When to climb and when to descend when faced with icing? (YouTube)

The flight faced icing at FL 90 inbound to Germany from the UK on a world tour. Earlier they were flying above an overcast layer.

After they saw the icing they requested FL 70, and asked what's the minimum altitude, which was 5000 feet. How do GA pilots decide that descending is better than climbing, as in climbing away from the moisture layer?

The plane is a Cessna T210L Turbo Centurion that should be capable of 27,000 feet.

While there may be guidelines as to whether to climb or descend when you're icing at your current level, individual pilots that I have known varied widely in the specifics of what they would do depending on their experience, their comfort level in icing, the route they were flying, and their experience with that route in bad weather. My take follows.

First, @acpilot's comment gives the general guidelines and is worthy of being an answer, I believe, except that I would be happy with just a couple of degrees above freezing rather than 5°C.

I put in a few hundred hours in Turbo 210s back in the 1970s. The ones I flew had boots on the wings and hot props. As such they were a reasonable machine to handle the icing of the U.S. Pacific Northwest. The things I wanted to know to decide whether to climb or descend included:

what's the rate of the icing
what kind of ice is it, rime ice (the most prevalent) or clear ice (basically freezing rain)
what kind of cloud am I in, is there convective activity
how high can I expect the icing to persist, are there pireps giving the tops
how much distance will I have to fly to either be in the clear, between layers, or get to where the minimum safe altitude is above freezing
what's my climb capability, in other words how heavy am I and how much has my climb capability already been degraded

Even with knowing those things, it's still as much a subjective decision as an objective decision, at least it was for me.

The distinction between rime ice and clear ice is particularly important. Clear ice (basically freezing rain) is the more dangerous, accumulating faster than rime typically and heavier. It always (almost) means there's air greater than 0°C above you, which would favor climbing.

Rime ice is minute drops of super-cooled water (water less than 0°C) that changes to ice when it hits the aircraft. More often than not, rime ice means there's no air above greater than 0°C, unless you happen to be in an inversion. Climbing out of rime icing usually means you've got to get high enough to get clear of clouds or get the temp down to -15°C or so.

Fortunately most icing is rime ice rather than clear ice. The rime usually collects slowly, although it can collect quickly in convective conditions. I once iced up with rime ice in a Cessna 340 in less than a minute to the point that I was having trouble maintaining altitude.

(skyvector)

Much of my T210 flying in bad weather was between Eugene, Oregon and airports in California's central valley (shown above left to right). V23 was the airway from the Eugene VOR to the Medford VOR to the Fort Jones VOR to the Red Bluff VOR and then south into the central valley. The airports were typically above freezing, but the MEAs were in the ice if the weather was bad, and you were guaranteed to get at least some rime ice. The MEA from Medford to Red Bluff was 10,000, and that was where the worst ice was usually encountered, but the MSA dropped rapidly south of Fort Jones.

If southbound from Eugene and approaching Medford you had already picked up more ice than you liked, and the Medford airport was above freezing, you could shoot an approach there and drop the ice before proceeding south of the Siskiyou Mountains. Once the OAT gets above freezing, the ice melts really fast, typically on the order of a minute or less. The windshield clears first. Importantly, for rime ice, the rough surface of the ice smooths quickly, permitting better airflow.

If it appeared icing along V23 was not going to be flyable, we would file over to the coast and go down over the beach so to speak with the attendant low MEAs and more airports, all usually well above freezing.

My point is, a lot of things enter into whether you should climb, descend, go another route, or get on a jet.

A couple of good (IMHO) links (pdf's) to icing information are:

Aircraft icing, noaa.gov
Aircraft icing, aopa.org


How much freight can a Boeing 737-800 NG carry if the majority of seats are removed? In the context of the COVID-19 crisis, I read reports of underused Boeing 737-800 NG aircraft being converted (by technicians in airports, not by factories) to cargo planes for medical equipment. How much freight (in tons) can such an airplane transport? How much weight can the place below each seat withstand? The flight distance would be about 5000 km. It appears that one such airliner carried only 9 tons also, as can be seen in the images, it was loaded close to its maximum capacity. I guess the freight (200000 masks FFP2) was light.

