very differently than what the pilot is used to. Sometimes the difference may be slight but at high speed and high altitude the same stick deflections can result in a significantly higher roll rate.
Additionally the stick does not provide tactile feedback as to how much force is required to move the control surfaces. With conventional flight controls "roll or pitch rate would more directly correlate with the force applied on the control wheel, and not the amount of deflection required."1
These things combined can result in the pilot over-controlling. The control inputs can get out of phase with aircraft movements resulting in pilot induced oscillation in the roll axis. This occurred in both of the above accidents.
It must be terribly disconcerting for a pilot when the A/P disconnects and they are handed control of an aircraft they fly every day, but doesn't respond the way they are used to. I'm wondering how much practice they get at flying in this configuration, either in a simulator or in the aircraft itself. Do most airlines flying FBW aircraft require this regularly? Does Airbus - or anyone else - recommend any amount of further training or practice? Have these recommendations changed in light of the accidents?

1Bill Palmer, Understanding Air France 447

It appears in both situations that the PF had trouble stabilizing the aircraft roll attitude.

Note that roll control is secondary when recovering from a stall.

  Restoring normal pitch and roll, he stresses, are "of secondary importance"

The situation required that the pilots heed the stall warnings, be aware of the stall and put the nose down to trade altitude for airspeed.

  Have these recommendations changed in light of the accidents?

You can read the recommendations in the official accident reports. These usually explicitly state what such changes should be. I believe it is very rare (except when investigators disagree) for regulators, manufacturers and airlines to ignore any such recommendations.

AF447
QZ8501

From what I recall, the major problems in these two incidents were not that the pilots were simply unfamiliar with how control inputs affect flight in alternate law. major problems were that the pilots did not seem to be aware that they were in alternate law or even aware that the aircraft was stalled. In both cases the pilots were confused, failed to follow procedures and failed to coordinate their actions effectively (i.e. CRM failures)

Airbus Flight Crew Training Manuals do cover alternate law.

Recommendations included

  5.3 Aircraft Manufacturer
  1. The KNKT recommends that Airbus to consider in developing a means for
  flight crews to effectively manage multiple and repetitive Master Caution
  alarms to reduce distraction.
  2. The KNKT recommends that Airbus to consider and review the FCTM
  concerning the Standard Call-Outs in all phases of flight.

The Indonesian report lists  in some detail the actions taken by the airline and includes an Airbus article "What is a stall? How a pilot should react in front of a stall situation". It also includes some guidance issued by Airbus which includes recommendations concerning upset training in various control laws

  After that how much opportunity do they get to practice this skill?

This depends on airline (and relevant regulators) more than on manufacturer. Upset recovery is probably something that all pilots should be trained in, and maintain familiarity with, regardless of who manufactured the aircraft.


Are some countries' civil aviation authorities safer than others? Okay, put down the pitchforks, I'm not pointing fingers at anyone!

I'm just aware that every country has its own civil aviation authority, and I wonder if there's any reason to believe that some countries perhaps have safer standards, and perhaps are more vigilant in enforcing them.  As opposed to, say...the opposite of that.

My assumption is that most countries, if not all, take aviation safety extremely seriously.  And that, as a result, aviation accidents are truly random occurrences that have nothing to do with the country of origin for the flight.

But I may be wrong, hence I'm asking.

NOTE:  I am not interested in opinion and personal experience on this.  If you don't have statistics and accident reports to back up what you are saying, please refrain from answering.

Lets take the EU for example. In the EU, there are 230 banned air carriers. The majority of these (228) are banned because the EU CAA has determined that the country in which these carriers operate do not have enough oversight to ensure the safe operation, regulation, and maintenance of the airlines.

Two of the air carriers are banned specifically because of poor maintenance standards, obsolete aircraft, or other reasons.

Similarly the US has the International Aviation Safety Assessment although a much shorter list because it really only evaluates airlines that are codeshare or operated by companies that serve flights to/from the US. For the EU, this is a larger list because it does not require crossing an ocean to get there...

Typically you'll find on these lists quite a few airlines that operate out of developing countries like the Congo or Angola. Its reasonable to draw the conclusion that countries that do not have regulatory oversight for much outside of running the government itself leaves the standards of safety to the operators themselves.

This isn't to say that the operators are automatically unsafe, they just lack government regulation and observation of safety standards.

So to mirror what vortaq7 said in his comment above, quantifying this is going to be difficult since the types of operations vary highly between countries. For example the US has a high accident rate looking at pure fatalities over the last 25 years compared to any other country, but this is because the US is biggest aviation market.

Looking at it in the context of incidents per million flight hours may be available, but it also groups a lot of different types of flights like cargo and commuter. I'm sure this type of data is available for more developed countries with reporting systems and formal accident investigations but may be difficult for countries that don't have that.

And now for a personal note... I know you're not interested in the personal aspect but Angola's air carriers are banned in the EU, so it gives a little credibility to what I say above, plus its a good story. :)

A part of my job is travelling the world and being flown out to oil rigs for work on the automation and safety systems. About 8 or 9 years ago I had to fly out to Luanda, Angola to be flown out to a rig a few hours off the Angolan coast. The flight there (and back) was on the Houston Express, a direct flight from Houston to Angola.

Upon arrival at the Luanda airport, I was taken to the heli-terminal for the flight out to the rig. Now understand that in the US Gulf of Mexico people travelling by helicopter to an oil rig are required to have egress training. In the EU, travelling out into the North Sea also must include cold water survival. In Luanda however, there were zero requirements (I know this because I did not have egress training, nor was I asked to prove I did).

After 3 or 4 equipment delays we were finally boarded on the helicopter, I sat across from the flight engineer and 3 or 4 other rig hands were on board. After take-off and after we flew over the coast, here is pretty much the actual words I spoke:

  ME->Flight Engineer: What is dripping on the back of my neck? Smells like fuel.
  FE->ME: It is fuel.
  ME->FE: Should I be concerned?
  FE->ME: Only if it stops dripping, that means we are out of fuel.

Be sure I spent the remainder of the flight studying very carefully how to pop out the egress window...

When we got back to Luanda the aircraft we were supposed to take back to Houston had "mechanical problems" which delayed my return by 3 days. I could see the aircraft on the tarmac and it was sitting on cinder blocks without any tires, literally like you would imagine leaving a nice car in a bad neighborhood overnight. I spent those 3 days in a compound run by the oil company that had 16 foot tall walls with barbed wire and a very hefty front gate.


When is an aircraft required to be pressurized? For example a Cessna Grand Caravan doesn't have a pressurization system, but a Phenom 100  is pressurized, so what are the main reasons a manufacturer has to build an aircraft with a pressurization system: altitude, speed, etc?

The main reason is that it's simpler than having everyone wear supplemental oxygen masks. There is no specific requirement for pressurizing an airplane but there are requirements for supplemental oxygen as per FAR 91.211

  (a) General. No person may operate a civil aircraft of U.S. registry--

  (1) At cabin pressure altitudes above 12,500 feet (MSL) up to and
  including 14,000 feet (MSL) unless the required minimum flight crew is
  provided with and uses supplemental oxygen for that part of the flight
  at those altitudes that is of more than 30 minutes duration;

  (2) At cabin pressure altitudes above 14,000 feet (MSL) unless the required
  minimum flight crew is provided with and uses supplemental oxygen
  during the entire flight time at those altitudes; and

  (3) At cabin pressure altitude above 15,000 feet (MSL) unless each occupant of > the aircraft is provided with supplemental oxygen.

So by pressurizing the plane to under 15,000 ft you can fly over that altitude and not need to supply the passengers with supplemental oxygen for the duration of the flight. There are unpressurized piston planes that fly well into the flight levels but are required to carry supplemental oxygen and everyone aboard needs to use it, it's a look some people in general aviation don't mind,

(source)

On a slight side note the engine does have an effect on this choice when it comes to light singles. Pressurization systems on turbo props/light jets pull their air from the compression stage of the turbine which is already compressing air so no extra compression hardware is needed. If you have chosen a piston engine for your design you need to drive some form of pressurization system which can add weight and complexity. There are a few pressurized piston planes out there like the Mooney Mustang, Cessna 210 (came in both pressurized and un pressurized) and the Piper M350 (the only one currently in production).


How does the fuel pump work in a PA-28 and when should I use it? Just started flying again after 14 years and I'm taking this to the finish line this time. All my time, up till 10 am today, is in a C172. I did fly a PA 28-151 today and liked it.
I do have some questions about Pipers (specifically PA 28-181 and 28-151):

Does the fuel pump has to be on the entire flight?
Is there a fuel pump for each tank?
If the pump doesn't have to be on the entire flight, how does the engine/carburetor get fuel considering the engine is higher than the wings?

The PA-28 Family of aircraft have two fuel pumps, an engine driven mechanical pump that is always pumping (so long as the crank shaft is spinning) and is considered the "main pump". The airframe also has an electrical backup pump that can be actuated by the pilot. The tanks do not have their own pumps.
You should check the POH for your airframe but generally speaking in the Archer and Arrow I fly the electric fuel pump is turned on:

checking fuel pressure before start
during take off
climb out
when switching tanks (I also run it for 30 seconds after switching tanks incase I either need to switch back and to maintain fuel pressure at a critical time in the event of a failure of the mechanical pump)
on approach
landing/go-around
in the event of a mechanical pump failure
while practicing maneuvers (S-Turns, turns around a point ETC)
during a practice stall
Engine power loss in flight
Loss of fuel pressure

Most of the above items can be found in this POH.
Note: You should not be running the electric fuel pump for the entire flight, like anything this is a component that can burn out and it should be used when needed or dictated by the POH.


