Which aircraft require that the vertical position of the centre of gravity is checked? We normally check the aircraft CG along the longitudinal axis and the lateral axis (sometimes referred to as the transverse axis) to ensure it is within limits. Until recently I was unaware of any aircraft that required a check and control of the CG along the vertical axis. None of the aircraft (B727, B747, DC-8, DC-9, L1011) that I have been involved in weight & balance detail for have required a vertical CG check. Recently, however, I have found that some B767 freighters do.

My question is: Are there aircraft other than some 767s that require a vertical CG check?

Edit in response to a comment: My purpose in asking the question was to learn how others might have handled a vertical CG check. You can see the method I used by going to http://terryliittschwager.com/WB/B767/load.html and searching the page for 'vertical'.

Additional edit in response to additional comments: Some comments have expressed the opinion that there is little or nothing that can be done about the vertical CG. In practice, the vertical CG of cargo (and thus the airplane) is easily affected by specifying which ULDs (unit load devices—the so-called "igloos") will be used for loading. ULDs come in various standard heights. 96", 64", and 45" are oft used heights but there are others, and an array of custom heights. A usually safe assumption is that the mean vertical CG across ULDs is no higher than half their height. In the case of the 767, the published weight & balance documentation gives tables for average CG heights of 42" and 36".

In many aircraft the vertical (Z axis) placement is of little impact on weight distribution.  However, in some aircraft it affects maneuverability.  So for an aircraft like a C-5 it is noticeable, yet a C-5 is not considered an aircraft where vertical CG would be a factor, as the nature of the operations are, well, cargo hauling.  In a helicopter, which tends to have a higher (Z-axis) CG with the top placement of the engine and transmission, the considerations are greater. It doesn't impact loading in a normal sense, but there is a noticeable impact in handling particularly when roll moments are involved.

Stated perhaps more simply, static stability is normally bounded by longitudinal and to a lesser degree latitudinal forces. However, a higher CG will impact dynamic stability where the consideration is not primarily limited to wing (rotor) and airfoil loading.

So the simplest answer is that it does make a difference, which is more than just measurable in some applications, but normally the difference is of small consequence and only a tertiary consideration.


Is it legal to fly over Vatican City? In February 1922 one or more aircraft flew over the Vatican low enough to disturb the papal conclave then underway. In response, authorities forbade flights over all of Rome until the conclave was concluded.
About seven years later, the Lateran Treaty between the Holy See and the King of Italy included a provision forbidding flights over Vatican City.
Various changes both in technology and in regulation of aviation have happened since that time. Ninety-one years after the aforementioned incident, Pope Benedict XVI announced that he would resign as of the last day of the month, and on February 28 he boarded a helicopter in Vatican City to fly to Castel Gandolfo.
What are the current rules and practices on flights over Vatican City?

Yes, it is legal to fly above Vatican City, but not below 2500 FT if flying IFR or 3500 FT if flying VFR.
There are several Prohibited Areas established over Rome. Prohibited area 212, mentioned by @JanHudec, is not actually above Vatican City, instead it is there to protect the Regina caeli prison.
However, there is a prohibited area covering the Vatican City: P243, in which IFR traffic is prohibited below 2500 FT and VFR traffic is prohibited below 3500 FT. The AIP (ENR 5.1.1-14) notes that the area is "subject to overflyng restriction for security-safety reasons". While not mentioned directly in the AIP, presumably, there is a way to get permission to fly into the area in special cases, for example to access the Vatican City Heliport.

The red area in the image above shows prohibited area 243. Marked with blue is the heliport.
There appears to be no restrictions for traffic above 3500 FT. There is even an airway (L5) almost directly above the Vatican City (black line on the map), which has a lower limit of FL95 (9500 FT) - which is a completely normal lower limit of an airway, Vatican City or not - so it seems IFR traffic will routinely fly directly above, albeit not a low levels.


How much would it cost for a large airline to switch engine manufacturer for their planes? I am trying to understand how much of a hassle and cost it would be for a large civil airline, who already had lots of installed Rolls Royce engines on its fleet, to switch over to GE engines on new planes they have ordered. For example if they opted for the RR Trent 1000 on a 787 initially but then changed new orders of 787's to have the GEnx-1B.

Very intangible I know, but would it be fair to say that switching would require some difficulty including:

retraining of pilots
changing maintenance schedules
additional uncertainty over performance and fuel efficiency

Alternatively, I could be wrong, and the airline will likely just switch to the GE if it thinks it can gain a small fuel burn advantage, in which case RR's back book of engines does not mean clients will opt for the RR engine choice going forward.

Many (especially larger) airlines have a history and infrastructure for engines from a specific manufacturer and are unlikely to change that, the cost would extend far beyond just the cost of changing the contract. E.g. KLM has a massive investment in maintenance facilities for GE engines, workshops, spare parts warehouses, highly trained specialised technicians. Retooling to RR would be prohibitive for them.

Of course a smaller operation without maintenance facilities of their own might be more flexible in that and just buy whatever is cheapest at the time, contracting out maintenance to various companies as needed. But even then, it's often more economical to have a single maintenance provider doing all your work for you, which might well mean a provider who's specialised in a single series of engines, for example if you're based at Schiphol there's a good chance you'll be contracting out your maintenance to KLM and their GE specialised shops.

So there are many good reasons for airlines to stick with a single engine supplier (and to a degree with a single aircraft manufacturer), even if the cost of buying those engines is higher than selecting competing engines from another manufacturer.
Say you're going to save 0.1% in your fuel bills for the 2 new aircraft you're buying to increase the fleet of 20 with other engines you have already (and that's a big if, those engines are pretty close between manufacturers for a specific aircraft). You'd need to fly a LOT for it to be worth building an entire duplicate maintenance network just for those few new aircraft.
Of course if it's part of a long term plan to replace the entire fleet with aircraft using that other manufacturer's engines, that's a different story. But mixed engine fleets tend not to be the most economical way to go, even if on one type manufacturer A is marginally more fuel efficient and on type B manufacturer B is marginally more fuel efficient (and that's what you're going to see, marginal differences only).
If you're talking an operation like Delta or United, with hundreds of aircraft of each type, it might be a different story. But then too you're talking about more aircraft with each manufacturer's engines than most other airlines have in total, and they'll have multiple maintenance shops themselves, making it easier to have a facility for each manufacturer (and of course in case of the big US airlines, they tend to have ended up with that in the course of gobbling up other airlines anyway).


How to determine if an aircraft type is currently under investigation? The Boeing 737 flew for several years despite rudder issues which had not been fully understood and resolved. It's conceivable that flyers who avoided the aircraft had had less probability of injury or death while flying.

How does one identify which aircraft are currently under some sort of investigation for similar, on-going issues?

All aircraft are constantly under investigation. Service bulletins and airworthiness directives are issued for every certified plane and engine. These are optional and mandatory checks and maintenance to be done in additional to the normally scheduled maintenance. Every accident and important incident is investigated by the NTSB, with a public report available online.

There is a period of time before an airworthiness directive is made final that might be considered an investigative phase, but information on this is generally not publicly available. One notable example pertains to the rudder controls on the Airbus A-300.


What can be considered under 61.113 (c) for passenger equipment? Does 14 CFR 61.113(c) allow a passenger to pay for rental equipment for themselves (e.g. headset or parachute) in addition to their share of the operating expenses, or is that considered compensation?

e.g.

Occupants = 2
Hobbs cost = \$100
Headset rental for passenger = \$10

Can they pay either:

\$50 (Only allowed to split Hobbs costs)
\$55 (Allowed to include the headset in the total cost - i.e. included as 61.113 (c) rental fees)
\$60 (half the cost of the Hobbs + 100% of the headset)

Incidental costs specific to the passenger, such as O2 for the passenger's medical condition, may be paid by the passenger and need not be factored into the cost of the flight for flight sharing costs.

In your example, the Hobbs charge for the aircraft is the flight cost.  If the passenger wants a headset, or anything else that is not a flight expense, the cost need not be shared.

Landing fees, navigation fees and the like are grey areas, and I would split those.  But something like a tie-down or hangar fee is not a flight expense, and the passenger may pay for in full.

If you read cases where the FAA has prosecuted pilots for conducting operations "for hire" or similar, they tend to be egregious.

So in your example, $60 would be the right answer.  I would suggest that you could even split the cost of the charts you might buy for the trip.

As a final word, I would make certain that your passenger understands that you are just sharing expenses, not that they are "paying for a ride."


How do I find the magnetic bearing to the station in this question? In my PPL trial test, I have the following question regarding an NDB:

  Consult Figure 3, Illustration 6. On a magnetic heading of 160°, the
  magnetic bearing to the station is:

  045°
  205°
  165°
  270°

  (See the attached figure, illustration 6)

Looking at the figure, I see a needle pointing to 045°. Therefore, I conclude that when my heading is magnetic north (as the heading of the aircraft is shown in the figure), the magnetic bearing to the station would be 045°. Consequently, I deduce that if I am heading 160° magnetic, then the bearing to the station would be 115° to my left, i.e. 360° - (160° - 45°) = 245°.

The reason I think this is because, as I am turning eastwards the magnetic bearing to the station will decrease until I reach a heading of 45° magnetic at which point the magnetic bearing to the station would be 0°. As I turn eastwards more, the magnetic bearing to the station would flip around at 360° and keep decreasing. I need to make a starboard turn 160° - 45° = 115° more to reach the desired heading 160°, so by that time the bearing would decrease from 360° by 115° to reach 360° - 115° = 245°.