Boeing 737-800 NG

VIDEO with such a Boeing 737-800 NG full of medical equipment

Press release:

  9 (nine) tons of medical equipment brought from China by a Tarom aircraft - Bucharest, April 3, 2020. In the middle of the crisis generated by the coronavirus pandemic, a Boeing 737-800 NG aircraft has been reconfigured to import from China 9 tons of medical materials. The plane took off from Bucharest Otopeni airport on April 2 and through Almaty (Kazakhstan) reached Shenzhen (China) and then returned on the evening of April 3. Two Boeing 737-800 NG passenger planes have been reconfigured.

At 160 seats, a -800 carries 16 tons of people in the cabin, plus freight/luggage in the belly, plus fuel. At 180 seats, that becomes 18 tons of pax. If you assume one 50# suitcase per person, add a quarter of that again, so 20 to 22.5 tons.  Much more than that you're probably starting to run up against the Max Zero Fuel Weight limit, although I don't have the right charts in front of me to give an exact answer there.

If the flight is long enough, max takeoff weight becomes an issue, as your fuel load starts to cut into your lift. Obviously lots of factors go into how much fuel a flight requires; "today's" winds & temperature & destination weather (alternate required? How far away?) will change how much weight today's flight can take.

Ballparking it with back-of-the-envelope math, anything up to about 3-4 hours, you can probably get those weights I mentioned above (i.e. 22 tons or so). Much more than that, I'd expect max takeoff weight to limit you, so more distance = less payload.

If the legs were 5000km between refueling stops, then you're well into the limit being max takeoff weight, rather than what the jet could carry on a short-haul.

Depending on the nature of the cargo, the limits on floor loading in the cabin + the cargo compartments may be limiting, or volume may be limiting. If you're hauling pingpong balls, it's all about volume. If hauling gold, floor weight capacity & total weight will be more limiting. If it's all rucksacks, then you can probably leave the seats in & load around them, and only total weight will play.

Disclaimer, doing this with no references in front of me, and I may have a number off in there. When there is a specific need to haul "this" from A to B, the guys with the charts & load plans come up with a far more specific answer. The above is simply a rough ballpark.


Why is the pitot tube located near the nose? The pitot tube located near the nose could be affected by the boundary layer and could increase the error. Then why is the pitot tube located near the nose?

There are several reasons to have the pressure sensors in the front section of the fuselage.

The further aft the more turbulent the airflow is and the thicker the boundary layer caused by the fuselage. This would cause strong fluctuations of the measured pressure and the reading could be inaccurate. Far forward the boundary layer is the thinnest because the boundary layer thickness grows with the distance traveled along the surface.
The position on the nose should be less sensitive to angle of attack and side slip angle changes. Having the sensors on the side of the fuselage would cause significant changes in pressure measurements depending on the aircraft attitude relative to the incoming air.
The pressure sensor in the nose is close to the cockpit instrumentation and avionics computers. Pressure tubes can be short and cannot be disconnected by mechanical forces that easily.
There is no ground equipment moving close to them. Having them further aft could easily damage the sensors during baggage or passenger loading/unloading.
During winter months airplanes have to be sprayed with de-icing fluids. Up front they are not flooded by these substances and are spaced far away from the affected areas.

Some aircraft have the sensors located on the wings. The Cessna 172 pitot tube is one example that comes to mind.


Can anyone identify what/where this photo is of? This is a photo by the Real Photograph Co. (Liverpool) No.684.
I think it may be a crashed WW1 Biplane (lots of the photos with it were WW1 biplanes).

This could be a Sopwith Camel without its guns.  At least these details don't disagree:

cowling's shape and front opening
9-cylinder radial engine (tappett rods lost when crushed by the cowling)
8-bolt prop hub
wicker seat's weave spacing (many photos if you search "sopwith camel seat")
fuel tank's position and shape match that in the Haynes Manual
location of the remnants of the cabane struts.

Although some of these details are common to many aircraft of the period, it's noteworthy that this many details agree in one place.  A statistical argument may be the best one can do with what's been given.


What are these markings on the runway at Haneda Airport? I haven't seen these markings in other airports in the world. Look:

They also appear in other Japanese airports. What do they mean?

I dug around for a while but couldn't find any official documentation on this (I suspect someone whose Japanese is better than mine will have to do that.)

But!  I did find the markings on all of the Haneda Airport runways and I noticed they were roughly halfway along the length of the entire physical runway.  A little bit of measuring confirmed my suspicion, it appears the marks show you exactly where the halfway point is on that particular runway.

I also looked at other Japanese airports and took measurements of the same markings at Narita, Osaka and Kobe.  All airports turned up the same results.  (You can use the measure tool on Google Maps to confirm if you'd like to)

I also find out that not all airports in Japan have these markings.  Neither Nagasaki nor Osaka seem to have markings like the ones mentioned above...