In RTCA DO-260B, what were the reasons GVA was added ? Is it a replacement for something? After digging up and reading through many documents, I have a rough picture of the evolution of DO-260 versions, particularly for its ADS-B Quality Indicators. For instance, in DO-260 (Version 0) integrity & accuracy were expressed in a single parameter called Navigation Uncertainty Category (NUC). In DO-260A (Version 1), more quality indicators were added such as Navigation Integrity Category (NIC), Surveillance Integrity Level (SIL), Navigation Accuracy Category for Velocity and Position (NACv & NACp), and many more. In fact, there were a lot of new things introduced in this version. In DO-260B (Version 2), more were added such as SIL supplements, System Design Assurance (SDA), Geometric Design Assurance (GVA), and many more.
It is important to note that the majority of the quality indicators in DO-260A were reliant on vertical components. For example, NACp is derived from Horizontal Figure of Merit AND Vertical Figure of Merit, NIC and its Vertical Protection Level, NACv and its VFOM, etc. However, in DO-260B, the vertical components from the quality indicators were removed and a new parameter called GVA was added.
I found too little info on GVA. What I know is that GVA shall be set by using the Vertical Figure of Merit (VFOM) 95% from the GNSS position source used to encode the geometric altitude field in the ADS-B message. Thus, each number on Figure 1 has a meaning. I searched about VFOM and found a schematic of it (Figure 2). Would this be a valid representation of GVA?

Figure 1

Figure 2

Sources:
 Figure 1 : RTCA DO-260B DRAFT Version 4.2
 Figure 2 : GNSS Height on Airbus A/C, Slide 5
 1. Analysis of Quality Indicators in ADS-B Messages by Simon Tesi & Stainslav Pleninger
 2. Study on Airworthiness Requirement for the Position QUality of ADS-B System by YAN Fang

The philosophy behind the the quality indicators is that the ADS-B transmitting system sends the data it has, as well as information about the quality of that data. The receiving systems can then decide, based on the quality indicators, whether the data is good enough for their intended application.

The reason that more find grained quality indicators were developed in subsequent ADS-B versions had mainly to do with two factors:

new insights in the type of ADS-B applications were being developed
better performance and quality information delivered by GPS receivers

Note: at the time of DO-260 development, GPS had Selective Availability (SA) still turned on. This deliberately degraded the accuracy of the civilian GPS signal. This meant that both in the horizontal and the vertical plane the accuracy was very poor compared to current levels.

The big difference between version 0 (DO-260) and version 1 (DO-260A) was the split between accuracy (95% error under nominal conditions) and integrity (protection against non-nominal performance).

DO-260B then split the quality indicators between the horizontal and vertical component (hence the introduction of Geometric Vertical Accuracy (GVA)), added a finer grained resolution to horizontal position integrity and distinguished between between measurement integrity and avionics faults (the concept of system design assurance (SDA)).

GVA is intended for ADS-B applications that depend on the accuracy of the geometric vertical position. One such application is RVSM compliance monitoring using ADS-B. They compare the reported flight level to the geometric height from ADS-B, using an meteo model to compensate for the non-standard pressure distribution in the atmosphere. GVA helps to assess whether a potential mismatch between geometric and pressure altitude is caused by the baro altitude system, the meteo model or the GPS receiver.

The representation in your question shows a cylinder with hard boundaries to represent HFOM and VFOM. Whilst the numbers indeed define clear boundaries, it is important to understand that these are the statistical 95% level of the position errors, as estimated by the GPS receiver. This means that

position errors may occur that are outside of the cylinder (5% of the cases, if the estimate is correct)
the estimate may be (and usually is) wrong. Receivers tend to be conservative in estimating the position error magnitude.

Disclosure: I was involved in the international combined Working Group 51 / Special Committee 186 that wrote the ADS-B standards, however I joined at the time when the work on DO-260B was almost finished. At this moment I not involved anymore in ADS-B standardisation and implementation.


What does this vehicle transport? I've seen on many airports a lot of vehicles transporting some "brown boxes".

Any idea of what is this?
 (I don't ask what's exactly inside this box, just in general)
Source: 123rf.com

What that may be?  Baggage? : in case that boxes contain baggage, why don't they put it as loose baggage or in ULD's. May that be other type of cargo?

Your question is rather broad and somewhat unclear, but what you have is a picture of a baggage tug and cart.

The carts are loaded with passenger baggage or cargo, then strings of them are pulled to the aircraft by the tug before being loaded on to the plane.

Based on the additional pictures, I think what you're after would be called palletized cargo. It's stuff, put in boxes, placed on pallets, wrapped with stretch wrap, with a net thrown over the top and hooked down to keep it from sliding off the pallet.

It is most likely not passenger baggage. That's either loaded one piece at a time by hand, or in a ULD.

If you're asking what's in those particular boxes, you'd have to zoom in quite close to find a label on a box to even have a chance of figuring it out. Just about anything can be and is shipped in a brown cardboard box, from anywhere to anywhere in the world. This question is far too broad and off topic for Aviation.SE, and probably any SE.


How is the range/fuel computed when there is a 3-hour difference for the same route? I am not a pilot. I recently took a trip SIN-SFO 14 hours (tailwind) and SFO-SIN 17 hours (headwind), flying United.

According to the Dreamliner Wikipedia page, the 787 range is 7,635 nmi (14,140 km), where the distance SIN-SFO is 8,447 mi.

How is the range rating computed? In the example above, there are 3 hours difference in flight time for the same range.
What is the difference in fuel consumption on the two trips (rough average)?

According to 'How does wind affect the airspeed that I should fly for maximum range in an airplane?':

  If you have a headwind, the longer you stay aloft, the more you are carried back, so you better hurry up. With a tailwind, it helps to slow down because now the wind is helping you to cover even more distance.

*

SFO = San Francisco

SIN = Singapore

If there is a 100 km/h headwind, you can't hurry by that much. That will exceed the plane's capability. Likewise for a tailwind, that would be too slow for the plane to stay aloft.

So, the increase and decrease of speed in head- and tailwind does not nullify the wind. The 14-hour flight was the one in tailwind. Think of the wind as a fast wave, with the plane riding it, the faster the tailwind, the faster the plane gets there.

The keyword is air distance. Wind increases (headwind) or decreases (tailwind) the air distance flown. And therefore the 3-hour shorter flight, would have saved 3 hours worth of fuel, roughly.

From Wikipedia, the Dreamliner cruises at 903 km/h. Using the total time in the air, you can arrive at a rough estimate of how long the air distance was.

From the table here 'For a B787 in cruise, what is the altitude, speed, and angle of attack?', we are looking at roughly 15 tonnes of fuel saved on the 14-hour flight.

The distance between SIN and SFO is 7340 in nautical miles, always use the same units. The plane's range you found is for a typical payload, if there are fewer passengers, the plane flies for longer, as it is lighter in payload and can carry more fuel. And with the wind accounted for, the actual air distance is then known.

See these posts:

Is this ferry flight route possible for a B737-800 WL?
Why can't Norwegian's Boeing 787 operate from Vegas during hot weather?
When does the airline check if an airplane can reach its destination?


Why is the initial short-field climb below Vₓ in a Cessna 172? Short-field takeoff procedure for a Cessna 172R is

  Wing Flaps – 10°.
  Brakes – APPLY.
  Throttle – FULL OPEN.
  Mixture – RICH …
  Brakes – RELEASE.
  Elevator Control – SLIGHTLY TAIL LOW.
  Climb Speed – 56 KIAS (until all obstacles are cleared then $V_y$).
  Wing Flaps – RETRACT slowly after reaching 60 KIAS.

However, best angle-of-climb or $V_x$ at is 62-67 KIAS depending on elevation. Why is the first stage of the climb slower than $V_x$, and if the slower speed gives better performance, then why isn’t that the published figure?

The published Vx is in the clean condition (no flaps): 62.

With the flaps down, for a short-field takeoff, Vx is 56.

This is the same reason why the stall speed (bottom of the white arc) is lower with flaps than without (bottom of the green arc)


How much does it cost to lease a jetliner? As the title says, how much would it cost to lease a airplane for an airline. Let's say a B737 and a B747 or similar aircraft.

I tried to contact several aircraft leasing companies but so far I've been ignored, and searching this site hasn't resulted in any answers either.

Slide 19 of this IBA presentation (PDF) shows how the aircraft's age is a factor:

Other factors such as global outlook, supply and demand, fuel prices, etc., all have an effect.

Anyway, that presentation confirms the accuracy of the data on an airliners.net thread from 10 months ago, from which:

For a 737-800 you are looking at USD 120,000–350,000 monthly.
For a 747-8i you are looking at USD 640,000–1,250,000 monthly.


What should I do with fuel drained from the sumps during preflighting? What should one do when done sumping a small amount of fuel during a pre-flight?  Should it be dumped on the ramp? Or is it considered necessary to find the container far away from the airplane? Is it considered safe to dump it right back into the plane if it is uncontaminated?