Nevertheless, the test claims that the correct answer is 205° (which I'm guessing is, for some reason, calculated as 160° + 45°). Could you kindly explain what is wrong in my thought process or interpreting the ADF instrument?

I think the figure is just intended to illustrate the bearing to the NDB relative to the aircraft, and not the magnetic heading of the airplane. So when they give you your magnetic heading of 160, you're still using the bearing of 045 relative to your aircraft, which gives 205 magnetic.


What is this on top of the Air Force One? Clarification: It wasn't Air Force One that visited Switzerland, it was the 747-200 E4 carrying Ashton Carter. Source

What is this thing on top of the plane (In red circle):

On other "Air Force One" Google image searches, I can't find this thing. I know they have more than one 'doomsday-plane', but whats the purpose / use of this thing?

The aircraft is the Boeing E-4B Advanced Airborne Command Post 'Nightwatch', which serves as the survivable mobile command post for the US National Command Authority. The aircraft are based on the same Boeing 747s as the VC-25s used by US President.
There are four identical aircraft performing this function in the US. The "bulge" on the top is a radome that houses a steerable SHF SATCOM antenna, as can be seen in the following image.

Image from cnet.com
The following image shows the aircraft systems in a little more detail.

Image from http://everythingisahoax.tumblr.com
As a side note, Air Force One refers specifically to USAF aircraft carrying the US President. From US DoT ORDER
JO 7110.65W

When the President is aboard a military aircraft, state the name of the military service, followed by the word "One".
EXAMPLE−
“Air Force One.”
“Army One.”
“Marine One.”

For civil aircraft, it is Executive One.


Remote access to Area Forecast Discussion Is it possible to access Area Forecast Discussion via SMS?  I fly to remote areas frequently (Wilderness), and after several days, it is almost impossible to get a good weather forecast, (one you're willing to bet your life on).  I wish I could get the Area Forecast Discussion via SMS over my inReach.

I have an old Garmin 496 & XM which works great for this. XM lets you suspend your subscription in winter if you want to. Tom M


What is the difference between SUA and SAA? I would like to understand the difference between Special Use Airspace (SUA) and Special Activity Airspace (SAA). I found the following definitions.

Eurocontrol Lexicon for SUA (from FAA):

  Airspace of defined dimensions identified by an area on the surface of
  the earth wherein activities must be confined because of their nature
  and/or wherein limitations may be imposed upon aircraft operations
  that are not a part of those activities.

Eurocontrol Lexicon for SAA (from FAA):

  [U.S.] Any airspace with defined dimensions within the National
  Airspace System wherein limitations may be imposed upon aircraft
  operations. This airspace may be restricted areas, prohibited areas,
  military operations areas, air ATC assigned airspace, and any other
  designated airspace areas.

Can you please help me to understand what the difference is?

SAA is a project being undertaken to unify how SUA's (and ATCAA's) are defined, managed, and shared.

  [SAA will] define SUAs and ATCAAs in a consistent manner using an editing tool
  Use a Service Oriented Architecture (SOA) to distribute that data to interested users
  Manage the schedule and status of those airspaces through a SOA.

SAA is part of the System Wide Information Management (SWIM).

In short, SAA encompasses SUA, ATCAA, etc.

Source: SAA Modernization (.ppt file)


How do you enter the hold after a missed approach at Riohacha (SKRH)? I have a question regarding this Missed Approach Procedure

Im in my final approach segment and reach DME 1.1 and the runway is not in sight so I start my Missed approach, how should it be executed?

Since I have to hold I'd start a tear drop entry turning to heading 125° and then left turn intercepting 275° course inbound. This option sounds viable to me
I first do a right 360° as charted (???) and going to the VOR and then starting probably a parallel entry into the hold. This doesn't sound viable to me

Textual description is also clear about that 360°
If it 1.) why would they chart that 360° there?

The missed approach is a climbing right turn to the VOR and then hold while climbing to 3000 ft.  The purpose of the 360 is probably to gain some altitude before entering the hold (someone with TERPS knowledge can address this).  The MSA where the hold is is 2000' and you start the missed at 390'.  The right turn will be protected starting at 390' wheras the hold is not, so you need to gain altitude before entering it.  Part of the issue here is that you can fly any entry you want, so the protected airspace for a hold (which determines its minimum altitude) is a somewhat large area.

To fly it:

Execute your missed approach items (flaps, gear, etc)
Enter a climbing right turn direct to RHC VOR.
Upon reaching RHC VOR continue to climb and fly the holding entry of your choice
Hold as published.


What is the length of an A380 aileron? I work as a paint shop prepper at an airport in the UK and have currently worked on a aileron of a 757-200 that was 14ft long and wanted to compare it to that of a A380

A good source of public information is the aircraft characteristics document that the manufacturers typically publish for each model, including the A380.

A scale planform drawing can be found on page 310 of the PDF, subject 8-0-0. Based on this drawing, the rough size of the ailerons can be estimated.

The aileron is divided into three sections which move independently (see Why do some aircraft have multiple ailerons per wing?). The outboard section is about 4m (13 ft) long, the middle section is about 3m (10 ft) long, and the inboard section is about 2m (6.5 ft) long. In total they are about 9m (29.5 ft) long.

Derived from Airbus document:

Ailerons viewed from cabin:

Image source (other question)


Are a gyrodyne and a compound helicopter the same thing? Both the gyrodyne and the compound helicopter are typically rotorcraft with driven rotor, additional stub wings and thrust provision. Are they two terms for the same thing or do they differ in some way?

I have now found a couple of pretty authoritative definitions for a gyroplane:

FAA Rotorcraft Flying handbook: For Gyroplane Use Only:

  "The 'gyrodyne' is a hybrid rotorcraft that is capable of hovering and yet cruises in autorotation. The first successful example of this type of aircraft was the British Fairy Rotodyne, certificated
  to the Transport Category in 1958. During the 1960s
  and 1970s, the popularity of gyroplanes increased with
  the certification of the McCulloch J-2 and Umbaugh.
  The latter becoming the Air & Space 18A."

Electronic Code of Federal Regulations (e-CFR) Title 14. Aeronautics and Space Chapter I. FEDERAL AVIATION ADMINISTRATION, DEPARTMENT OF TRANSPORTATION Subchapter A. DEFINITIONS AND GENERAL REQUIREMENTS Part 1. DEFINITIONS AND ABBREVIATIONS, Section 1.1. General Legal Information Institute, Cornell Law School.

  "Gyrodyne means a rotorcraft whose rotors are normally engine-driven for takeoff, hovering, and landing, and for forward flight through part of its speed range, and whose means of propulsion, consisting usually of conventional propellers, is independent of the rotor system."

So it may be that the gyrodyne fully offloads the rotor and relies on autorotation during cruise, while the compound helicopter drives its rotor throughout. But I can find no authoritative definition of a compound helicopter.

Technically, under autorotation the rotor disc tilts backwards and air flows up and over the blades from below, while under powered drive the rotor disc tilts forwards and the blades drive air downwards from above. It would be nice to think that this was reflected in the two definitions, but there are obviously borderline cases where the disc angle and loading are not quite textbook.


How serious is the problem of "stopping too far" next to the jet bridge? In the answer to the recent question "How does an airliner adjust ...", there is the mentioning of the signal: "STOP TOO FAR". But what to do if you see this?
In other words, if the airliner stops too far next to the jet bridge, how is this problem typically resolved? I was thinking about the possible approaches:

In my car, I would just drive backwards. But seems that the usage of the reverse thrust is somewhat frowned upon.
The aircraft will be serviced without the jet bridge (movable stairs, etc).
The aircraft will be towed backwards where it should be.
The jet way most often can be adjusted to compensate the unexpected location, the sign is just the warning.
No ATPL holder would ever make a mistake of this kind anyway.

Also, how often does this happen? Is this a very rare problem or something more common?

I can't tell you how often it happens, but the procedure is quite simple: stop the aircraft immediately, and alert the ground crew. It is then up to the ground crew to decide what, if anything, needs to happen. If the aircraft has just slightly passed the intended stop position, maybe it can just stay where it is. If it has overshot significantly, it may be required to push the aircraft back to the correct position with a tug. Worst case, the aircraft might have actually hit some obstacle (building, vehicle, jet bridge) in which case the aircraft or surrounding equipment might have been damaged.


Do helicopters carry radar equipment? Is so, where is it placed and what type is used? Do helicopters carry radar equipment and if so, where is it? (e.g. in the nose section like a fixed-wing aircraft).

Also what type do they use? (e.g what type of radar would a maritime helicopter carry)

Most helicopter don't carry radar; there is no need for it.

In civil helicopters radar is typically used for weather detection. In most use cases there is not real need for it because helicopters are usually operating under daytime Visual Flight Rules (VFR) and are staying away from clouds. For maritime helicopters that are used in search and rescue operations or off-shore operations on oil-rigs, weather is often installed. The typical location is in a radome in the nose of the helicopter. Examples are the Honeywell Primus 660 or Rockwell Collins RTA-4100

Military helicopters use radar for detecting objects. Depending on the nature of the mission the helicopter is designed for, the radar is underneath (usually for navy operation, providing sea surface detection capabilities, see for example the navy version of the NH90) or on top of the rotor (to look over obstacles such a hills / trees  and provide fire control to their missiles). An example is the AN/APG-78 longbow fire control radar.