Not sure why they mark the halfway point...someone with better Japanese than me will have to look into the history of that decision.

Edit:  mins found some documentation and posted it in the comments (thanks mins!):

This is an English translation of a report done by the Japan Transport Safety Board regarding a runway overrun.  They mention the meaning of the markings (in passing) on page 2.

Here's a capture mins created of the relevant part of the report:

So, there we go, more than just circumstantial evidence we have the aviation authority of Japan saying it's true.  It's the halfway mark for the runway.


Do negative flaps increase glide ratio? If I understand correctly, gliders uses negative flaps while crossing between 2 thermals, but I don't understand why. For me:

positive flaps decrease sink rate but increase drag and thus worsen glide ratio
best L/G ratio is reached when flaps are in neutral position.
negative flaps decrease lift and thus require a greater angle of attack, maybe increasing drag.

The flaps on a glider wing are mainly for shifting the laminar bucket of the wing to the right lift coefficient. Yes, they help to increase maximum lift, too, but for gliding this is less relevant.

The goal in glider flying is to get in the shortest time from A to B. This can be achieved by flying faster than the best glide speed, because it gives the pilot more time to climb up in the next thermal. The stronger the thermal, the faster the optimum glide speed. You can read a great discussion about the finer points of this speed-to-fly theory here, courtesy of John Cochrane.

A fixed airfoil needs to have a wide laminar bucket, so drag can be minimised both when gliding and thermalling. This, however, will result in a less deep bucket which means that the drag coefficient within the bucket is higher than for a flap airfoil with optimum flap setting. Since adjusting the flap allows one to shift the laminar bucket to the desired lift coefficient, the laminar bucket of a flap airfoil can be made narrower and deeper.

The direct answer to your question is: no, negative flaps do not increase the maximum glide ratio but help to fly fast at low airfoil drag. If you look at the L/D at high speed / low lift coefficients, then yes, negative flaps improve the L/D at that part of the polar. They also help to limit the maximum lift a wing can produce and allow to fly faster in gusty weather.

Positive flaps reduce both drag at high lift coefficients and sink rate, and they are essential to reach the polar point of minimum sink in high performance gliders because that optimum is at very high lift coefficients due to the high aspect ratio of the wings. If you fly fast with positive flaps you will greatly increase drag and the sink rate, however.


Why do light taildraggers swirl uncontrollably on runways in a simulator? When a pilot chooses to run at a speed way above its takeoff speed on the runway in a simulator without taking off, the plane begins to skid and spins 360 uncontrollably around the yaw axis, what is the cause?

A taildragger is more or less a shopping cart being pushed backwards, or a car driving fast in reverse.  It's dynamically unstable when rolling and wants very badly to switch ends, requiring constant input to the steering.


What is being depicted w.r.t. AoA in this video? I was looking at this video of the AF447 crash simulation and at about 4:45 the angle of attack diagram shows a few things I don't understand (frame above).

I get the orientation of the airplane, pointing a few degrees above the horizon, but what does the green line represent in layman's terms? Is that the direction the plane needs to go for the wings to generate lift?

Also, I see the red line has to do with the warning, but how does that play into it as well?

There are several colored lines.  Which one are you asking about?  I'm assuming you are referring to the (cyan?) and red lines in the upper right inset.

The cyan line shows the current direction of the airflow.  When compared with the orientation of the airplane can show the angle of attack.  At this time, the relative wind is coming at a very steep angle to the airplane and the wing is stalled.

Another way to see it is that reversing this arrow shows the direction the airplane is moving through the air.  In a no-wind situation, this would also be the direction the plane is moving with respect to the ground.

The red line is at a constant angle with respect to the airplane and shows how low the airflow can go without causing a warning.  As it is below this line, the stall warning has sounded.


Why is the use of reverse thrust not considered when calculating accelerate-stop distance on dry runway? In calculating the accelerate-stop distances, what are the possible reasons why the use of reverse thrust is not taken into account on a dry runway, while it is taken into account on a wet runway? What's the rationale behind this diffentiation, when at least as I see it there's nothing that prevents the use of reverse thrust on a dry runway for an aborted takeoff?

If your thrust reversers are operational and debris is not a concern, you're right for an in-service rejected takeoff (RTO):

  There's nothing that prevents the use of reverse thrust on a dry
  runway for an aborted takeoff.