The FAA issued an Aviation Safety Program publication called All About Fuel which talks about fuel preflight, including sumping fuel.  It contains the following:

  Preflight Action     As pilot in command, you have the responsibility to determine that your aircraft is properly serviced.
  Check your aircraft before each flight and be sure you have the
  correct type of fuel. This practice may save your life! Take the time
  to inspect your aircraft thoroughly. Good preflight procedures:

  Be sure all of the fuel and oil tank caps and covers are installed and secured properly after you visually check the fluid level.
  Observe the color and odor of the fuel as you check the tank.
  Draw a generous sample of fuel from each sump and screen drain into a transparent container. Check for the presence of water, dirt, rust
  or other contaminants. Unless you have the proper kind of fuel tester
  (e.g., GATS jar with intact fuel strainer), do not attempt to save the
  fuel drained from the sumps by pouring it back into the tank. Note: If
  you are not equipped to safely pour the drained fuel back into the
  tank, please be sure to dispose of it in accordance with EPA
  regulation.
  Check that each fuel tank vent is clear of restrictions; i.e., dirt, ice, snow, bent or pinched tubes, etc.

Any FBO that sells fuel should have a way to dispose of it.  Some of them make it more obvious than others by putting sump barrels out on the ramp, but ask them what to do with it before you go out to the airplane and they should be able to help you out.  For environmental reasons, you definitely shouldn't dump it on the ground, because even if it does evaporate, it will come down somewhere.  At some airports you can even get in a lot of trouble (and have to pay a big fine) by dumping fuel on the ramp.  Don't take a chance!


What are the following documents about aircraft maintenance? What is a service bulletin? How does it originate?
What is a rapid revision in aircraft maintenance?
What is a non-conformation report?

What is the difference between these 3 documents?

A service bulletin is a document that the manufacturer of an aircraft issues directly to operators of related aircraft. It mandates some maintenance / replacement / check activities.

A rapid revision is a change to the maintenance documents, that is not released officially according to normal procedures, yet.

A non conformation report is any discrepancy found in the aircraft / system such that the related system does not look / operate / behave / function as expected. This document is mostly a form that the ground crew / technicians / pilots generate.

Disclaimer: these are general information and details would change depending on whether the aircraft is a civil or military one, and the certification type. This answer is written in the perspective of military aircraft, for general knowledge.


Partial Serial numbers on Military Aircraft - what combination is unique? Modern US military aircraft have partial serial numbers painted on their tails. Interestingly, it seems these serial numbers are actually not quite enough to be unique on their own, being comprised of only the year the aircraft was ordered, followed by the last 3 digits (usually) of the serial number. On occasionally this is not a unique combination by itself. Is there a specific combination of tail markings that is guaranteed to be unique? For example, if one combines the branch markings (such as 'AF' for Air Force) and/or the base code (such as 'AZ' for Arizona National Guard), would that be guaranteed to be unique?
Thanks so much for any insight.

I can only address the AF system, but I assume the other services systems are similar.  The AF serial numbers for vehicles (the system applies to all vehicles including ground vehicles) are made up of the letters "AF" followed by the contracted delivery year (2 digits) followed by a sequential number that restarts at 1 every year.  The pic below is of an F-22 assigned to the 1st Fighter Wing (the FF code).

So this aircraft was purchased in 2008. The sequence number is 161, and would have been assigned from a block of numbers provided in the contract.  The contracting officer was given the block by the office that keeps the master list.

It's worth noting that the paperwork and the data plate is where it has to be complete. What's painted on the a/c is just used for the convenience of the crews. For some a/c such as KC-135's and B-52's whose production run was less than 10 years, they often just us the last digit of the year and the sequence number.  Most of the tankers I see have a 5 digit number that starts with 2, for 1962.

In a fighter squadron, most 'local' documentation just refers to the last 3 digits.  example: 061 and 813 were how we referenced two of the F-16s in the test squadron.


Are there any locations where I could launch a rockoon with minimal red tape? I'm currently working on a research project on hypersonic control surfaces, and as part of it, I'd like to launch a rocket-assisted lifting body from a high-altitude balloon.

Are there any locations in the US where I could do this? The first thought that comes to mind is launching at sea, but recovering a small craft (~ 1 meter wingspan) from the open ocean seems like a logistical nightmare. Ideally, I would prefer to launch over land that can be accessed with relative ease by car or offroad vehicle.

At one point, I read about a section of airspace in Alaska with basically no regulation but I can't seem to find that information now.

Thanks!

NAR / Tripoli
I would recommend getting in touch with NAR or Tripoli about your needs. They handle these sorts of things all the time. TBH if you or your range officer hasn't been certified by one of these bodies you shouldn't be launching anything larger than an E sized engine.

Both organizations offer memberships including training, certification, participation in sanctioned events, and liability insurance (some of which may carry an additional fee.) The minimal red tape stipulation will be achieved if you can join an existing sport launch which meets your needs. The biggest issue will likely be whether or not that particular launch is cleared up to your intended altitude.

NAR and Tripoli have had agreements in place with the FAA since the October Sky days and coordinate launches with the feds to make sure everything is safe and above board. If you can't join an extant sport launch, you can set up your test as a sport launch of its own. The fee you pay will be tiny in comparison to FAA fines and probably also cheaper than relocating to wherever there aren't rules.

FAA Waiver
You can, of course, choose to set everything up for yourself and apply for an FAA waiver on your own. It takes about as much patience as a moderately complicated DMV procedure. Generally speaking you can be approved for any class G or E airspace and for Class A airspace that is not in or near an air route.


ATC interruptions, when is it okay/overlooked? Having recently completed my first solo flight, the tower gave me my taxi instructions, and then congratulated me, as my instructor was hanging out in the tower with the controllers at the time.

It's relatively busy airspace, so there were quite a few pilots on frequency, and multiple landing at the same time. However, before I could repeat/confirm my taxi instructions, a half dozen other pilots got on frequency congratulating me as the "new-blood". Seeing as this took about 30 seconds, it delayed my taxi readback, and potentially delayed other pilots approaching.

Controllers were fine with it, so it seemed, but I would assume that in certain situations this could be considered abuse of airport communications. Are there any guidelines for when the tower has to / should report such incidents, or is it merely up to the towers discretion?

It didn't prove any large issue in my case, other than a delayed taxi, but I'm wondering where the "line" would be drawn.

Since you have not stated any jurisdiction in your question, I am basing this answer on the EU, which is what I am familiar with. Rules in many other parts of the world are going to be similar if not nearly identical.
Regulation EU 376/2014 lays down rules on the reporting, analysis and follow-up of occurrences in civil aviation. On the list of Mandatory Reporting you will find:

(v)   communication-related occurrences;

which I think, among the possible categories, would be the best fit for the kind of event you describe. However, it is not detailed further what is encompassed by this category, so whether something like what you describe falls under mandatory reporting, I don't know for sure. In theory, rules were violated (unrelated chatter on the radio is, quite simply, not allowed). In practise, it doesn't sound like any harm was done.
In any case, there is also Voluntary Reporting, which is designed to capture

(a)  details of occurrences that may not be captured by the mandatory reporting system;
(b)  other safety-related information which is perceived by the reporter as an actual or potential hazard to aviation safety.

The rules concerning reporting can be quite difficult to comprehend; my general advice (and what I have learned at school) is: if in doubt, file a report. If you feel the safe environment was in any way compromised, you should report it. Remember, the aim of a report is not to appoint blame, but rather to improve safety. I know from experience that it might be difficult to report something done by someone you know, since it might feel like you are pointing fingers or ratting someone out, but the truth is that the reason aviation is so safe, is because the people who work with aviation takes safety seriously, and reports violations.
As for whether the controller is going to report your situation, I doubt it - they is the one who initiated in the first place after all. But again, if you feel safety was violated, please write a report. Your writing today may save lives tomorrow.


Descending while turning inbound on a non-precision approach I'm writing my instrument rating examination soon and one of the questions that I came across was:

  While executing a VOR A RWY 01 approach for Alexander bay, 2700ft is reached at 8DME on the outbound leg. The most appropriate action would be to:

  level off and turn inbound before descent
  continue outbound and descend to 2500ft before turning inbound
  turn inbound while descending to 2500ft >>> Correct

Now some background info: Alexander bay's VOR approach outbound leg ends at 8DME 2500ft.

I would say answer 1 is correct, however the answer is stated to be no. 3
Is it because it's a NPA? I was taught never to descend while turning inbound and that it is sloppy flying/poor planning. So this answer doesn't really make sense to me.

I tried looking it up in 8168 and the ICAO docs, but I haven't had any luck.

This is the plate:

Source: caa.co.za (PDF), click image to view

It's because 2500 feet is the minimum altitude prior to starting the inbound descent at 8 DME and you can descend to that altitude pretty much anywhere between ABV and 8 DME inbound, within the protected airspace of the procedure turn.

When you started the outbound leg at 3000, you can pretty much descend to 2500 sooner or later, as long as you don't descend below 2500 until you are 8 DME inbound on final.  So if you find yourself at 2700 at 8 DME outbound and you're ready to turn in, you can keep descending to 2500, leveling at 2500 until you cross 8 DME inbound.