In the old days, why didn't aircraft use simpler systems to land on a Carrier? In the old days (during and before WW2), Carriers used arresting wires to help aircraft land. In fact this is still used today for jet-based carriers, but I want to ask about the old days with piston-prop aircraft because they go slower and generally make this easier.

I'm pretty sure even back in WW2, most carrier aircraft had tailhooks that grabbed the wire. This is so you "brake in the back", not the front. If you braked in the front, you are much more likely to nose forward or spin around.

Well, I've seen plenty of footage where that happens because something in the front of the aircraft, the nose or front wheels, gets tangled up in those wires, causing bad accidents. Here's a good compilation that has all sorts of landing accidents, including with the wire. What tends to happen is the wire is snagged wrong and the aircraft nose dives into the deck, wrecking the propeller, and/or the aircraft goes spinning around.

I can think of several alternatives to an arresting wire:

Reverse Pitch Propellers (reverse thrust)
Powerful Brakes in the rear wheel, or all wheels
Drag Chute
Air Brake

To me the simplest solution seems to be the last one, an Air Brake. I've seen this on modern jets like the Su-27 (@ 5:06). It's basically a big flat panel that swings out and creates enormous drag. In the Su-27 they keep it near the center of mass so it doesn't induce large torques, but you can put it in the rear half of the aircraft too. That will "brake in the back" and I think it won't require any more airframe strengthening than a tailhook, since that brakes in the back too.

So why not use this simpler solution? For reference, an Essex Class Carrier has a runway of about 260 m. I actually could not find normal "landing distance" that a small aircraft (bf 109, spitfire, etc.) needs once it touches down. If someone can find that, we would be closer to an answer.

The benefit of a cable is that it's a mechanical means of arresting the aircraft. You either catch the cable and stop, or miss and go around. Aside from breakage and malfunction, there is no in-between. If braking is left entirely up to the pilot, it's up to them to engage all the right functions quickly enough, and in sufficient amounts. If they misjudge, they end up going into the water.
Cables are also effective at stopping aircraft quickly. This means less deck space is needed for landing, and with the introduction of the angled landing area, allows aircraft to be launched and recovered at the same time. Only a tail hook is needed to be added to the aircraft, which adds great braking effectiveness for relatively little weight. Other options have reasons they would be less effective than cables, with greater penalties in areas such as weight.
Reverse Pitch Propellers (reverse thrust)
This can be an effective way to help slow a propeller aircraft down. The downside is that it's up to the pilot to engage the reverse pitch and in a sufficient amount. Single engine aircraft can be hard to control especially if higher power settings are needed while in reverse.
Powerful Brakes in the rear wheel, or all wheels
Brakes are limited by the friction between the wheel and the ground. On carriers, this was generally wood, later replaced by metal. Especially when wet, this does not provide very much friction. Increasing downforce (see the air brake section) can help, but only to an extent. And as you noted, especially with tail draggers used on early carriers, aircraft would nose over if they braked too hard. The tail wheel doesn't have much weight on it, so would not be effective in braking.
Drag Chute
You'll notice that pretty much the only aircraft that use drag chutes are aircraft that land at high speeds and already need long runways. This is where you get the most benefit from a chute; long landing rollouts at high speed. As speed is reduced, they become less effective. You need to spend the time recovering, repairing/replacing, and re-packing the chute after each landing. They are also going to be less reliable than a tail hook. If it fails, you can't land on the carrier, you may just have to bail out.
Air Brake
Air brakes are similar to drag chutes in that they work best at high speed. While they are are not a consumable item, the drawback is that they have much less area, and therefore much less effectiveness than chutes. They work best when they can also provide downforce to improve braking friction, but still need high speeds and long distance to really provide benefit.
Carrier aviation in World War II was certainly dangerous. Many pilots died in accidents rather than in combat. But the military was still learning about how useful carriers would be in combat, and figuring out what the best configuration for carriers and their aircraft would be. Carrier aviation has come a long way since then, and cables are still the go-to system for recovering aircraft that don't at least have some kind of STOVL capability.


If a warning emerges, why not land as soon as possible? This question is based on this episode of Air Crash Investigation concerning crash of Lauda Air flight 004.

Reverse Thrust Indicator warning is shown at 21 minutes and 10 seconds into the video. At the time of flight, such a warning was deemed a non-emergency, though wrongly. Yet should the pilots still have continued the flight?

I would think it safer to err on the side of caution, and try to land instantly whenever a warning emerges? The video depicts the pilots' verbalised guesses and thus perplexity over on the warning's causes.

I anticipate two counterarguments:

Some warnings may be too minor to justify an immediate landing, but where is the line drawn? Anyways, this warning seems dire enough?
Pilots may fear reprisals from their greedy employers for effectively cancelling the flight, for possibly a false negative, but a principled virtuous pilot would prioritise safety over monetary interests.

As rbp explained, information provided to the pilots is differentiated based on how serious the problem is, from informational up to emergency. Part of designing an aircraft is determining how serious different failures will be.
As technology has advanced, aircraft have become more and more complex. New devices and systems allow aircraft to become faster, safer, more capable, and more comfortable. However, more complexity means more things that can fail. The airplane's systems are analyzed to understand the ways that a system may fail, and how it will affect the rest of the airplane. The results are used to determine how serious different indications may be, and to provide instructions to the crew about what procedures to follow.
With the complexity of modern aircraft, it is much more likely for at least one system to have an issue. It does not make sense to ground an aircraft just because a coffee maker isn't working. Aircraft will have a minimum equipment list (MEL) which allows the aircraft to depart with certain systems inoperative. Circumstances such as ETOPS will further limit which systems may be inoperative. Once in the air, the crew is informed by their checklists about the correct actions to take. Some indications are not critical, others will be. Modern aircraft go even further in determining the severity of indications. Certain less critical information will be suppressed during high-workload phases like takeoff and landing to avoid distracting the crew. Some aircraft will even make it simple for the crew and give them a message like "LAND ASAP".
The basic FAA regulation for crew alerting systems on transport aircraft such as the 767 in your example is 14 CFR §25.1322 (b):

(1) Warning: For conditions that require immediate flightcrew awareness and immediate flightcrew response.
(2) Caution: For conditions that require immediate flightcrew awareness and subsequent flightcrew response.
(3) Advisory: For conditions that require flightcrew awareness and may require subsequent flightcrew response.

It is up to the group certifying the aircraft to convince the FAA whether something requires immediate awareness and/or immediate response.
Some specific alert systems are covered in different sections, such as §25.1305 (d):

(2) A position indicating means to indicate to the flightcrew when the thrust reversing device—
(i) Is not in the selected position, and
(ii) Is in the reverse thrust position, for each engine using a thrust reversing device.

There isn't much more specific than that, but there are other rules that affect the warning system such as §25.729 (e):

(6) Failures of systems used to inhibit the landing gear aural warning, that would prevent the warning system from operating, must be improbable.

So in order to make a failure of the warning improbable, the system may display a false indication rather than fail to indicate a valid problem. Most indications for things such as fires are false. It is much better for the system to assume the worst if in doubt, and for the crew to assume the indication is valid and act accordingly.
So, to answer your questions:

The systems are analyzed to determine which indications are
important enough to alert the crew, and what the crew's response should be. These decisions will be reviewed
at certification to convince the FAA (or local authority) that these
decisions are sound.

As always, the ultimate responsibility of the aircraft lies with the
captain. If the captain feels that diverting is the safest option,
he or she can make that decision. An operator with a good culture of safety will never cause a captain to
fear reprisal for making such an informed decision in the interest
of safety.


Can drone with two propellers above one another fly vertically upward without spinning? Basically, like the propellers setup in this image without the tail end bit:

Where the propellers will be spinning in opposite directions. This would, in theory, counteract the angular momentum to a degree right? So would this be able to fly vertically upwards without spinning?

Yes, because spinning in the opposite direction produces counterbalancing torque. Those rotor over rotor models adjust yaw by small variations in the speed of the rotors, so that one produces a bit more torque than another.

There are a number of large helicopters that use two main rotors to the same effect, such as the CH47 Chinook (front and rear) and the Kamov KA31 which uses the same rotor over rotor configuration as the model you picture.


What is the hierarchy of aviation regulatory bodies? What does the hierarchy of the aviation bodies look like?

Does it look like this:

International: ICAO

Continental: EASA,ECAC,...?

National:?

As pointed out by vasin1987 in a comment to the question, each country's regulatory body has control of that country's airspace: These agencies exist in a "mesh" of coequals as opposed to a hierarchy.

The EASA is a EU agency and applies to all of EU, very similar to how the FAA is a US agency that applies to all of the US.

ICAO, in turn, deals primarily with situations where air traffic crosses regulatory borders. In its own words, borrowing from the page About ICAO on their web site,

  The International Civil Aviation Organization (ICAO) is a UN specialized agency [which works] to reach consensus on international civil aviation Standards and Recommended Practices (SARPs) and policies[.]

Like pretty much the rest of the UN, the ICAO is more a forum for cooperation between regulatory bodies than a regulatory body in its own right.


Is a hold in lieu of a procedure turn specifically one hold on an RNAV approach? I was looking at RNAV (GPS) RWY 6. I considered that conceivably ATC could have me enter the hold at an altitude requiring a rapid descent to get to the initial approach altitude.