Most in-service RTOs utilize reverse thrust if available (see this video for a dry-runway example).

However, in order to certify a transport-category airplane, one must show compliance with 14 CFR 25.109, paragraph (f), which precludes the use of thrust reversers on dry runways in calculating the accelerate-stop distance. The demonstration required by paragraph (i), therefore, cannot be conducted with reverse thrust on a dry runway. From a certification standpoint, the accelerate-stop calculation is made the way it is because that's what the regulations say. But why is the law written this way? See this video at about 1:12:

  The whole intent is to demonstrate [that] under [the] worst conditions you can safely stop the aircraft.

"Worst conditions" encompasses at least inoperative thrust reversers and worn brakes.


Is there a rule-of-thumb for estimating distance to an aeroplane? It's generally hard for humans to estimate how far away an aircraft really is.
Question: Is there a standard rule for estimating the distance to an aircraft?
For example:

The entry/exit hatches are 2.4 metres high and your thumbnail at arms length translates to a 2.4 metre high door at 400 metres

Adults with normal 20/20 eyesight can make out an airliner's passenger windows in normal sunlight, so the plane is under 500 metres away, and over 500 metres if you can't make out individual windows.

Aircraft's registration code must be XX cm high and is therefore readable at up to 1000 metres.

Of course lighting and elevation differences, paint designs and model of plane, and even heading differences would mess it up a bit, but this is a rule of thumb in lieu of any other information.

For example, the popular fallout character is not giving a thumbs-up.  Instead he's comparing the size of a distant mushroom cloud to his thumb to see if he's outside the nuclear fallout zone.

Inspired by How close can planes fly to each other over the ocean?

A rule-of-thumb would imply frequent usage. Such as this list on SKYbrary.aero: Rules of Thumb.

For usage by pilots and/or controllers, a distance-to won't be beneficial even if there was a rule-of-thumb (there isn't).

Imagine two planes on approach to a runway, the controller uses his thumb, now the following plane is also higher, so we have different slant angles: it just won't work to judge their separation (and on a foggy day it's of no use of course).
Imagine a pilot flying VFR, at night. The pilot spots the navigation lights of a plane, but nothing else to judge the plane's size: again won't work.


What happens during Engine Performance Restoration What is the typical process during an Engine Performance Restoration?

I am trying to find a resource that gives an overview of what happens but I have not been successful.

Any help would be much appreciated.

An engine performance restoration is a scope of work ("workscope") for a turbofan engine shop visit, where the engine is removed from the aircraft and sent to a engine maintenance facility and the engine case is opened.

An engine performance restoration involves the inspection, rebalancing or replacement of specified parts in the core gas flow path, that is, HPC, combustor, and HPT. These parts are most critical for the fuel burn and EGT margin of the engine. If any of these parts are life-limited, they will be changed at this point.

It also should include any mandatory inspections or replacements, covering the whole engine, dictated by the manufacturer or regulators.

It excludes any optional refurbishment or inspection of the fan, LPC, LPT, and gearbox. It may also exclude inspection of parts deemed non-performance critical in the core.

Because of this, the cost of the visit is reduced, but the engine will not be as efficient and potentially require another visit sooner than a "full overhaul". Thus, an engine performance restoration is the most basic scheduled engine shop visit. It is common that a performance restoration be alternated with a full overhaul.

See Engine Maintenance Concepts or Basics of Aircraft Maintenance Reserve Development and Management for a more detailed overview.


What turn to intercept a radial and how to tune in B737ng sim? Forgive my very amateur questions, I have been out of flying for years now and I am getting fit to fly again after slight setback.

I have an opportunity of doing some refresher flying in 737NG simulator soon. I was given a mock clearance for practice and would like your input. Take off is from runway 30, runway heading till 2000ft and then track and intercept radial 020. The VOR station is right behind the runway at the airport.

Does this clearance allow me to fly a reciprocal of 020 inbound to the station? Or will I just have to fly away from the station on 020 radial?
Will a left turn be more appropriate than a right turn to intercept radial 020?
In a Cessna 172, I would twist the OBS to get a from flag and then select 020, how do I ago about this in a 737 sim? Is it the same thing?
Do I select the radial before or after take off?
Finally, the b737 uses ADI with a movable pointer or "sky up"which is different from what single pistons and light twins use. I will appreciate any tips to help me nail the movable pointer ADI/attitude indicator on the sim.

I will really appreciate any tips to help me make the most of the sim session and sorry for the terribly novice questions.