Basically, the airspace between ABV, the 10 mile circle, and 30 degrees of the approach path is the protected airspace with a floor of 2500, and technically, you can do whatever you want within that zone.  Also the altitude is a minimum.. you don't have to descend to exactly 2500, technically (I'm assuming SA IFR rules are similar to NA)


Can you identify this engine? I am studying Aircraft mechanics currently, and in our academy we have some aircraft engines and different types of them, and I need please your help to know the reference of one of them, and if it is possible the engine manual of it, because it is essential for us to study it deeply.

It appears to be a Rolls Royce Viper turbo jet engine made in 1966. The maker's mark (BSB) derives from Bristol Siddeley, formed from Armstrong Siddeley (the company that originally developed the engine) and Bristol Aero-engines. Bristol Siddeley were later taken over by Rolls Royce.

Source: Wikipedia


Does load factor limit actually change with aircraft weight? Does load factor limit in reality change with aircraft weight? Is it fixed to make certification and operation more simple? Now unloaded aircraft has lower Va speed. This makes sense to keep constant maximum G. But if aircraft can sustain +2.5g at MTOW will anything break if it sustains +3.0g completely empty? As I understand wing bending force will be the same and this should be limiting factor in most aircraft? What else can break? Engines fall off? Tail breaks off? Theoretically possible but can this really happen?

Yes, the limits are always set by the worst combination of weight, loading, flight speed and expected gust speed. They are valid over the full range of parameters for simplicity reasons.

Note that the load factor limits are not directly proportional to the aircraft's overall weight, but the weight of the non-lifting parts. An empty fuselage and full wing tanks allow the aircraft to pull more gs or fly faster until the maximum wing bending moment is reached when flying through the maximum expected gust. Calculating the precise load limit during each stage of a flight would put excessive demands on any pilot and increase the risk of errors.

All components should be sized so that they fail at 150% of calculated maximum loads. In real aircraft some secondary considerations like the availability of sheet metal gauges or extruded profiles put a limit to this local optimization, but technically the chance for breaking is the same for all structural parts. All the failures described in your question have happened in the past when aircraft were overstressed or improperly designed.


Can I use motorcycle engine in aircraft I am trying to build an Aircraft, The weight of the Airplane will be around 40 Kg with engine and 8 liters of fuel and with me the total weight will be around 90 to 100 kg and I want to use a motorcycle engine in the aircraft, the aircraft will be one seater and I have a question that which type of engine is best to use,

Displacement    123 cc
Engine Type 4 Stroke, Air-Cooled, SOHC
Max Power   10.7 bhp @ 7500 rpm
Max Torque  10.40 Nm @ 6500 rpm

The engine produces a maximum peak output power of 11.00 HP (8.0 kW) @ 9000 RPM and a maximum torque of 9.80 Nm (1.0 kgf-m or 7.2 ft. lbs) @ 7000 RPM

So should I use 1070 or 1065 size wood propeller and if this size is not suitable so please recommend me the best size,

If this plane successfully flies so what would be speed of it and will it give me same mileage as the engine gives in Motorcycle? And for how long I can fly nonstop with these engines,

Mine Airplane design would be like this

Please explain me as simple as possible, and Sorry for Bad English. thanks

You will need a fixed landing gear and a braced wing to keep your mass at your target of 40 kg. Expect a maximum L/D of not more than 10, maybe even 8.

You say that the flight mass will be 100 kg. The weight will be 980 N. 1/10 of that is 98 N. To arrive at a power from that will need a few more heroic assumptions. Your wing loading is probably 12 to 15 kg/m² and your lift coefficient around 1.0. This equates to a flight speed of 15.5 m/s (= 56 km/h). I assume a propeller efficiency of 70%, so your power for straight flight will be 2.2 kW.

For decent take-off performance, add 1.5 m/s climb capability at an air density of 1.1 kg/m³. The extra power to climb with those 1.5 m/s at 15.5 m/s and 70% efficiency is another 2.2 kW.

The sum is 4.4 kW which is 5.9 HP. Running the engine at this power level means 54% of maximum power. Make sure you have a gearbox which reduces prop RPM to not more than 2000 in order to make those 70% realistic. The wing will need to have about 8 m² area and the tail another 1.6 m². Will that all fit within your 40 kg?

I expect that 80 kg is a more realistic empty mass. Now the wing will measure 11 m² and the power demand climbs to 3 kW for level flight and an extra 3 kW for climb. Total power will be 8 HP or 75% of maximum power. Ask yourself how long the engine does this and how big your radiator needs to be in order to avoid overheating.


How much water does a passenger airliner carry? How is the amount of water for (a) toilet and (b) drinking to be carried by a flight calculated in the industry? Is there any formula or thumb rule using which we can roughly estimate the amount of water required for toilet and for drinking calculated if the aircraft type $A$ is known and the flight has $P$ passengers on board and is scheduled to fly for $H$ before its next landing?

It is going to vary according to flight mission, passenger count, season, or what part of the world the flight takes place.

Here are the capacities from a company document.

MD88/90 47USG (177.9L)
MD11 250USG (946.4L) Estimated

732/733 40USG (151.2L)
737 60USG (226.8L)
747 3x 110USG = 330USG
757 66USG (249.8L)-Fill valve closes automatically at 50USG (189.3L)
762/763 120USG (453.6L)-Fill valve closes automatically at 102USG (385.5L)
763ER 160USG (600.6L)
764ER 224USG (847.9L)
772ER 300USG (1135.6L)

CRJ 200/700 31.5USG (galley 8USG/30L, lavatory 5USG/19L, toilet 18.5USG/70L)

As mentioned, the term "potable" water seems to be a minor overstatement. Some companies warn employees to not uplift any potable water overseas (which means not outside the US, Canada, or UK). But based on what I've heard, I wouldn't drink it wherever it came from.

Why You Should Think Twice Before Drinking Tap Water

Why you should never drink water on a plane, according to a flight attendant

Airline's drinking water fails hygiene tests

The "potable" water is usually chlorinated water, but you´ll never know about the truck or tank service overseas. Usually the galleys contain an anti-biological filter (silver ions) for the taps and coffee makers, which get changed regularly. Also the tanks will be disinfected every few weeks, normally during an A-check. Part of a C-check is to remove and test them.

On a B737, the potable water tank is in the aft cargo pit behind the last wall under the aft galley just right in front of the pressurized bulkhead. The tank itself is made out of composite materials and fiberglass. The tank is about 4 feet long and 2 feet tall. The 737s only have one potable water tank.

The B747 has three 110 gallon filament-wound fiberglass tanks attached to the forward side of the center section front spar (rear bulkhead of the forward baggage compartment). The water is moved from the tanks by pressurized air. This pressure is provided by electrically driven air compressors mounted next to the tanks. Not all operators use all three tanks and some have deleted some tanks. 747 AMM, 38-11-00, page 1, pg 2.

Passenger MD-11 hold up to four 63USG tanks, the freighter usually has all removed except for one. This water also serves for flushing the vacuum toilets if installed. The tanks sit in the utility tunnels (behind the sidewall) of the forward cargo compartment.


In wing design, how is factor of safety related to G-limits? What does factor of safety mean with regard to the maximum G force an aircraft is rated for? Does a rating of 3Gs mean the same as a safety factor of 3? Or is there some extra margin?

Structural Factor of Safety is the margin between the normal limit load and the ultimate load the structure has to take.  It is normally 1.5 times the yield load or limit load of the structure.
The structure is designed to withstand the Limit Load that is applicable to the category, in the case of Transport airplanes 2.5G, without permanent deformation.  The Factor of Safety is applied on top of this value as a minimum, below which the structure may permanently bend, but not break completely.
So an airliner has to be good for 2.5G without permanently bending the wings, with a Safety Factor value of 3.75G before the wings break off, and you have to design your structure taking into account of the elasticity and ductility of the material, to achieve both those requirements.
The Limit Load is the maximum load the airplane is expected to encounter out in the world doing its thing.  The 1.5 Safety Factor been been in use since the 1930s and is a fudge factor to account for all sorts of variables, like assembly defects, calculation errors, aging damage etc.
So to answer your question, although you said 3G, we'll say the value you're interested in is the value for light planes in the Normal Category, which is 3.8G, which would be the design limit load that it's expected to be able to take with no permanent damage.  The Safety Factor, which by industry convention is 1.5, is added on top of that value to get 5.7G and is expressed as design ultimate load for Normal Category. So you have to make sure your wing can take 3.8G and still return to its original shape, and 5.7 Gs before breaking off.
You try to get as close to these limits as you can in your design, because every pound of weight above the minimum requirement is wasted ballast as far as the airplane operator is concerned. However, designers may add additional structure on top of that calculated to meet the regulatory minimum just to be safe (especially if the materials are new), or to improve fatigue life. This is why wing structures typically get tested to failure after their fatigue test program is completed, to see what will happen and to prove they can meet ultimate load requirements, and usually fail well above the design ultimate limit.
You might find this report from the 1950s helpful.


Is it required to be an ATP and work for an airline to be a Captain? I'm just curious if the ATP certificate and working for an airline are prerequisites to being a Captain, or could you have such a title as a PPL or a CPL?