I know that unless NoPT is specified on the initial segment or from the TAA sector that you must do at least 1 hold, but what I wasn't sure was could you do more than one hold at your own discretion?

So, say, you were told to cross the IAF at 4000 and you felt like losing 2000 feet in half a hold would make your passengers uncomfortable, could you just make two laps?  Would you have to ask ATC for permission first?

It looks like crossing the hold fix is your initial approach fix and then it's subsequently an intermediate fix so it seems like the design is that you enter the hold and then cross the holding fix on your way into the approach, so it seems like it's designated that you cross the fix only twice, but I wanted to be sure.

Edit:

Certainly there are exceptions.  When directed by ATC to fly a straight in approach, the hold-in-lieu is not required.  If NoPT is designated for your TAA region or on your initial segment, then it is forbidden.

To rephrase my question, given that your hold-in-lieu is required then your lower bound is 1 hold.  Is there a designated upper bound of 1 hold as well?

I.e., if a hold -in-lieu-of-procedure turn is required, is it 1 and only 1 hold?

Sorry for the confusion.

When flying a hold-in-leiu-of-procedure-turn and the exemptions from that procedure are not met, you must fly that procedure exactly once. From the AIM, 5-4-9(a)(5):

  The  holding  pattern  distance  or  time
  specified in the profile view must be observed. For a
  hold−in−lieu−of−PT,  the  holding  pattern  direction
  must  be  flown  as  depicted  and  the  specified  leg
  length/timing  must  not  be  exceeded.  Maximum
  holding  airspeed  limitations  as  set  forth  for  all
  holding
  patterns apply.
  The holding pattern maneuver is completed when the aircraft is established on the inbound course after executing the appropriate entry. If cleared for the approach prior to returning to the holding  fix,  and  the  aircraft  is  at  the  prescribed altitude, additional circuits of the holding pattern are
  not necessary nor expected by ATC. If pilots elect to make additional circuits to lose excessive altitude or to  become  better  established  on  course,  it  is  their responsibility to so advise ATC upon receipt of their
  approach clearance.

Further notes: I'm not sure quite what you mean by losing 2000 feet in "half a hold", but if you got a clearance to maintain 4000 until established, you could start your descent immediately after crossing ODAZO, giving you 8 miles plus a one-minute turn to descend. At 120 kt, that's 4 minutes for the 8 miles, so 5 minutes for 2,000 feet or 400 ft/min - perfectly reasonable.

Finally, you mention that "When directed by ATC to fly a straight in approach, the hold-in-lieu is not required." Your statement should be stronger: as when NoPT is printed on the chart, a straight-in clearance forbids the procedure turn/HILOPT unless you ask for and receive approval from ATC.


What are these parts on the wing of a Boeing 737? In the picture above:

What are the parts numbered "1" and and "3"?
Is "2" maybe the reinforcement of the pylon?

I try to improve on the guesswork of @aeroalias. Here we go:

Parts 1 and 3 are the two exhaust gates on either side of the aft engine fairing, which is part number 2. For a reference, see The Boeing 737 Technical Guide, page 116, by Chris Brady (Google Books link).

The flap actuation mechanism is quite a bit ahead of the flaps, and the same goes for the spoilers.

The engine on the Boeing 737 is placed very close to the wing, so the flaps would sink into the jet blast when lowered to the landing position. This would cause high vibrational and thermal loads on the flaps next to the engine exhaust, especially when the pilot increases thrust in landing configuration for a go-around. To protect the flaps, the exhaust gates were added so this part of the flap does not extend down as much as the rest of the wing flaps.

Drawing of the inboard wing flap of the Boeing 737-300/400/500. The inboard flap track sits within the wing root fairing, and the outer track sits in the aft engine fairing (picture source).


Does pumping fuel to each wingtip affect roll trim in a stabilized turn? If an aircraft is stabilized in a constant-bank, constant rate turn with no sideslip, and then we pump 200 gallons of fuel from a fuselage tank to a tank on the tip of each wing, without changing the position of the ailerons or rudder, and without changing the aircraft CG, will we find that the aircraft tends to roll into the turn, roll out of the turn, or neither?

The intent of this question is that we get the aircraft fully stabilized in a coordinated turn at the original bank angle after pumping the fuel to the tips, and then we put the controls back in their original positions and see if the aircraft tends to roll.  This isn't meant to be a question about the effects of suddenly redistributing the mass and the way that this might possibly create a temporary yawing tendency due to yaw rotational inertia, like we see when a skater throws out their arms horizontally while spinning around.

Not if the fuel pumped to the wing tips in equal amounts so that the lateral CG is unchanged.  But... there are effects related to the inertia of the fuel mass (the wings become spokes in a flywheel, you could say).

I have some flying time a plane that has all of its fuel in the wing tips, in tuna fish shaped tanks, 25 usg per side.

The difference in handling qualities with the tips nearly full and the tips nearly empty is all related to inertial effects of the fuel mass; roll rate, roll inertia, yaw inertia, and roll response to yaw.  The roll rate you can generate by skidding the plane is much lower with the tips full.  The influence of dihedral in damping and restoring roll deviations is reduced.  Disturbances are amplified because of the flywheel effect of the tip mass and this is especially noticeable in yaw; there is more of a tendency for the tail to wag in bumps.

But in a stabilized bank in smooth air, the amount of fuel in the tips made no difference in increasing or decreasing overbanking/underbanking, except to the extent that the interia of the fuel dampens or inhibits the usual restorative forces.  So if you had an airplane that can be banked and holds the bank angle with no input, there is no major effect of fuel in the tips in changing that characteristic, until you hit a bump and the inertial effects start to work.  However, where the airplane has an overbanking or underbanking tendency, the flywheel effect will inhibit it starting and tend to keep it going once started.

So to answer the basic question, if you had a plane that naturally wants to roll out of a turn when inputs are removed, the effect of fuel in the tips will be to reduce that effect somewhat, so it will still roll level but take longer, and once level it will tend to overshoot more.  It it wants to overbank into a spiral dive, same thing that way.


How often is the exterior of airliners cleaned? Do airlines such as American Airlines or Emirates clean their planes regularly?

If so, how regularly? After every flight or at scheduled maintenance points?

How long does this activity take? Do they apply any wax or polishing to the plane?

I am interested in the general upkeep of the exterior of the plane and what major commercial airlines do to upkeep their planes.

According to Lufthansa Technik, with the new cleaning agents, once every 6 months.

  Until just a few years ago, wide-bodied aircraft had to be washed four times a year with foam and up to 13,000 liters of water. Today, thanks to newer cleaning agents, a resource-friendly dry wash every six months is enough.

It takes between 8 and 24 hours to wash or dry wash an airliner.


Where inside a black box is the data storage located? In a news article I saw this picture of a black box from a crashed A320. The article mentioned that finding the black box had lead to some disappointment due to the black box having been damaged:

This got me wondering where the actual data storage is located inside the black box. The cylindrical piece on the left looks like it is designed to be more robust than the rest of the hardware, so I am guessing that is where the data storage is located.

Are the reports of the damage exaggerated in this case? Or is any of the critical data stored in those parts of the device which have been damaged?

The image of the recovered "black box" is showing the Cockpit Voice Recorder (CVR). The data is stored in the cylindrical housing that was relatively undamaged in the crash.

The housing consists of a stainless steel outer shell to withstand impact forces. Beneath it is a layers of high temperature insulation to protect the data storage from post crash fire.


What are pro/cons of steam as lifting gas for airships? I was reading Wikipedia page about hot air airship and another Wikipedia page about the first powered and steerable airship which was powered by a steam engine. So I ask myself "why didn't they use the exhaust steam from the steam engine as lifting gas?"
After quick research, I found this home made experiment of steam used as the lifting gas. This website even gives a comparison with other lifting gasses:

+-------------+----------+------------+-----------------+--------------------+--------+-----------+-------------------+------------------+
|     GAS     |   M.W.   | Temp. (°C) | Density (kg/m3) | Lift (N/m3) in ISA | Safety |   Cost    | Ease of provision | Buoyancy control |
+-------------+----------+------------+-----------------+--------------------+--------+-----------+-------------------+------------------+
| H2          | 2        | 15         | 0.084           | 11.19              | bad    | fair      | fair              | no               |
| He          | 4        | 15         | 0.169           | 10.36              | good   | very high | very bad          | no               |
| CH4         | 16       | 15         | 0.676           | 5.39               | bad    | low       | fair              | no               |
| NH3         | 17       | 15         | 0.718           | 4.97               | fair   | low       | fair              | no               |
| hot air     | 29 (avg) | 110  (avg) | 0.921 (avg)     | 2.98 (avg)         | good   | very low  | good              | yes              |
| steam (H2O) | 18       | 100        | 0.587           | 6.26               | good   | very low  | good              | yes              |
+-------------+----------+------------+-----------------+--------------------+--------+-----------+-------------------+------------------+

(source: http://flyingkettle.com/jbfa.htm)
Given the conclusion of this website, steam should now be more widely used as a lifting gas:

it is available since the first lighter-than-air (plenty of time to develop the technology)
it has a good lifting power
it is cheap and safe

Yet, on 2020, almost all airships and balloons use helium or hot air, and no other lifting gas. So there must be drawbacks I fail to imagine. I hope finding those drawback will orient my research for my first question ("why didn't they use exhaust steam as lifting gas on airship fitted with steam engines?"). Moreover, there may be advantages I also fail to imagine.
Thus, what are pro and cons of using steam as a lifting gas?