Does this clearance allow me to fly a reciprocal of 020 inbound to the station? Or will I just have to fly away from the station on 020 radial?

A radial is always pointing away (radiating) from the VOR. The clearance therefore instructs you to fly away from the VOR on the 020 radial.

  Will a left turn be more appropriate than a right turn to intercept radial 020?

I think the image should answer that: right turn is better.

  In a Cessna 172, I would twist the OBS to get a from flag and then select 020, how do I ago about this in a 737 sim? Is it the same thing?

You can tune the VOR frequency on the ground using the NAV tuning panel on the center pedestal (left image) and then dial in the course of 020 on the Mode Control Panel (MCP) on the glareshield (right image):

(737 NG FCOM)

  Do I select the radial before or after take off?

You can do it before takeoff because you will depart using runway heading (in HDG SEL mode) initially.

  Finally, the b737 uses ADI with a movable pointer or "sky up"which is different from what single pistons and light twins use. I will appreciate any tips to help me nail the movable pointer ADI/attitude indicator on the sim.

You depart in HDG SEL (8 in image below) with a heading of 300 (or runway heading) until 2000ft and then turn right onto a 065 heading (still in HDG SEL). This will set you up for a 45° intercept of the radial. Note: With strong winds, you need to correct this heading to get a 065 track.

To arm the intercept, you select the VOR LOC mode on the MCP (5 in the following image):

(737 NG FCOM)

This will arm the VOR intercept (should show VOR LOC as an armed mode on the FMA (Flight Mode Annunciator), which is located at the top of the Primary Flight Display). Now all you have to do is follow the flight director (or turn on the autopilot and watch it do all the work).

If you don't want to use the flight director, you can switch the Navigation Display (ND) to VOR mode on the EFIS (Electronic Flight Instrument System) on the glareshield (2 in the following image, when turning 1 to VOR you also get an ADF like pointer towards the VOR):

(737 NG FCOM)

The ND will then look something like this:

(737 NG FCOM)

Here, you find familiar things like lateral deviation (10), TO/FROM indicator (12, also shown as arrow inside) and the selected course (3). It is using heading up or track up, but it works exactly like in a C172. The easiest way to fly the intercept is however to use the flight directors.


How much air does a 777-300ER's pressure cabin hold? I've been wondering how much air a 777-300ER's pressurised cabin holds, but, while most of its specifications are fairly easy to find, the amount of air it contains is not.

Does anyone know what volume of air is held by the pressure cabin of an empty 777-300ER?

I once read it's about 1 metric ton (1000 kg) of air for a large widebody.

Here's a ballpark calculation to verify this.

The 777's cabin altitude is 2430 meter (8000 ft) when it's cruising at max altitude.

Wolfram Alpha tells us that a cubic meter of air weighs 0.96 kg at that pressure.

DimensionInfo shows that the 300ER is 73.9 m long and 5.86 m wide.

For simplicity, consider the 777 to be a perfect hollow cylinder.  The volume of the cylinder is:

$$\pi \cdot \left(\frac{5.86}{2}\right) ^ 2 \cdot 73.9 = 1993\: m^3 $$

So that's about 1900 kg of air for the cylinder.
You have to subtract a large amount for the air lost to non-pressurized volume, tapering of the cylinder, non-circularity of the fuselage, and volume not occupied by air (structures, furniture, insulation, wiring, luggage, passengers).

I'd guess it's somewhere between 1000 and 1500 kg.

Whether people are on board or not has very little influence.  People float or sink in water depending on having air in the lungs or not.  So the density of a human is around that of water: 1000 kg per cubic meter.  [In other words: you could fit 10 people weighing 100 kg in a cubic meter if you pressed them hard enough].  So the density of air is a thousand times lower than that of the passengers.  Three hundred people - weighing 30000 kg combined - displace less than 30 kg of air.


How do I mount an iPhone X in the cockpit to record flights, as a student pilot? I'm a student pilot and would like to mount my iPhone X in the cockpit to record my flights. Don't want to get an extra GoPro. However all the mounts I've seen online are designed for iPhone 6 or lower, and the RAM mount seems slightly troublesome to set up, in an aircraft that I'm only spending two hours in every day (and that too not the same aircraft every day).
Any suggestions as to how I can mount my phone? Any help would be appreciated!

if you were using one aircraft you could get an adhesive mount and then buy a mount that has the adhesive side and a phone mount, but since that is not the case you could try using a suction cup. i have done it a few times, it worked out OK for me.