The key phrase, legally speaking, is "pilot in command".  On a multi-crew aircraft the PIC is normally given the honorific title of Captain, but what really matters is the legal status of PIC where an aircraft legally requires more than one crew.  In general to be PIC of a multi-crew a/c you need an ATPL.  Operators may operate single-pilot aircraft like Caravans as if they were multi-crew with two pilots and call the PIC Capt, but again that's just an honorific title and in that case the PIC can be just a CPL.

Like DLH says, a PPL or CPL can call themselves anything they want; Captain, Great Exalted Poobah, Zontar The Munificent, whatever.


How does the a movement of the cyclic control stick in a helicopter affect the swashplate? Yesterday I learned that the surface that the helicopter blades create while rotating is very similiar to a gyroscope, and thus gyroscopic precession works there as well. My question is, when the pilot moves the cyclic pitch controls forward, how does that affect the swashplate? Does it rise it in back and lower in front or does it rise on one side and lower on the other?

Horizontal displacement: Tilt the rotor

With the rotor blades rotating around a vertical mast, the total lift vector of the rotor is vertical and the helicopter hovers. If a translation is desired, this vector must be tilted so that an horizontal component can appear. This is done by tilting the rotor disc. The rotor disc is the plane of rotation of the blades.

Rotor tilted forward: The helicopter moves forward.

Rotor tilt: Change blades pitch

The rotor disc is tilted using the lift difference between individual blades. For instance to tilt the rotor forward, each blade creates more lift when it is aft of the mast, less when it is forward of the mast, and the change is progressive between these two positions. The lift difference creates a torque moment on the mast, tilting the mast and the helicopter with it, at least this is our intuition.

As you mentioned, the force doesn't produce its effect instantaneously due to the gyroscopic precession. It is effective only 90° later. So the lift variation must have an advance of phase of 90°. For a forward translation, the lift is maximum when the blade is on the left, and minimum when it is on the right. The momentum is stored into the blade and released 90° later.

The variation of lift is done 90° in advance.

Lift depends on the angle of attack of the blade. Varying the amount of lift according to the position around the mast leads to varying the AoA around the mast: Increased AoA on the left, decreased on the right.

Pitch varies according to the radial position of the blade.

Change blades pitch with the swashplate

Changing the pitch of a blade is done using a system of swashplates. A stationary plate is oriented in space by the pilot using the cyclic stick.

Another plate is maintained in contact with the first one, but can turn freely with the rotor. This second plate is forced to rotate in the plane of the stationary plate, thus it oscillates constantly as it rotates. In turn, it creates an oscillation of the vertical rods (horn links). Each horn converts the rod oscillation into a pitch oscillation on the blade. The blade is now pitched according to its instantaneous radial position relative to the mast, and according to the cyclic stick command.

MBB Bo105, based on photo by Burkhard Domke (source)

Horns are linked to different angular sectors of the swashplate

The vertical rod transmits the stationary swashplate height at its current axial position. By locating the rod 90° ahead of the blade, the link takes care of the rotor precession. The mechanism is so that when the pilot pushes the cyclic forward:

The swashplate is tilted forward.
When a blade moves left of the mast, its horn link measures the height of the stationary swashplate in its aft sector, where the plate is the highest.
The horn is pushed by the link to the maximum, and the blade in turn gets the largest pitch angle.
The situation is reversed when the blade arrives on the right. The link is in contact with the forward sector of the swashplate, where the latter is at the lowest position. The blade gets a low pitch angle.

(Redrawn from source)

Advance for pitch order can be adjusted as required

By adjusting the offset of the horn link which interfaces the horn and the plate, one can adjust to the exact amount of advance required for a given blade.


How close is a pilot allowed to get to a runway without landing clearance? I heard it was 100 feet above minimums, I'd just like verification.

From the Pilot/Controller Glossary:

  CLEARED  TO  LAND−ATC  authorization  for  an aircraft to land. It is predicated on known traffic and known physical airport conditions

A clearance to land is exactly that: a clearance to land. A pilot without landing clearance hasn't violated ATC instructions until their aircraft has, in fact, landed. So the answer to how close you can get is: anything that isn't touching the surface.


What does %MAC mean? A document said that the optimum CG position is from 13% MAC to 33% MAC. I'd like to know what that means, and what would this look like on an actual plane.

%MAC (Percent Mean Aerodynamic Chord) is a mathematical calculation that shows where the center of gravity is over the wing.  The calculations is rather straight forward and the generic formula is the same for all airplanes.  All one has to do is to figure out the numbers and plug them into the formula.

How is %MAC calculated?

Pilots, dispatchers or load masters will calculate the weight and balance with respect to the aircraft datum.  Unfortunately, we need to change where the reference point is from the (datum) to the front of the wing called leading edge mean aerodynamic chord (LEMAC).  There is also a trailing edge mean aerodynamic chord (TEMAC) that is calculated.  TEMAC - LEMAC = Wing width.

LEMAC and TEMAC are easy to calculate for straight wing aircraft.  It is simply the measurement from where the wing starts and ends from the datum.  For swept-wing aircraft it isn't that simple.  As a result, aerodynamic engineers figure out where the average of the LEMAC by averaging the wing root and wing tip.  The same process happens for calculating TEMAC.

%MAC is simply a calculation of how far the CG is from the LEMAC.  If the CG is at the LEMAC, we can say it is at 0% MAC.  CG at TEMAC means the CG is at 100% MAC.

In the picture below, we can eyeball the %MAC to be around 10% MAC.

Here is the generic formula:

$$ \%\mathrm{MAC} = \frac{\mathrm{CG} - \mathrm{LEMAC}}{\mathrm{WingWidth}} \times 100 $$

The formula is great because all the numbers are relatively the same for every airplane.  For example a 13% MAC means that we are most likely at or close to our forward CG limit.  This should instantly give the pilot a sense of how the airplane is going to respond versus a pilot trying to comprehend  what a CG of 2,341.0 inches aft of datum means for that particular aircraft.  The last number can be different for every aircraft type but 13% MAC will be the same.  The only difference is where the forward CG limit is located for that particular aircraft.

A little background, and perhaps an engineer can expand on the thought.  The basic weight and balance formula is $ \mathrm{Weight} (W) \times \mathrm{Arm} (A) = \mathrm{Moment} (M) $.  What may not be obvious is that we can use this formula in figuring out why %MAC is important.

Take a look at the picture above.  There is a moment between CG and CP and another moment between CP and tail down force.  These moments must equalize for the airplane to be in equilibrium.  If the CG moves forward, Moment #1 gets larger and to compensate the tail down force must increase.  We can increase the tail down force with nose up stabilizer trim.  The same analysis can be made when the CG moves aft.

If this is the case, there is a specific pitch trim setting for each %MAC and airspeed combination.  Most jet pilots will set a specific takeoff pitch trim setting for takeoff based on %MAC so that if the pilot loses an engine after $V_1$ the trim will be set for a speed around $V_2$.


What is the relation between aircraft stability and control? :)
My teacher gave the task to figure out the common relation between stability and control. I know definitions and only thing I could find on the internet was "Control is directly opposed to stability" but I am not sure if I get it. Thank you for your answers :)

You are right.

Stability is the tendency of a system to return to its old state after a disturbance, be it a gust or a control input. It needs more control effort to change the state of a more stable airplane, such that stability can be measured by measuring control forces instead.

A famous example which is often mentioned in literature is the BE.2, a British two-seater which was designed as a training aircraft in 1912. As a trainer, it had ample stability, so its control responses were sluggish. That proved to be its undoing when war broke out in 1914. The BE.2 was easily outmaneuvered by dedicated fighters, so it had to be withdrawn from frontline service.

Quote from Wikipedia:

  While it proved fundamentally unsuited to air-to-air combat it had a
  relatively low accident rate, and its high degree of inherent
  stability actually proved helpful in its artillery observation and
  aerial photography duties (although it also made it more difficult to
  maneuver quickly)

BE.2 (picture source)


Is there a modern single-seat turboprop? There seem to be a number of tandem seat turboprop trainers like the PC-9/Texan and the Super Tucano. Are there any single-seat turboprops? I am asking about modern production planes.

Yes. Although most aircraft to find good use for a turboprop need to have more than one seat, some single-seat uses do exist. For example, agricultural aircraft like the Air Tractor AT-802, Thrush models, and the PZL-106 Kruk are all currently in production.


Is there a way to get descriptions of great circle routes? Given that I am blind and love to know where I'm flying over, is there a online service that describes a great circle route in text form? e.g. in a format like this:

  A great circle from CYVR to LTBA goes NNE over British Columbia,
  Alberta, Northwest Territories and Nunavut, passing over Arctic bay in
  the northernmost part of Baffin Island. Then across the middle of
  Greenland, crossing the Atlantic to Norway then Sweden, passing abut
  30 miles west of Stockholm. Then across the Baltic Sea and over the
  Kaliningrad Oblast, which is the small exclave of Russia that sits
  between Poland and Lithuania. Then across eastern Poland, western
  Ukraine, Romania, the very corner of Bulgaria, across the very western
  edge of the Black Sea and arrives in Istanbul from NNW.

(thanks to @TomMcW who left this in my previous question).

This way I can stop asking these great circle questions here.