The main disadvantage is the energy required just to get off the ground. Let's do a back-of-the-envelope calculation, comparing this to a hot air balloon.

Let's heat 1m³ of air. To get equivalent lift, we need about 0.5m³ of steam.
The density of air is about twice the density of steam. So, for a given amount of lift, we need to heat approximately the same mass of lifting medium.

To heat 1kg of air to 100°C above ambient, we need about 100kJ (specific heat of air is about 1kg/(kJ K)).

To heat 1kg of water to 100° above ambient, we need about 420kJ just to heat the water. To vaporize the water, we need an additional 2250kJ, for a grand total of 2670kJ.

The most used hot air balloon size is 2800m³. For that we need to heat 1400kg water. Multiplying gives about 4GJ of energy or 75kg of propane just to get off the ground. Since we established steam can lift twice as much per unit of volume, we can maybe get away with 2GJ or 35kg of propane.

Some more negative thinking... Condensation on the outer shell seems like a very efficient mechanism to transport heat energy to the environment, only partially offset by the reduction in surface area compared to a hot air balloon due to the cube-square law. This may require more energy in flight than a hot air balloon. Closed-cycle operation (only heat condensed water, not 'new' water) is preferred due to the high energy requirement, so descending means you have to wait for enough water to condense or be willing to pay the price in fuel. Superheated steam is not safe (despite what your table says); the high heat capacity and enthalpy of condensation can result in serious scalding burns compared to hot air. All of these effects are of course much more pronounced when comparing to a helium-filled blimp.
Still, it's a really neat idea, and sometimes you don't need a practical reason just to build something neat.


Why does load shedding still occur in jetliners (e.g., 787) when there is so much power available? I had a few questions about how electrical generator works on an aircraft, the 787 to be specific although I know it is similar for all aircraft.

So the 787 engines have 2 VFSG each which are rated at 250 KW each which is 1000 KW -> 1 MW which is said to power a 2000 population town. Since the power is generated using the rotation of the engine, im assuming it does not always produce 1 MW , only at peak rotations?

What on earth in the plane sucks that much energy, I know the 787 is a more electric aircraft, but still how much power does it really take? If the total power output of the plane is 1.45 MW, I'm assuming the APU Gens are able to provide 0.45 MW in total to the airplane.

If there is so much power available, why does load shedding still occur in aircraft?

Load shedding always occurs during engine start, according to the FCOM of the B789. Both engines are allowed to start simultaneously. The B789 engines are started using both VFSGs of each engine, which are mechanically connected to the N2 shaft via the accessory gearbox. So, the VFSGs are using power instead of delivering during engine start. The APU cannot provide enough power to prevent load shedding.

If the engines are started using external power, at least two 90 kVA external power sources are required. Optimal start performance is achieved using 3 external power sources (2 on the left forward fuselage and 1 behind the left wing on the fuselage). If only 2 external power sources are used, significant load shedding can occur (e.g. First Officers displays blanks and even the audio control panel receive and trasmit selections may be lost).

The APU is normally switched off during flight, unless e.g. one VFSG is U/S.

Load shedding should normally not occur when both engines are running and all VFSGs are operational (I haven’t experienced load shedding during flight).


Which of these is true for a glider flying with constant angle of attack? I'm trying to answer the following question, but I'm not sure of which one is correct.

A glider is gliding with constant angle of attack along its path. We can assume the hypothesis of small angles. Under these conditions, the following has to be fulfilled:
a) None of the other answers is correct.
b) $C_L=C_{Lopt}$
c) $\hat{D} =$ constant
d) $\alpha=\alpha_{max}$
e) V must remain constant

Note: D with hat means dimensionless drag.
I've been trying to discard some of them but I'm still doubting about if the correct answer is (a) or (e). Can you help me guess which one is the right one?

Two things are important here:

It is about a glider, so no fuel is consumed, and the mass is constant over time.
The angle of attack is constant. This means also that the lift and drag coefficients are constant.

If the mass and the lift coefficient are constant, and we assume a constant load factor (implied by the gliding along its path description), the dynamic pressure is also constant, and the airspeed changes with the square root of air density. Gliding implies that the altitude decreases, so the indicated speed will be constant, but the true speed will decrease. Unfortunately, the condition e) is not precise enough to make this distinction. e) could be correct or not, depending on the definition of V.

I would suggest that also c) could be a correct answer: If the angle of attack and the mass are constant, so should be the drag coefficient. However, gliding down also implies that the Reynolds number changes because true speed decreases and temperature increases over the glide. Strictly speaking, this would also mean a very slowly changing drag coefficient.

Answers b) and d) are false and I agree with your reasoning.


Do regulations permit an airliner be equipped with a radio transmitter which cannot be controlled by the crew? In response to the case of MH370, an idea which has been floated is that of a device to transmit an aircraft's GPS position. Not specified is whether such a device would be operable by the flight crew.

Among the scenarios suggested to account for the aircraft "going dark" and its apparent flight path is intentional action by someone aboard.

To provide aircraft position (or any other parameter) reporting free of any possible interference by anyone aboard the aircraft would require installation of equipment inoperable by and therefore in a location inaccessible to anyone (crew included) during flight. That means such equipment could never be powered down while the aircraft is in flight.

Have there been/are there currently any regulations which would prohibit the installation of such a device for any reason - e.g. fire safety, possible interference with other radio equipment essential to safe flight (navigation, communication)?

All equipment on board needs to be interruptable, either by direct control input (turning it off) or via circuit breaker. This especially holds true for equipment that is capable of broadcasting or emitting radio waves, such as radio telephony equipment or the transponder.

If the transponder for any reason became damaged and started to send incorrect replies to ground interrogation, it needs to be shut off in order to not confuse ATC systems or other aircraft's TCAS. Electronic or electric systems also need to be shut down in case of eletrical fires or other safety hazards to the aircraft.


On an IFR flight, should I avoid class B and include approach fixes/procedures in the route? When creating an IFR route, should one route around and avoid class B airspace?  Should one include the IAF (Initial Approach Fix) of the desired IAP (Instrument approach Procedure) in your route?  Should one include the desired STAR and/or SID in the comment section or route section?

You don't need to.  If enter a start & end point at https://skyvector.com/, it can suggest standard routings for you.  You can follow those, or tweak it up some.
For example, 6B6 to WWD, standard routing will put you on V1 and will take you out over the water quite a ways, farther than I prefer to be in my single engine 4 seater. I'd tweak that to route over JFK, Colts Neck VOR, Atlantic City instead, having been cleared for that route several times by NYC approach. IFR at 6000, and VFR even, at 6500 feet.

I've also talked to pilots going north & eastbound who were IFR and routed north around the NYC Class B, sometimes quite a few miles out of the way. So it really depends on what the weather is doing and how busy the Class B is at the time of your transition.

For the approach part, I'd leave that out as conditions can change and your approach will be determined by the weather at the landing end and ATC will assign what to use as you get closer, or you can request a Visual Approach if the weather is nice and you don't need the approach. One thing you can do is put "No SID, No STAR" in your flight plan if you don't have them on hand.


What are the benefits of using Krueger Flaps? I mean why using Krueger flaps in some aircraft instead of using slats all over the leading edge?

There are different types of Krueger flaps used, with increased efficiency coming at the cost of increased complexity.

The simple ones are basically flat panels that folds out from the wing lower surface into a very steep, deployed position and seals against the wing leading edge; though these are extremely simple, they don't offer much in the way of stall protection.
The next type has large folding 'bull' nose that offers increased performance at the cost of slightly increased complexity. This type of flaps is used in B727, B737, and inboard of B747, as shown in the figure below:

By Arpingstone - Photographed by Adrian Pingstone in August 2002 and released to the public domain., Public Domain, https://commons.wikimedia.org/w/index.php?curid=118310

The most complex type of Krueger flaps are the variable camber type, which has a flexible panel (which forms a slot at the trailing edge) and a bull nose. This provides a better $C_{L_{max}}$ performance compared to a slat, but gives poorer takeoff L/D performance compared to the slat. However, the main problem is that the mechanism is exceptionally complex, as can be seen in the figure below. As a result not many aircraft use these.

Image from aviation.stackexchange

This means that simpler Krueger flaps are used in the inboard with slats in the outboard region, so that the flow remains attached in the outboard regions at higher angles of attack compared to the inboard region.


On a centrifugal compressor impeller what is the difference between the radial vanes and rotating guide vanes? I thought the radial vanes were a machined part of the impeller that just force the airstream radially? And that another name was rotating guide vanes as they are rotating with the impeller disc.. But the picture from a textbook I have is referring to them as 2 different things? Or have i misunderstood something?

Thanks

The radial vanes are the main compression channels, where air gets squeezed into 5 - 10 bar and gets slung out of the tip traveling at 500 m/s.

The rotating guide vanes scoop the incoming air into the impeller vanes, kind of like a pre-whirl tower. They’re at the inside diameter and travel at much lower linear speed, subject to lower centrifugal force.