Can we always trust the PAPI lights? I have a question about the PAPI system. One of the first things student pilots learn during their ground school is the PAPI. You get two whites and two reds on a visual approach, you're on the 3 degree glide path. If you get three reds and one white, you're a little low. If you get four reds, then you may consider going around since you're way too low.
My question is: Can we always trust the PAPI lights? Is there no chance the PAPI lights might be wrong, so for example, when you get three whites and one red on the 5 mile final, but you are actually below the 3 degree glide path?

Yes, you can trust the PAPI lights. As usual in aviation, safety is priority #1, this also applies to airport lighting, which is checked and monitored continuously.
PAPI lights are calibrated regularily to ensure their indications are correct. As with all technical equipment, there is a non-zero risk PAPI lights may fail, however, given how simple they are (literally just a light bulb, a colour filter and a lens - no moving parts) a failure is very unlikely.
You should note that not all PAPI lights are calibrated for a 3° glidepath. While 3° is most common, there are runways where the normal approach angle is different and the PAPI lights will be calibrated to that. So make sure to check the airport charts before using the PAPI system, or you may risk coming in steeper or shallower than you are used to.
Also note that, as pointed out by @wbeard52, approaching with the PAPI showing two whites and two reds is not a guarantee of obstacle clearance if you are far from the runway - it is only a guarentee that you are following the nominal glideslope of the approach procedure with which the PAPI is aligned.


What is wrong about this interpretation of QFE and QNH? I'm a little confused about the QFE and QNH definitions, and its application on aviation. I learnt that:

$QFE$ = pressure measured by an altimeter which is adjusted to ground level (it gives a height of 0 when the airplane is on the ground).

$QNH$ = pressure measured by an altimeter which is adjusted to sea level (it gives the field elevation when the airplane is on the ground).

So, by those definitions we can say that on ground level $QFE=p_0=101325Pa$ and $QNH=p_{ISA}(h_e)$ where $p_0$ is the SSL pressure and $h_e$ is the field elevation. $p_{ISA}$ is the pressure on a standard atmosphere associated to the height $h$. For the troposphere we have:

$$p_{ISA}[Pa]=101325\left(1-\frac{0.0065 h[m]}{288.15}\right)^{5.25588}$$

So, pressure drops with an height increment. For example, if the field elevation is positive (i.e. it's above the sea level) and the airplane is above the field we have to have $QFE>QNH$, because the height measured by $QFE$ configuration is lesser that the height measured by $QNH$ ($h_{QHN}=h_{QFE}+h_e$ and $h_e>0$)configuration, and by the relation between $p$ and $h$ if $h_{QFE}<h_{QNH}$, then $p_{QFE}>p_{QNH}$.

What I have seen in many problems of QFE determination by QNH and vice versa, is that the definitions used for QFE, QNH were exchanged. See for example this question How can I calculate QNH from QFE?.

I've seen this type of resolution done in many exercises, and so I presume that I'm wrong! But I still didn't get it! What am I missing here?

I can even show you a figure that was used on the theoretical slides of my Aircraft Performance course:

As you can see this figure says that $h_{QFE}=h_{QNH}+h_e$ which should instead be $h_{QNH}=h_{QFE}+h_e$ (the opposite) according to the $QFE$ and $QNH$ definitions.

Your question is quite not clear, you are challenging definitions and use of definitions, but you may start with a misunderstanding:

  QFE = pressure measured by an altimeter which is adjusted to ground
  level (it gives a height of 0 when the airplane is on the ground).

Definitions

To ensure clarity, QFE/QNH:

are reference pressures, not altitudes
are not read/measured by the altimeter, but entered by the pilot in the setting window.
when mentioned in an altitude would be mentioned like, for instance: "I'm flying at 1,000 ft referenced to QFE", and not "my QFE is 1,000 ft".

QFE and QNH are not what the needles of the altimeter indicate, they are only the pressures that are to be entered in the setting window to calibrate the altimeter for common low level flight references.

How is QNH determined

While QFE is a direct measurement of the pressure, where the temperature is ignored, QNH is never measured, it is "retro-computed" from QFE:

Determining QNH requires first to know the local pressure (QFE) at the relevant location (e.g. airfield reference point)
The actual altitude of this location is then converted into a pressure difference, using the ISA model (+1 hPa per 27 ft at low altitude), without taking into consideration the temperature
The offset (which is positive unless the airfield altitude is under sea level) is added to QFE, the result is QNH (hence QNH pressure is usually > QFE pressure)

Said otherwise: QNH is the theoretical pressure value at MSL. but as the temperature wasn't taken into account, and actual distance between isobaric surfaces vary with actual temperature, then the actual pressure at MSL (if it could be measured) would be likely different.