Thread
In your Manchester to Vancouver question, I wrote that a great circle is like a taut thread on a ball.
If you have a tactile (Braille) globe, get two pins and a thread. Place the pins on the departure and arrival cities, and wrap the thread around the pins so that the thread is taut. You can then follow along with your finger.
Coordinates
Another method is to go to Route Finder website.
Enter the departure and arrival airports ICAO codes (you can get them from Wikipedia). And plot the flight plan.
The generated flight plan will be close enough to a great circle. The waypoints will be listed by their latitude and longitude. The distance and track (heading) for each leg will also be listed.


How long after the last departure shall the airport stay open for an emergency return? For how long after the last take off for that day, will an airport stay open for emergency return of that last departed aircraft? Where can I find that recommendation?

To clarify: an airport needs to be closed during the night due to work-in-progress on a runway, so it can't stay open 24/7 for emergency landings.

Where I work, we have to stay in the tower for 15 minutes after the last departure in case they need to return. To my knowledge, there are no international regulations about this, so the rules may well be different from place to place.


How much force is needed to tow an airliner? Airliners are often (almost always actually) pushed back by a tug or a tractor to leave the gate. The tug can also tow an aircraft that cannot taxi by itself.

How much force is required to pull or push a large airliner on the ground? In other words, what is the horsepower of these tugs?

EDIT: By large airliner, I mean Boeing 747 / 777 / A380 type, the ones which ATC append "Heavy" (or "Super") to their call sign.

Tugs do not need that much horsepower, even the biggest tugs only have around 400bhp. What they do need is lots of torque for push/pull power, lots of weight to give it plenty of inertia, and big grippy tires to transfer the power to the surface.

The T80 in this link is pretty typical, 50-70 tons, 408bhp, and 1500 pound feet of torque. That's more than triple the torque of a hefty pickup.


Can a US airline add an employee passenger as crew? Suppose an airliner flight is full with passengers and the airline staff realize afterwards that there are employees who the airline wants to send to their shifts using that flight.

The airline staff decides to ask for volunteers to surrender their seats in exchange for compensation, but they don't get enough volunteers to make room in passenger seats for their employees to take.

Would it be legal for the airline to instead have one or more of the employee-passengers take the flight as "extra crew" (assuming there is an extra seat to strap into in the crew areas)?

First of all it depends on what type of "employee" and if the jumpseat is in the cockpit or the cabin.

According to the answers to this question there are no regulations on who may sit in a cabin jumpseat as long as it meets the specifications set out in FAR § 121.211 for proper airline seats. So the airline is free to set policy for cabin jumpseats.

As for jumpseats on the flight deck the restrictions are quite stringent. If the employee is a pilot FAR § 121.547(c)(3) allows off-duty pilots to fly in a jumpseat on the flight deck, so there would be no need to make them an active crew member.

FAR § 121.547(a)(1) allows an on-duty flight attendant to fly in a jumpseat on the flight deck. The person would have to be a certified flight attendant, so you couldn't just make any employee a crew member. Remember also that flight attendants have hours of service restrictions, so any time they are considered on-duty has to comply with the h.o.s. regulations.

Certain other types of employees are allowed to fly in the flight deck, but they have to be performing a specific function on the flight that requires their presence in th cockpit. For example, a mechanic could fly in the cockpit if he's there to monitor something on the aircraft. A flight dispatcher could be on the flight deck if they are observing flight operations as a part of training.

Even some persons who may be given access to the cockpit for operational need, like air marshals or training personnel, still must have a seat available in the cabin.

So they can't put someone in the cockpit just to transport them from place to place and they couldn't just plop a salesman or a ticket agent there and say they are a "crew member."

There are regulations stipulating the minimum number of flight attendants on a flight but not a maximum number. So, technically they could make a flight attendant part of the crew, but if that person was just listed as a crew member and wasn't actually performing any duties, the FAA would certainly interpret that as a violation.


Runway landing with a front skid instead of wheel - how does steering happen? (Dreamchaser) The Spaceflight Now article Sierra Nevada’s Dream Chaser performs critical glide test flight shows the Dreamchaser performing a runway landing using a front skid and two wheels.

The skid extends backwards from the point where it connects to the aircraft. I am guessing that it can turn for steering once in contact with the runway during landing. Are front skids an established method for runway landings? Can they be used to steer by rotating? If so, is there a simple way to understand how this works - how would rotating the skid actually change the direction of the aircraft; how does the frictional force from forward motion produce a lateral force that changes the direction of the aircraft's motion?

Screen shot cropped from Dream Chaser Spacecraft Extended Free Flight

Images (cropped from originals) from here.

Like many aircraft that dont have a steerable nose wheel they most likely use differential braking. Here is a pretty interesting paper that covers some of the testing they did on the airframe. Steering is not really a large concern for the Dream Chaser (or the space shuttle for that matter) as it has no power to maneuver on the ground anyway. Generally speaking it will be towed off the runway.

According to this source SNC notes similar reasoning to the X-15, simplicity as well as concerns around tires in a space environment (which is interesting seeing as the space shuttle had them).

  Speaking of the rationale for opting to use a skid strip, SNC noted it
  is a simple, light, safe option. They also added that there had been
  some issues with the analysis of the performance of tires in the space
  environment and this eliminates one of the tires – with the other two
  tires easier to control than the nose wheel if there’s a problem with
  one of them.

  SNC noted they do not expect any crosswind limitations from the skid.

Although here is a fairly interesting NASA press release on testing the "nose gear" for the Dream Chase implying there may have at one point been gear there.


What license woud an ex-military aircraft (E.g, L-39) go under according to the UK-CAA, an EASA licence or a non-EASA license? I've been looking around for the sale of ex-military jets, to own and fly, namely an Aero Vodochody L-39 Albatros, and so out of interest had a look at the CAA website for infomation on licenses, training and general ownership etc, and across the terms EASA and non-EASA licenses, I rougly understood the meaning of them, but am unsure as to which license I would need to own and L-39.

Whilst asking this, I would be appreciative if anyone could give me some general tips and advice on owning an ex-militatry jet trainer and the kind of courses/training I'd have to undertake to own and fly it. Thank you in advance.

Please edit if I have left something important out or anything like that.

According to CAP 632

  6.1 Ex-military aircraft on the UK register with a ‘Permit-to-Fly’ may be piloted by either private or professional licence holders. Pilots must hold a current civil licence with a current civil class or type rating appropriate to the ex military type[s] being flown or where no civil type rating exists, an Aircraft Type Rating Exemption from the need to hold a type rating.
   Any questions relating to licence matters should be addressed to CAA PLD

...

  Any turbine-powered aeroplane - To fly an ex-military turbine-powered
  aeroplane a pilot must hold an Aircraft Type Rating Exemption. See paragraph
  6.7 for details of obtaining an Aircraft Type Rating Exemption

and so on.

Ex-military aircraft are non-EASA aircraft. Ref


How does landing performance differ on water vs. ice vs. paved/grass runways? It's a follow up from How do aircraft brake when using skis?

Can someone give me data about difference between landing distance for each environment ?

Paved
Grass
Water
Snow/Ice

Any aircraft is fine, provided it's fitted with brakes used during wheels landing.
Ideal answer would be comparing the same plane fitted with different landing gear on similar wind condition/ pressure

The takeoff and landing performance on different surfaces varies with condition - for example, a wet or snowy paved runway will have different braking and rolling acceleration performance than a dry paved runway. We can make some general/broad statements, but it's important to note that the specific conditions on any given day will play a large part in aircraft performance.

That said, here are some general/broad statements:

Water (and snow on skis)

Water is the odd surface in the group: It is not a runway in the traditional sense. If you "land" an aircraft with traditional hard-surface landing gear (tires) on water it is not a landing, but rather a ditching and is covered under emergency procedures.

Landing a seaplane (either a "flying boat" hull or something with floats) on water is a different beast from a landing on land, and indeed an entire different rating on your pilot certificate. Everything you ever wanted to know about seaplanes can be found in the FAA Seaplane handbook (FAA-H-8083-23), which also covers aircraft equipped with skis for landing on snow (which is in many ways similar to landing a float-equipped airplane on water).

Pavement (Asphalt, Concrete, etc.)

Paved surfaces are the baseline standard by which "land" landings are judged.  The performance charts and tables in your POH are generally given with the assumption of a dry, paved, level runway that is free of contaminants.

Wet or Contaminated Pavement

Wet, snowy, or icy pavement primarily affects braking performance: An aircraft will not be able to stop as quickly on wet pavement as the tires may hydroplane/skid with aggressive braking. This video gives a good overview of the effects of wet pavement, and while it's primarily aimed at Part 25 Transport-category aircraft/operators the same basic considerations apply for light General Aviation aircraft.
Your aircraft performance charts may discuss operations from contaminated runways, but if they do not cover this situation some rules of thumb for landing performance on wet or contaminated pavement are:

Wet Pavement (no puddles): Multiply landing distance by 1.4
Wet Pavement (w/ puddles): Multiply landing distance by 2.3
Snow on Pavement: Multiply landing distance by 1.6
Icy Pavement: Multiply by 4.5 (expecting braking to be largely ineffective)

Similarly allowances must be made for takeoff performance (additional resistance on the tires from contamination on the runway) - The usual takeoff rules of thumb work here ("Have 70% of your takeoff speed by the time you've used 50% of the runway."), with the caveat that you need to consider braking distances in the event of an abort: Aborting halfway down a 3000 foot runway when you expect to need 1800 feet to brake to a stop would be a Bad Thing.