This picture was used in an earlier answer, and illustrates the different roles of the channels and vanes. As the pressure ratio on the horizontal axis increases, we can observe the following:

Efficiency decreases: losses are higher for higher single stage pressure ratios, which is the main reason that commercial aircraft turbine engines use axial compressors.
Impeller height increases. The curved shape allows for a more gradual change of direction of the airflow: inflow is in axial direction, outflow in radial direction. A large potential source for efficiency loss.
Inside vane shape points more and more in radial direction: it scoops the airflow into the impeller, and pre-whirls into the radial direction that will be imposed by the impeller.

The right-most impeller combines an axial stage with a centrifugal stage. In an axial compressor the rotor is followed by a stator, in our axial-centrifugal combination there must be no stator because we want whirling flow now.

With advances in production technology, this has resulted in impeller shapes with double curved inlet vanes, such as above. The inner bits of the impeller are subject to much lower stresses and may be shaped much more funkily than the impeller outer diameter.


How does the slotted flap work? Slotted flap is an improved version of the plain flap, generating more lift (a fact I don't challenge). However the principle is air passing between the wing and the flap.

Source

This seems counterproductive as this airstream could also prevent the upper stream to follow the airfoils shape, and lead to air being less rotated downward than with a plain flap.

How does the slotted flap happen to be superior?

The slot helps to prevent early separation, so the flap stays effective for larger deflection angles. What that means in reality can be read in this old NACA report which compares two NACA23012 sections, one with a plain flap and the other with a slotted flap. I have copied the pressure distributions below from that PDF:

Now I need to explain a little how to read the pressure distribution plots. The solid line shows the local pressure on the upper surface while the dashed line shows the pressure on the lower surface of the airfoil. The X-axis runs along the chord. The Y-axis represents pressure (suction is up) and y=0 represents ambient pressure. The area between the dashed and the solid lines is proportional to the lift coefficient, which is helpfully shown below each plot.

Now to the curvature: As long as the lines show a gradient, flow is attached. Separated flow can be recognized by a horizontal line slightly above the X-axis. As you can see, we have already fully separated flow on the upper plain flap surface at 30° flap angle and 8° angle of attack. At the same angles the slotted flap still shows fully attached flow on both surfaces. Attached flow creates a suction peak at the nose of the flap, which in turn allows a higher suction value at the end of the fixed part of the airfoil. This in turn allows a much higher suction peak at the nose of the airfoil, so that the suction force over the whole chord of the airfoil with the slotted flap is much higher than on the airfoil with the plain flap.

Only at 50° flap deflection will the slotted flap also show flow separation over the rear part of the flap, but there is still a suction peak which helps to stabilize the flow over the fixed part and increases lift. Unfortunately, the plain flap was not measured at the same angle, but from the small change between the 30° and 45° flap angles you can see already that moving the flap to 50° will change little.

The reason for this difference is simple: While the plain flap has only the old, thick and tired boundary layer coming from the fixed part of the airfoil to work with, the slot allows the slotted flap to create a new, thin and vigorous boundary layer which can tolerate much higher pressure gradients. Another benefit is the unchanging direction of the local flow behind the fixed airfoil: This allows to use a thin, highly cambered flap airfoil which maximizes lift production. The old NACA airfoil did not take advantage of this, but modern airliner flaps certainly do.


Autorotation: Positive Pitch during descent phase possible? I was wondering if a rotorcraft, that descends very slowly compared to its nominal rotor rotation rate (for example an RC Helicopter vertically autorotating with 3m/s descent rate and 2200RPM on the rotor) can maintain the rotor rotation rate even though it is having a positive collective pitch.
Is this possible? If so can it be sustained or is it just for short time frames?
The following image might enhance the understanding (Source)
look at the speeds and look at the lift/drag curve, if the lift/drag ratio is big enough this should work, right?

During autorotation there are two main angles which are important, the inflow angle created by the rate of descent flow and rotational airflow plus the pitch angle (normally minimal on initial entry to autorotation). It is these angles which produces the lift. The second angle produces the auto rotative force, or the force which maintains the rotor blades turning in auto. This is created by the forward tilt of the main lift vector (or the horizontal component of the lift vector) which is shown on your diagram. During autorotation it is important to maintain control over rotor rpm. Density altitude has a marked effect in auto, which is your consideration albeit to a greater extent. As density altitude increases rotor rpm for the same pitch angle will increase, this can’t be allowed to continue to increase and is contained by increasing blade pitch, hence the greater the DA, the more pitch is required including positive pitch if required. Another consideration which may help is, autorotation appears to be an aerodynamic effect of rotorcraft that is considered in isolation when a helicopter loses power, rather like an on/off switch. This isn’t the case, helicopters will experience a degree of autorotation in a normal descent due to the rate of descent flow but the rotor rpm will be maintained within normal limits due to the rotor governing system. It is a progressive process from the start of a descent to fully established auto when the torque to the rotors becomes zero.


What is the difference between MCP and METO? I was reading the Cessna 150H POH earlier today and noticed that the manual mentions something like "Normal cruising is done at 65% to 75% of METO power."

The Cessna 172S POH charts refer to cruising power as "75% MCP."

Given that METO stands for "Maximum Except for Take Off" and MCP stands for "Maximum Cruising Power", are the two terms the same thing or do they have differences?

They're the same indeed. Power settings that are not time limited and can be used continuously. The answer is in the model, Cessna 150 vs. 172.

METO is an outdated term, like reading artificial horizon vs. attitude indicator. You'll also find METO in DC-3 documentation, but not MCP.


Are there experimental aircraft regulations in Germany? The title is the question.

Are there any special regulations regarding experimental aircraft in Germany (or the EU)?

Those "special regulations" would (hopefully) be looser set of regulations in contrast to the LTF-UL regulations for ultralight planes (MTOW < 600kg).

Yes, there are. Everything is regulated in Germany.

Your regular experimental aircraft would have to follow JAR 23 and the LBA has transferred the supervision and certification of experimental aircraft to the Oskar Ursinus Vereinigung (OUV). Before you begin work on your experimental, be sure to contact them and talk to an OUV assessor. This assessor will follow the progress of your work and will regularly check if all work is done properly. You need to prove that you have a heated workshop and document your work sufficiently (like keep samples of every resin mixture that was used for building composite parts). This assessor will also help with contact to experts if you got stuck or with the insane amounts of paperwork, which - this being Germany - often seem baffling and pointless.

You do not need a full certification but at least a load test to 100% design load. The aircraft will get a vorläufige Verkehrszulassung (VVZ; provisional airworthiness certificate) which needs to be renewed regularly. Also, operation of that experimental will be restricted to overflight of sparsely populated areas. But others have succeeded before, so good luck!


How has weight been reduced in commercial aircraft design and how will this trend continue? Reducing weight of aircraft has become a priority as the aviation industry seeks greater fuel efficiency and commits to strict emissions targets. How much weight has been cut from single and twin aisle aircraft in last 10 yrs and where? How much further can OEM's and suppliers cut weight in the next 10 years and in what components/equipment/furnishings will the greatest savings be made? I am looking for some fun and relevant facts on this subject for an article I am writing.

Perhaps the biggest weight reduction in aircraft across the board has been the use of composite materials over aluminum. This extends from small GA aircraft all the way up through airliners like Boeing Dreamliner. In the retro fit game, some older planes have been certified to take newer composite propellers. Even small savings matter according to this article

  LED modules give on average a 40% weight saving over the incandescent
  modules they are replacing, which improves fuel economy

Glass cockpits and modern computerized avionics tend to weigh less and offer more than their earlier steam gauge counter parts. Even in the space shuttle.

  The system also provides greater backup capability, weighs less and
  uses less power than the original design.

Another thing that has made a fair dent in take off weight is how fuel efficient engines have gotten. While there is a practical limit to this as engines have gotten more efficient the need to carry fuel has dropped or planes can go farther on the same fuel. In a similar vein as weather planning/predicting has gotten better fuel loads (while still in compliance with FAA reserve regulations) may be slightly lighter as they can now be more precisely calculated.

The one thing that has gotten heaver is the passengers...


Can a commercial pilot fly passengers for hire without an air carrier certificate? I have my private multi instrument and almost done with my commercial multi. A collegiate school's football team has requested that I fly the coaches (2-4) to their away games and other travel events throughout the year. Once I receive my certificate, am I legally permitted to fly them for hire without needing an operator's certificate? Maybe through a contract agreement?

If you are referring to the USA with the regulating body being the FAA, this link will give you your answer. All text below came from this site:
http://www.cfidarren.com/cpinfo.htm

Here is the answer I think you are looking for:

  If your customer approaches you, with his own airplane (either he rented it himself, or he owns it).  In this case you may fly for hire, because you are not offering your services as a package or "holding out" as a commercial operator -- air taxi or charter.

Answer details:

  Commercial Pilot Certificate

  A certificated Commercial Pilot is paid for services as a pilot, provided you meet the qualifications which apply to the type of operations you are doing.  Some of this flying is  controlled by Part 91, but most commercial flying is controlled by Part 119, 121, 125, 129, and 135.

  Do I need a new medical certificate?   To exercise your commercial pilot privileges, you must get a new medical (at least 2nd class) every year.  If your 2nd class medical expires (and becomes a 3rd class), you cannot exercise the privileges of a commercial pilot.