Altimeter readings

After calibration with QFE or QNH value, any change in actual pressure -- that can be due to an actual altitude change, a local temperature change or a local pressure change -- will be sensed by the altimeter, the pressure change being indicated by an altitude change at the ISA rate of -27 ft for each added hPa.

By principle when the altimeter is on an isobaric surface equal to any pressure entered in the window, the value read is zero. As by definition the QFE is the pressure at the airfield level, then if the altimeter is at the airfield, set to QFE, and QFE has been just updated, then it indicates 0 altitude above ground level (AGL).

More generally (I hope this is accurate, else correct me):

Note that the pressure scale on the left has the higher value at the bottom, while the altitude scale on the right has it at the top.

  As you can see this figure says that $h_{QFE}=h_{QNH}+h_e$ which should
  instead be $h_{QNH}=h_{QFE}+h_e$ (the opposite).

You or the book may be correct, depending on the sign of $h_e$. But it's not clear. If this quantity is positive, you are correct.

Remember that QFE and QNH are not altitudes, they are the altimeter reference values. For an airfield above MSL, QFE < QNH and an altimeter reading (h) referenced to QFE ($h_{QFE}$) will be approximately the height above the airfied (AGL) while when referenced to QNH ($h_{QNH}$) it will be approximately the altitude above MSL (AMSL).

Indeed altitude above sea level is larger than height above ground. Possibly in your example above, the author may have $h_e$ as a negative value.


In ARINC 429 where are the bits indicating the hex value of Equipment ID? I'm a newbie in ARINC-429 standard; the following might be a very simple issue for you.
In ARINC 429 the LABEL field  is covered by the bits 1 through 8; which bits are covering the Equipment ID?
For istance: Equipment ID 002 hex (Flight Management Computer) would require 12 bits to be represented. Where are located these bits in the ARINC word, so that the receiver could read the it?
Thanks for your assistance.

ARINC 429 is a unidirectional one-to-many (broadcast) transfer bus with a maximum of 20 receivers.

SourceDesign with four unidirectional ARINC 429 buses

There is normally little need to identify the source, as there is only one. So there is no field in the protocol word dedicated for ID transmission. As visible in the previous image, bidirectional exchanges require two buses and several ports to connect them. The receiving unit (LRU) can determine from the port used which source feeds the bus.

However label 377 (octal) can be used to transmit, from time to time, the equipment ID if desired. Example: ID 10D (hex):

Source: ARINC specifications

Note: On the other hand, SDI bits can be used by the source to select the receiver.


What are the design goals for NASA's X-57? There have obviously been work going on prior to the X-57 announcement (see LEAPTech and Joby Aviation http://www.jobyaviation.com/LEAPTech/). However, what does NASA actually think they will achieve as far as specifications after completing the X-57? I have seen the 175 mph cruise at 1/5th the energy cost numbers thrown around, but the real numbers that matter are not stated by NASA. For instance, will the entire aircraft except for the pilot seat be packed with batteries? What is the useful payload? What will be the range on a single charge?

I just found an answer (pointed out by @RoboKaren), on another question. The only reason I am copying this is that the original question was not really about goals. This is from Mark Moore:

source: What are the advantages of the NASA LEAPTech propeller on wing technology?

  The lift augmentation has nothing to do with Magnus lift (or the
  Coanda effect). It is simply an increase in the dynamic pressure
  across the entire streamtube of the wing.

  At the low takeoff and landing speeds (slightly above a 61 knot stall
  speed), the induced velocity of the propeller almost doubles the
  velocity that the wing sees; and the lift is a function of the
  effective velocity squared. But due to swirl and other effects the
  wing doesn't experience a 4x increase in lift, but about 2 to 3 times.

  The objective of the inboard propellers is not to achieve a high
  propulsive efficiency, instead we want those inboard props to achieve
  high induced velocities - think of them as part of the high lift
  system (which happens to also provide thrust redundancy).

  In fact landing is the critical case, and having lower propulsive
  efficiency (and a poorer spanwise lift distribution because of the
  swirl effects) is helpful in creating sufficient drag. The inboard
  props are not active in cruise flight, but simply fold against the
  nacelle (many motorgliders already do this type of folding on the
  nose).