Grass

Grass is the traditional "soft field" surface, though well-maintained grass with good drainage on a dry day may not be all that "soft" when you land on it.
As with a contaminated runway the major difference is rolling resistance: You will stop a little faster on dry, level grass than you would on an equivalent paved field because the grass offers more resistance to the tires, and you will take longer to accelerate to takeoff speed for the same reason.

Again, your aircraft performance charts may include figures for soft field / grass field takeoffs/landings (this is often given as a percentage of the paved surface figures). If your performance charts don't include soft field / grass field data there are again some rules of thumb you can use:

Short grass: Add 10% to the takeoff roll
Long grass: Add 25% to the takeoff roll
Wet grass (Soft Field): Add 35% to the takeoff roll
Muddy field: Add 50% (or more) to the takeoff roll

Paved surface landing distances work well for grass fields, with the exception of wet grass (which can be treated as a soft-field landing on wet pavement to account for your tires skidding on the wet grass).

Gravel

Though not strictly a subset of grass, considerations for gravel runways are similar: Increased rolling resistance during takeoff (as your tires push the gravel around), and diminished braking effectiveness on landing (in particular extreme braking on gravel can lock a wheel and leave a rut, as it would on a soft grass field).
Soft field takeoff and landing distances would be appropriate to use for gravel runways.

Ice

Ice is an interesting surface: It's hard, but it has a very low coefficient of friction. On clean ice your takeoff distance will be similar to pavement, but your landing distance will be significantly increased since your wheel brakes will be largely ineffective.
As for icy pavement you can plan on a landing distance of 4.5 times your paved runway landing distance (or more), and you will likely be nosewheel steering or differential braking will have reduced effectiveness for directional control (tailwheel pilots will be at a distinct advantage here, as the rudder work they do in a normal takeoff or landing is very similar to what's required to keep the aircraft going straight on ice).

Contaminated ice (standing water, snow, etc.) impose penalties similar to contaminated pavement, with the added caveat that water on ice is an incredibly low-friction surface (to illustrate this drag your finger across an ice cube dry, then do the same with a little bit of water on the surface of the ice cube) -- the increase in landing distance required would be substantial if landing on a wet ice runway.

Unique surfaces

While the list above covers the vast majority of runways you're likely to encounter there are some "unique" surfaces out there (for example perforated steel planking which is sometimes used to construct temporary runways can become a special challenge when it rains as friction on the metal is reduced by water and mud squirting up through the perforations with certain types of underlying soil).
If operating from a "unique" surface the best resource you have is other pilots who routinely operate from that surface, or at least have some experience with it -- they can tell you what to expect under different conditions, and you can plan accordingly.


What is the distance between RLLS lights? I've looked everywhere for a few days now, but I can't seem to find the distance between lights on an RLLS approach lighting system. The specific one I am looking at is for the 31C approach at KMDW.

Approach Lighting Systems (ALS) can have many variations, but each variation will be a combination of certain components.
For example, green runway threshold lights extend across the runway threshold, 5 feet apart (center-to-center), and extend outward 45 feet from the runway’s edge on either side. This line of lights can be up to 10 feet from the landing surface before requiring a waiver.

Centerline lights are rows of 5 lights that span 13.5 feet wide and are usually spaced 100 feet apart extending away from the runway threshold out as far as 3,000 feet in some cases.

Many other lights may be incorporated, including sequenced flashers, runway alignment indicators (RAILS), side row bars, and wing bars.
In general, rows of lights will usually be 100 feet apart for at least the last 1,000 feet, and 200 feet apart beyond that.

For more detail on all of the various types of lighting and specific measurements between each, check out this article on Code 7700.


What is the difference between a propeller and a rotor? I have been making the mistake of using words "propeller" and "rotor" interchangeably when they are not the same thing. What is the difference between a propeller and a rotor?

Short answer:

A propeller is used to propel something, the force at stake being called thrust. Thrust is usually horizontal (but not always, think about rockets). This device comes from the navy domain.
A rotor of a rotorcraft is used to control anything, including the altitude, the generated force being called lift. Lift is generally vertical (but not always, think about a rudder). Rotors have generally more than two blades.

Lift and thrust involved are components of the same physical force (the aerodynamic force) to which we refer to differently based on their purpose after an arbitrary breakdown based on the direction they act (compared to the direction of the aircraft or to the gravity field).

For our pleasure, let's also talk about a turbine rotor, which is a rotor/propeller which doesn't produce lift or thrust, but convert the fluid velocity (actually momentum) into torque. For example the wind turbine rotor. A turbine rotor is certainly a propeller, but we don't refer to it as a propeller, as it propels nothing.

Rotor is the most generic name for something spun by a shaft. It can be used for a propeller, the rotating wing of an helicopter, the rotating elements of a compressor or a turbine, etc. You can't be wrong using rotor for any spinning device. Rotor (rotating) is the antonym of stator (stationary). These terms are not specific to aerodynamics, in a electric motor there is usually a stator and a rotor as well.

The rotor of a rotorcraft comprises blades and is comparable to a propeller, if we except blades additional freedom in flapping and leading-lagging to limit vibrations created by repeated force swings encountered while performing a 360° turn.

Source: Wikipedia

But actually the main rotor is a (rotating) wing, and the anti-torque rotor is a (rotating) rudder. Their blades are meant to produce lift like the wing and the rudder they replace. Because lift is involved, we tend to name them rotors rather than propellers.

A propeller also comprises blades, fixed on the rotation axis.

Source.

The purpose of a propeller is to propel the aircraft, that is to push it in the direction of its flight.

Both a rotorcraft rotor, a propeller and a wing create the same aerodynamic force. However the conventions are:

A rotor and a wing are used to eventually counter weight and control altitude, and a propeller to eventually counter drag and control velocity along the flight path (I say eventually because to gain altitude you usually temporary increase thrust, that's a detail)
A rotor and a wing create lift, a propeller creates thrust.

Wing, propeller blade and rotorcraft blades work exactly the same from an aerodynamic standpoint. The resultant of their action is the total aerodynamic force, which is then broken down arbitrarily into lift, drag, thrust and torque to facilitate mechanical analysis.

Notes:

Moving an aircraft along its path (thrust) is easier than maintaining its altitude (lift), and therefore a main rotor is far larger than a propeller for aircraft of the same mass.
The horizontal displacement of the rotorcraft is also produced by the main rotor (cyclic control), so some part of the aerodynamic force can also be seen as thrust, and the rotor also acts as a propeller.
Propeller is used for simple propellers and not in turbine engines. While the fan of a turbofan is comparable to a propeller, participates to thrust production and aircraft propulsion, the fan is still only one part of a system (guide vanes, fan duct and exhaust nozzle greatly maximize final thrust). Bottom line the fan is a rotor, not a propeller.


Is there an FAA ruling about the airworthiness of piston engines with less than 60 on the compression test? We've been having some discussions around if there was a specific ruling from the FAA that explicitly said that if a cylinder was showing less than 60 on the compression test, the FAA deemed it unairworthy.  I've heard that, it's a rule-of-thumb and not a ruling, but I don't know for sure.

Has this been conclusively made into a ruling from the FAA, or does it exist as a rule-of-thumb and there are other factors to consider?

This would be specific to United States/FAA and 4-6 cylinder Lycoming or Continental piston engines.

Long story short is that the manufacturers instructions take precedence over all. FAR Part 43 Appendix D requires that at each annual and 100 hr the compression be checked and that if there is "weak" compression further inspection of condition and tolerances of the engine have to be made. Now the term "weak" is kind of broad. FAA Advisory Circular AC43.13-1B states:

  If a cylinder has less than a 60/80 reading on the differential test gauges on a hot engine, and procedures in paragraphs 8-14b(5)(i) and (j) fail to raise the compression reading, the cylinder must be removed and inspected.

However, it also states:

  This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when there are no manufacturer repair or maintenance instructions.

So the gist of it seems to be that if the compression is below 60/80 on a hot engine IN GENERAL, then yes it needs to be removed and inspected, UNLESS the manufacturers instructions state otherwise. Keep in mind Advisory Circulars are guidelines and not regulatory in nature although they are usually used as best practice guides. There is a great AVweb article The Savvy Aviator #13: Putting Compression In Context that has more in depth analysis.


How is unusable fuel taken into account in weight and balance calculations? Unusable fuel is considered part of the empty weight for aircraft so when the aircraft is weighed the undrainable fuel is already included. Is there any time unusable fuel would be required to be listed either added or subtracted on a weight and balance form?

Since you've used the word "required", you may need a current specialist on the subject to get a truly valid answer.

That said, here's what I can tell you based on having written weight & balance software that was FAA approved for use at three different freight carriers in the period from 1988 through 2016 for B727, B747, DC-9, DC-8, and L-1011 aircraft.

That software made no provision for unusable fuel, as that was included in the basic operating weight. Also, the manual systems the software replaced had no provision for unusable fuel.

However, as I remember, the forms that came to me from the last weighing of the aircraft when adding an airplane to the program sometimes listed the unusable fuel, and sometimes the undrainable fuel as well, depending.