  What can I do with a commercial certificate?    The following are things commercial pilots can do under part 91 -

  Local scenic flights within 25sm of departure airport.
  Ferry Flights
  Crop Dusting
  Banner Towing
  Aerial Photography
  Fire fighting
  Go on to earn your flight instructor certificate

  Can I fly people around for money?  You can certainly be paid for being a pilot with a Commercial Pilot Certificate.  You may work for an Air Carrier certificated under Parts 119, 135 or 121.  You may not be the Air Carrier conducting the business without getting an Air Carrier Certificate.  When you offer a package (airplane, price, pilot), you are no longer operating under Part 91 and would need an Air Carrier Certificate. An example would be "Can you fly me to Atlanta?"   If you provide the aircraft, you need to have an Air Taxi Commercial Operator certificate under Part 135.  There are different rules regarding maintenance, airworthiness requirements, crew member qualifications, and expanded VFR and IFR equipment lists that make Part 135 very different from Part 91.

  So the answer is NO, you cannot fly people around for money.  A Commercial Pilot Certificate simply allows a pilot to be PIC engaged in a commercial operation (airline) that has an Air Carrier Certificate.  A pilot certificate does not entitle the holder to become a commercial air-taxi operator.

  If your customer approaches you, with his own airplane (either he rented it himself, or he owns it).  In this case you may fly for hire, because you are not offering your services as a package or "holding out" as a commercial operator -- air taxi or charter.


Freezing cold cabin housing just under a window I flew on a Boeing 737 plane yesterday and was sitting next to 2 windows. When I tried to rest my arm on the inner housing of the cabin during the flight, I noticed that the inner cabin housing just below one of the two windows was very cold. It was almost like touching a surface in your freezer. I was under the impression that it is not a common thing and that it could only result from outside air being in contact with the inner cabin housing which I believe is not supposed to happen.

I spoke to a flight attendant about it and he came check the window, but I feel like his feeling all in all was that I was worried for nothing, and I’m not sure he actually shared the information with anyone - and he did not thank me in anyway for reporting a potential issue.

I’m curious of anyone views on this issue. My uneducated guess was that it was maybe not a critical issue now, but a defect that should be known by maintenance teams as it could be tied to a structural issue of the outside window.

It's not a structural problem, and nothing to worry about. The cabin of an airliner is a pressure vessel, the pressure outside of the airplane is far lower,  if there was a leak air would be going out, not in. The air outside is very cold, which makes the skin of the airplane cold, there's insulation between the skin and the cabin fixtures but if that comes off a bit you get cold spots.

By all means tell the cabin crew so they can log it for the next time the airplane's in for maintenance, it's important for passenger comfort.


Are tail strike landings preferable for an emergency landing on water? In this video, would it have been better if the pilot had attempted a tail-strike landing instead of a normal flare?

While there probably is an "ideal" attitude to impact water, and it would be nice to crash-land with exactly the correct attitude... it's very much a secondary consideration.

By far the most important thing, when landing on water, is to land slowly. As such, your aim should be to land (/crash) with whatever attitude gives the lowest possible ground speed/descent rate at the point of impact. If that means a "normal" flare, do a normal flare. If that means a harsher flare with a tail strike, do that instead.

The point is that you're in an emergency situation, and your biggest priority is to get the speed right: you likely don't have the luxury of trying to fine-tune your angle of impact and flare. Additionally, you always have the risk of over-flaring and stalling into the water instead, which is a worse end result.

So yes, it may - with some airframes - be beneficial to have a tail strike, but you're far better off just keeping the aircraft under the best control possible and reducing your speed as far as possible


Is the A330F main deck cargo compartment pressurized? If not, what is the purpose of the structural cargo access door? I know there is a related question already asking if cargo holds in general are pressurized.

My question is specifically about the A330-243F in the below photograph. That looks like a pressure withstanding door/bulkhead dividing the fore crew compartment from the aft cargo compartment. Is that correct, and does it mean the aft cargo compartment is not pressurized? It's also possible both are pressurized and it's an airtight door for fire/fumes control.

Source

As mentioned in the linked question, it's likely both compartments are pressurized because of the efficient shape of the pressure vessel and so that the same pressurization system can be used across both passenger and cargo variants.

You're correct- for cargo aircraft derived from passenger planes, the cabin is pressurized. This reduces the complexity and certification requirements. Also, this allows for easy conversion too. The thick door may be due to two reasons:

To improve load carrying/transferring capacity- airbus talks about

  ... the aircraft’s reinforced fuselage and doors, which increase shear and bending/running loads..

also, the thick door will help protecting the crew should the cargo move forward in flight.
To act like an airtight seal between the cabin and cockpit, so that it prevents smoke ingress into cockpit and to enable the crew to extinguish fire. FAA Class E (for cargo aircraft) requires isoaltion between the cargo compartment and cockpit.

Another thing to note is that even a number of military 'cargo' aircraft have pressurized cabins (An-124, I think is among  exceptions), as they are also used in Medivac and causality evacuation roles- these require the cabin pressure to be set higher than normal.


Can anyone identify this old airplane? Can anyone identify this old airplane, and/or what era it's likely to be from? The photo is from northern Italy, and it's the only photo I have. I'd be very grateful for the help!

Reasons that it would be just a pretend airplane, perhaps the body of a wheeled vehicle:

big awkward holes on the cowl sides for headlights
paint scheme is straight lines only, inexpensive
unusual tiny duct in the top of the cowl

But it does have an antenna poking out of the roof. Molto misterioso.
If one rotated the panel below the side window half a turn about the vehicle's pitch axis, its stripes would align with the cowl's.  So it is indeed the door, not something else.
Through the gap behind the door, one sees what might be grass.  The vehicle may not have a floor back there.
Era:

cowl top's deterioration suggests fiberglass, so after 1950
paint scheme is 1960's


Is the "auto-routing" created by online flight planning resources preferred by ATC over Direct routing when NO preferred route exists? I noticed that when I use online flight planning resources, they provide an "auto-routing" option for my flight. I also noticed that they are the same between both services so it seems like both are pulling the route from somewhere. Additionally, the auto-route appears to be different than a "preferred route" because, for example fltplan.com will say "No Preferred Routes or Stored Routes" then it gives me (a sometimes crazy) auto-route.

Are auto-routes predetermined or is it just an algorithm (i.e. nearest intersection/victor airways to destination)?

Extra Credit Question: If there is no preferred route for a given destination, is filing Direct or using the auto-route more likely to get you a "cleared as filed?"

I did some follow up research and also spoke to an air traffic controller and I have a partial answer to this question.

If the "auto-routing" doesn't make much "sense" and there are no "preferred routes" then the probability of "getting a direct clearance would be fairly high."
The "auto-routing" logic is simply using nearest victor airways from Point A to Point B. (I'm still looking for a source for this, but it seems pretty obvious to me now).


Why does the landing gear control have 3 positions (up/down/off) on Boeing 737-800? I've noticed that Boeing 737-800's cockpit has a 3-position landing gear lever. During takeoff, the pilot moves the lever from DOWN to UP and then to OFF position. What's the point of the OFF position? Why not simply have UP/DOWN positions?

With the gear handle in the UP position, the retraction side of the landing gear actuators remain pressurized. The gear will stay in the full up position, however they will not be hanging on the mechanical locks designed to hold them in the up position. They've simply hit the full up mechanical stop.

When the handle is placed in the OFF position, hydraulic pressure is removed from the retract side of the gear actuators, allowing the gear to settle down and onto the mechanical uplocks, which are essentially hooks that just hold the gear up.

There are a couple reasons why it's beneficial to select OFF. One is that if they remained pressurized, then the gear are essentially hanging in space, at a mechanical stop provided by the actuator. Hydraulic actuators are delicate components, and these weren't designed to hold landing gear for an extended time in the retracted position. Imagine the loads imposed on the actuator in turbulence for example, as the gear are loaded vertically with each bump. Removing hydraulic pressure allows the gear to hang on the mechanical uplocks that were specifically designed to hold them, without putting additional wear on the actuators.

The other reason is that gear actuators are major users of hydraulic fluid volume, and the actuators and the lines that supply them are a potential for leakage. If they were to remain pressurized for a several hours long flight, that increases the amount of pressurized lines and components that can leak and cause a complete loss of the hydraulic system.

Note that if you look at more recent Boeing airplanes such as the 777, 787, and 747-8, you will not see an OFF position on the gear handle, only DN and UP. The OFF function is done automatically on these models. After you select UP and the gear are all fully retracted, there's a short time delay and the system automatically depressurizes the retract side of the actuators. It removes one After Takeoff checklist item for the pilots to do.

To answer some of the questions posed in the comments:
If you selected OFF while the gear were still in transit in some intermediate position between up and down, they would simply freefall into the down position since you've removed the retraction power.
If there's a leak in the landing gear actuator or its associated hydraulic lines, it would cause a loss of that hydraulic system, and all other users of that system would be failed. The 737 has two (and a half) hydraulic systems. The half is a standby hydraulic system that's very limited in its function, but I digress.


How to determine the compressive force in the spars of a strutted wing? How does one calculate the compressive force that the inner section of a strutted or braced wing (from the strut attach point to the wing root) experiences?

I know how to calculate the tension in the struts (or flying wires), which is simply

$$ \mathrm{Tension} = \cfrac{\mathrm{Lift}}{\sin \theta} $$

but I don't know to calculate the compressive force that a strutted or braced wing experiences.

It's very easy. Consider that the point at which the strut is attached does not accelerate, i.e., the forces in all directions exactly even out.