In other words, forms or software involved in the everyday calculation of weight & balance typically do not show unusable fuel. Forms involved in aircraft reweighing, whether due to maintenance or regulation, oft times do.

Edit - Seeing @Anilv's answer, I realized I may have misunderstood the question. I took "unusable fuel" to mean fuel that can not be physically delivered to the engines as opposed to fuel that can be delivered to the engines but should not be. Ballast fuel fits into that second definition. It physically can be delivered to the engines, but should not be. Using that second definition the answer to the question:

  Is there any time unusable fuel would be required to be listed either added or subtracted on a weight and balance form?

is: Yes, there was an entry field for use for ballast fuel if such might be required to meet weight & balance requirements. On 747 freighters, for example, ballast fuel was on occasion used to move the zero fuel weight c.g. forward to be within limits. This was especially true for 747s that had been pax aircraft and had been converted to freighters. The addition of a cargo door well aft of the wing root produced a configuration susceptible to a c.g. that was aft of the aftmost zero fuel weight c.g. limit. The solution, if you couldn't arrange the cargo to take care of the problem, was to to put ballast fuel in the fuselage tank, which was the most forward tank.

While you were not supposed to burn that fuel, it was comforting to know that it could be used if you really needed it.


What is this metal plate below the door? What is this metal plate for?

(source)

You can also see it here:

(source)

Is found on many aircraft so what is that piece of metal right there?

@Greg is correct. It is a scuff plate, to protect the fuselage from damage due to GSE (like jetbridge etc). The image below shows it.

Image from slideshare.net

The image below shows an example made out of Ti from a manufacturer's site.

Image from exoticmetals.com


Do the landing gear modules rotate? I'm particularly thinking about larger aircraft with more than two sets of main landing gear.

It's pretty obvious you would not want them to rotate while landing and take-off, but while taxiing around or being tugged, the drag caused by fixed axis wheels would be significant on tighter corners, and would leave rubber on the tarmac.
As such one would presumably want the wheels to track the turn.
That effect would be even worse for something like the Antonov 225... which I have no idea how they turn on the ground...

If they do somehow pivot, is there some sort of locking mechanism that is employed prior to take-off and landing.
Note this has nothing to do with crabbing for landing, but for ground maneuvers.

For the 747 it does exist. It's called body gear steering. The two center bogies/trucks rotate when the nose landing gear exceeds 20° and the speed "decreases through 15 knots."

Both conditions would not exist during takeoff, and thus the system is locked. The 777 also employs body gear steering, as do many large aircraft. Such as the C-5 Galaxy. As for the An-225, someone says the aft 3 axles, but I couldn't corroborate it, but it is likely.

  Body gear steering operates when the nose wheel steering angle exceeds 20 degrees. This reduces tire scrubbing and lets the airplane turn in a minimum radius. Body gear steering is activated when ground speed decreases through 15 knots. As speed increases through 20 knots, the body gear is centered and body gear steering is deactivated (747-400 FCOM).

The system reduces the asymmetric thrust needed for tight turns, and reduces tire scrubbing.

Here's the 777's via YouTube:


In which situations can a Dual-Aircraft Platform (atmospheric satellite) remain nearly geostationary? The Dual-Aircraft platform described by William Engblom of Embry-Riddle Aeronautical University on this NASA feature page and in the first half of the video of the 2016 NIAC Symposium uses the wind sheer (difference in horizontal wind velocities) between two altitudes to allow a well-programmed pair of tethered solar gliders/planes to remain in a general area (e.g. 150 km radius) above the Earth.

The analogy is kite sailing, except in this case both fluids are air.

In the simulations presented, the aircraft have propellors which can generate forward thrust using electrical power, or reverse thrust when the motor is used as a generator (analogous to regenerative braking on electric vehicles).

I'm having problems understanding the point of this system - can it do something that a single solar-electric atmospheric satellite can not do? Can the horizontal wind sheer provide some special advantage for "hovering" near a certain location on the ground? Is a small (say 10 degree) difference in wind direction at the two altitudes enough to remain roughly geostationary - or is a much larger difference needed?

The second questioner (a bit after 00:20:00) seems to pose a simple but interesting question about tacking, and the third questioner's mention of conservation is also interesting.

Below: screen shots from the 2016 NIAC Symposium

Almost any situation

The only time you wouldn't be able to stay (near) Geostationary would be if the wind velocity is higher than your own maximum powered velocity, at all altitudes you are able to operate.

In any other situation, you simply fly in something resembling a circle over the area you with to loiter, or fly directly into the wind (zig-zagging if the wind is slower than your minimum airspeed). Which you choose will depend on your minimum/maximum airspeed, the windspeed, and how "stationary" you need to remain with reference to a point on the ground.

Then again, the above applies to almost any solar powered aircraft, so how about the specifics of this concept?

What are the potential advantages?

Winds at altitude are not always (or even usually) in the same direction, or at the same velocity. We can take advantage of this in some scenarios:

If you have, for example, a fast-moving stream of air near a slower moving body of air, or (even better) a body moving in roughly the opposite direction, you can fly slowly "upwind" in the slow moving stream (which doesn't use much energy, relatively speaking) then "surf" down the fast stream, harvesting energy as you go - you then use that gained energy to fly back into position, through the slow moving body.

If you get an even more ideal scenario, the slow moving body is in the opposite direction to the fast moving body, you can simply "float" around in the currents, harvesting energy most of the time.

The important part to note is that zig zagging (akin to tacking in sailing) - regular tacking requires you to change direction regularly to avoid going off course... with two aircraft tethered, however, they can "tack" away from each other without changing direction. Essentially, they can angle away from each other and vary the angle of incidence to remain relatively stationary within the wind: if the wind speed increases, you fly more "into" the wind, if it decreases, you fly more perpendicular to the wind (and therefore more "away" from your partner aircraft).

This combination of two aircraft tack-surfing (totally not a technical word, I definitely just made it up) mean you can "surf" the wind even when the windspeed  is lower than your own stall speed, without having to tack as a single aircraft would. Tacking has a couple of disadvantages - it's more involved, and it means you are constantly travelling laterally across the wind, while two aircraft surfing together can stay in almost exactly the same place, merely by angling more or less into the wind. They can also, by coordinating power between themselves, move laterally along the "face" of the wind

In short, you have much more fine-grained control over your position, in more conditions


Why are runway lights always on? By driving often near my local airport, I deduced that runway lights are kept always on as soon as the day light or weather conditions require it.
I also suppose that it comes from safety reasons established in the past ad that's a worldwide standard.
what I don't understand is why, given nowaday's instrumentation both on ground and airborne, it's not possible to switch them off when no aircraft is ever going to even fly at cruise altitude around it. Here at LIBP (Labruzzo Airport, Italy) we hardly have more than 8 transits during winter so probably 80% of the energy spent by lights it's totally wasted.

The answer to the question lies in your statements

  I deduced that runway lights are kept always on as soon as the day
  light or weather conditions require it

Typically when the visibility drops below a certain level airports will turn their lights on. You mention that the airport does not have a whole lot of arrivals but you would only be able to progressively turn them on and off if all the arrivals were planned. There are plenty of cases where they may not be, (here in the US GA traffic is one), emergencies, diversion, aircraft not operating on a flight plan etc. In that case its simple courtesy to leave them on.

In some cases the lights are actually part of the instrument system and the regulations may dictate that if the ILS is on so are the lights.

The lights may also not always be on, it may simply be that you happen to drive past them when they commonly are on. There are lots of variables here but different airports handle this differently. Here in the states smaller non-towered airports often have lights that are always off, a pilot can turn them on by keying the mic button on their radio 3 or 5 times. They are typically on a 10 or 15 minute timer. If you turn them on to far out and forget about it they may shut off on short final... Larger towered airports may keep some lights on and allow you to request others be turned on or off. This is only applicable if the tower is in operation. At night, if the tower closes they may simply leave the lights on as there is no way to remote key them.

At night, even in good weather (or especially in good weather) lights can help identify an airport from quite a distance. Since runways are lit in a predictable, standard way if you are on approach to an airport in a well lit city it can help to split the airport out from all the roads and highways when looking for it in the terminal area.


Is a wingtip vortex strongest at the critical angle of attack? Generally, wingtip vortices increase in strength as the angle of attack increases.  Is the critical angle of attack/Cl Max the peak strength, or do the vortices generally continue to increase in intensity once the plane is in a stalled condition? For the purpose of this question, assume the stall is not an accelerated stall and is entered from straight-and-level flight in smooth air.
If taken to an extreme, such as a 90 degree angle of attack, there would no longer be wingtip vortices, just turbulent flow and drag.
If it doesn't peak at the critical AOA, where does it peak?

do the vortices generally continue to increase in intensity once the plane is in a stalled condition? For the purpose of this question, assume straight and level flight in smooth air

These sentences are contradictory. Never mind.
The strength of the tip vortices post-stall depends on the wing's stalling characteristics. If the inboard section stalls first (which is desirable because it helps keep the ailerons functional) then the tip vortices will continue to increase with angle of attack until the tip section itself stalls. At that point lift near the tips drops sharply and the vortices will decrease accordingly.
If the plane is capable of post-stall flight at extreme AoA, it will not be travelling fast enough for the vortices to maintain significant energy.