The force in the spanwise direction exerted by the strut is

$$F_{strut}^{spanwise}=\cos\theta\cdot F_{strut}$$

This force must be balanced by the wing in compression, so the above expression is also valid for the amount of compression on the wing.

This assumes the strut only transfers longitudinal forces, while in reality it may have some bending stiffness. Also, I'm not sure where you got your equation for the tension in the strut, but it's not necessarily true (there's a lot more structural members exerting a vertical force on the wing, so the force- and moment balance is not as simple).


What is this delta wing aircraft parked at NASA Ames Research Center at Moffett Field? What is the aircraft shown at these coordinates? https://goo.gl/maps/VTc153EacEL2

This is at NASA Ames Research Center at Moffett Field. Per the Ames map it is located north behind building N-127 (Warehouse) and west of the VTOL pad.

It has a dome canopy and a delta wing configuration. It is approximately the size of a fighter aircraft. A very blurry picture of the tail is visible by using Google Streetview from the Moffett golf course and looking west across runway 41R.

It is a STOVL E-7 Model.. It is not an actual aircraft.
https://artsandculture.google.com/asset/e-7-stovl-fighter-model-testing-in-ames-40x80ft-wind-tunnel-tom-trower/RQEHhSh_zF3TuQ?ms=%7B%22x%22%3A0.5%2C%22y%22%3A0.5%2C%22z%22%3A9.425586796493714%2C%22size%22%3A%7B%22width%22%3A1.2166568742655705%2C%22height%22%3A1.2375000000000007%7D%7D

The enlarged photo shows the inlet duct covers. It looks like they left the lifting cables just laying on top. This is a model that never flew.


Are jet aircraft still at risk of hijacking by people wielding melee weapons? Are jet aircraft still at risk of hijacking by people wielding melee weapons (as opposed to explosives)?

I thought that aircraft were designed post-9/11 such that passengers can't enter the cockpit, but Wikipedia's list of hijackings mentions incidents this decade which involved melee weapons:

  China June 29, 2012: an attempt was made to hijack Tianjin Airlines
  Flight GS7554 from Hotan to Ürümqi. Six people tried to hijack the
  aircraft 10 minutes after take-off. There were 6 police officers on
  board. Four were in plain clothes, taking the plane for a business
  trip. The hijackers used aluminium canes with sharpened tips to attack
  the members of the crew. The police officers and civilians on board
  subdued the hijackers, all of whom were of Uyghur ethnicity. The plot
  was foiled and the plane returned to Hotan in 22 minutes after
  takeoff.

United Airlines Flight 93 history shows that melee weapons are not very efficient: the passengers that are likely to outnumber hijackers at least ten to one can win if they are willing to fight.

The question remains, how likely is that passengers will go into risks to show the resistance. If it is just about changing the course to land in the different airport, they may choose to be quiet. If they have nothing to lose, I think the hijackers have no chances.


Which models are these aircraft? Can somebody please identify the second aircraft. I tried reverse search but no use :C.
I'm trying to win a boeing 777 model :D

DC-9 (probably a DC-9-10 but not 100% sure), BAC-111, and Sud Aviation Caravelle, from top to bottom.


Why is it rare for small aircraft to have winglets? I've noticed that winglets are very rare on small aircraft. I wonder why this is the case. Wouldn't they have the same advantages, especially because they travel at low speed? Or is it just a wrong impression I got and they aren't that rare?

I've talked to a couple of aerodynamicists, and for small aircraft when you run the numbers the improvement offered by winglets is often less than the drag penalty caused by their weight (which also reduces allowable payload). For larger aircraft that fly long distances, the proportion of the aircraft's weight that is due to the winglets is much smaller than it is for small aircraft, so they don't have this problem and that makes winglets a viable strategy.


How are Airbus pilots trained for using the mechanical backup control systems? The A320 and friends have mechanical-only control capability (manual pitch trim + rudder pedals) for cases of total fly-by-wire computer or electrical failure.  Obviously, this would be covered in ground school when the various control laws are gone through.

However, is any further training given to line pilots on operating the aircraft this way? Or is it one of those "dusty corner" procedures that you learn about in ground school, but never really touch again?

Alternately, what would the chances be of someone who'd never flown a 'Bus in mechanical/backup law being able to keep the plane dirty-side-down and airborne long enough to get the FBW back going and/or perform a landing where the aircraft is reusable?  Clearly, it's non-nil as the Airbus test pilots actually tested flying the plane that way, but how difficult is the aircraft to fly in mechanical/backup law?

According to AIRBUS but more specifically Airlines, have training syllabus.
Airbus Company states, clearly how, and how often to train Mechanical Backup Flying.

Each airline, which operates airbus A/Cs has to adhere with these recommendations and/or dictations.

Several failures of systems, or combination of failures can lead to the mechanical backup "state".

Every six months airline pilots are going to the Simulator for checking and training (LPC/OPC/SIM REC).

  Of course there is training regarding mechanical backup.

How often?

It depends each time on the scenario, the seasonal systems to be trained, the systems failures, and many other factors.

Mechanical backup is a possible scenario by the Airbus, and well defined (but as always not exhaustive) in books, how can be happened. For this reason IMO all airliners provide this training to their pilots, and of course all the time (every six months) there is the possibility to fly a scenario leading to mechanical backup.

  Generally the aircraft can fly and land with mechanical backup as long
  as the situation permits, the failures, the pilot skills, and plenty
  of other factors.

How difficult is?

1) Pilot Skills

2) Environment (Weather)

3) e.t.c (The list is non exhaustive at all.)

  Aircraft manufacturers and airlines want to give proper and sufficient training to their pilots, and professional pilots want to
  receive proper and adequate training, and fly remote and difficult
  scenarios in the simulators, in order to be ready for "anything" that would be happened.
  So to summarise, and once again to answer your question, in general
  terms, YES airline pilots get training regularly, on the Mechanical Backup
  Scenario to FLY and to LAND the A/C  and the difficulty is
  something generic, for some people would be less difficult than some
  other.


Does a headwind affect the climb gradient? When studying for my German PPL exam, I came across this question:

For those that do not speak German I'll try to translate the question, but bear with me as it is kinda picky what I'm trying to get across here:

  During flight on a straight track with constant speed

  headwind will increase the gradient of climb
  headwind will decrease the gradient of climb
  headwind will increase the distance needed to descent 50m
  headwind does not affect the gradient of climb at all

As you can see answer 4 is supposed to be the correct one. I disagree, so I started searching for a reason, yet I couldn't find anything useful. Let me explain my train of thoughts:

I didn't really know the "gradient of climb", I always used the two terms "angle of climb" and "rate of climb". Thus I'm trying to figure out which one the gradient of climb is. Common sense makes me think it is the same as the angle of climb (gradient does just sound like an angle). This opinion is supported by SIDs, which have a PDG (Procedure Design Gradient) that is given in percent, just like the gradient of climb.

Assuming the gradient of climb is indeed the same as the angle of climb, the only way for the "correct" answer to be confirmed is by taking the distance traveled during the climb relative to the air, not to the ground (angle of climb is defined as the height gained divided by the horizontal distance traveled in a certain amount of time). This would mean that it is used with IAS, rather than GS, which in my opinion absolutely makes no sense ("Hey look, that mountain is 10NM away, we are climbing with 600 ft/min. and going 100 KIAS and need 3500ft more to go over the top, we're all good!" - Well, add a tailwind of 25kts, the pilot would still think he's good to go but it'd result in CFIT...).

The question I have is: How is answer 4 the correct one?

The gradient of climb is the ratio of the increase of altitude to horizontal distance through the air, not over the ground.
The definition used by the UK CAA in CAP 698 is:

  Climb Gradient

  The ratio, in the same units of measurement, expressed
  as a percentage, as obtained from the formula:
  - $$\text{Gradient} = \frac{\text{Change in Height}}{\text{Horizontal Distance}} \times 100 \% $$

If you have a headwind or a tailwind it makes no difference to your climb gradient because your airplane is moving relative to the air mass.

Climb gradient is not the same as rate of climb, although they are related. Rate of climb is altitude over a period of time, gradient is climb over distance travelled. There is another question which covers the difference in more detail.


Does aircraft nacelle length/shape/geometry affect the contrail formation? This is coming from Question 1 and further down Question 2.
I think the nacelle and exhaust cone do affect the contrail formation in some way, what is the relationship between them? Do the famous 787 Chevrons also affect the contrails? And How does it  compare with engines with exhaust mixer?

Ultimately the engine is dumping water into the atmosphere and this will quickly cool to ambient temperature.  Depending on the relative humidity at altitude it may or may not condense.  These factors are irrespective of the engine design provided that combustion is more or less complete, so the only factor that will significantly affect contrail formation is the extent to which the exhaust gases are distributed.  This will certainly be affected by the design but is hard to predict in general how; it’s mostly down to fluid dynamics well downstream of the engine itself.


What if an airspeed prescribed by a published procedure isn't safe for a particular aircraft's operations? Departure and arrival procedures often prescribe airspeeds which must be maintained at certain fixes. 14 CFR 91.117 sets rules for airspeeds but has a paragraph which states that the airspeeds don't apply if the aircraft can't operate safely within those limits. Does any regulation or published recommendation alleviate aircraft from the airspeeds prescribed on instrument procedures for similar reasons?

If you are unable to comply with a published procedure, you simply inform ATC and the controller will tell you what to do instead. The controller will most likely be authorized to make this decision, even if there is no written regulation covering the specific case.