  By only using the wingtip propeller at cruise, we're able to achieve a
  ~95% propulsive efficiency (versus 75 to 85% with a typical fuselage
  nose propeller installation). The reason for this is because we have
  lower blockage and scrubbing drag, as well as being able to take
  advantage of the strong wingtip vortex by rotating against it. Since
  electric motors don't experience a power lapse with altitude (because
  it's non airbreathing) we have far too much power at altitude anyway
  so by only using the wingtip motor it doesn't cause much penalty in
  motor weight (and electric motors achieve ~6x lower weight per
  horsepower than a reciprocating engine).

  In terms of batteries and range, the key is to achieve high efficiency
  cruise flight and it looks like with current batteries a 200 mile
  range is achievable. By adding a small <50 hp range extender motor,
  the aircraft will be able to achieve ~400 mile range. We're currently
  designing an X-plane that will fly in 2017 to substantiate all these
  numbers, with the ground test rig (wing and truck) providing an
  aerodynamic database to validate our analysis.

  Please note that I'm the NASA Principal Investigator of the LEAPTech
  Distributed Electric Propulsion integration approach, and the
  Convergent Electric Propulsion Technology (CEPT) X-Plane demonstrator.
  We have a team across NASA Langley and Armstrong, as well as two great
  small companies, Joby Aviation and ESAero who are retrofitting a
  Tecnam P2006T with a Distributed Electric Propulsion wing system.


Are any Boeing 720s still flying? A Boeing 720 looked like a 707 but was the size of a 737. I flew in one in 1961. I did not travel regularly until 1970 but I do not remember ever seeing a 720 again. Was it intended to have limited production? Are there any 720's still flying?

According to Wikipedia, no. The last one was retired in 2010.


Regarding N1 and N2, what does the "N" stand for? I initially phrased this question in a comment to another question regarding what N1 and N2 mean, hoping it would be that sort of low-hanging fruit that is easily answerable and not worth a proper question in its own right.  But then I starting searching and came up short.

This Yahoo Answers page has lots of guesses on it, from "nominal" to "Newell".  (And we all now are reminded why nobody goes to that site for real answers.)  Many other pages explain what N1 and N2 are, but not where the "N" part comes from.

I'm wondering if it might have its origin in German, as a lot of early developmental work on jet engines was done in Germany.  Or was it simply an unused letter that fit?

As far as I can tell, the N comes from the physics behind turbines in general, whether they be water pumps or wind turbines, a subset of mathematics called fluid dynamics.  Many thanks to ROIMaison's answer for steering me in the right direction.

In Turbomachinery: Basic Theory and Applications, Second Edition, Earl Logan, Jr. provides a symbol reference list at the end of each chapter starting in chapter 2, defining $N$ as rotor speed, in units of rad/s; an angular change per unit time, and $N_s$ as the specific speed, a dimension-less quantity derived from a ratio of the flow and head coefficients.

N1 and N2 seem to be derived from both.  Wikipedia mentions the speed as a percentage paradigm as a human factors consideration, though as of 5/14/2015 there are no references to support this.  However, there may be a scientific basis behind this as well, since a change in N is proportional to the resultant change in airflow (i.e. thrust produced):


How does a fixed-pitch propeller changes the blade's angle of attack? Studying I have found a question that states that in a fixed-pitch propeller in flight at a given TAS, the blade's angle of attack will change if RPM increase, but I don't understand how can the blade's AOA change if the pitch of the blade is fixed.
Am I missunderstanding the concept of fixed-pitch propeller or the question is wrong?

Question answer and explanation for source:

Q:For a fixed-pitch propeller in flight at a given TAS, the blade angle of attack will:
A:increase if RPM increases.
E:For a fixed pitch propeller the pitch angle cannot be changed (it varies from root to tip to maintain near-constant angle of attack along the blade).

The angle of attack as in the case of a aircraft wing is defined as the angle between the chord line and relative airflow. With a propeller however the relative airflow is the resultant of the airflow due to rotation and forward speed. Change either of these values and the angle of attack will change.

The pitch of the blade is fixed.  But the angle of attack depends on how the blade moves through the air.  That motion is mostly a combination of the forward speed of the airplane and the rotational speed of the blade.

In your graphic, the blade is attached to a plane that is flying up the page.  The blade is sticking out of the page and is being pushed to the right by the turning engine.

At some combination of airplane speed (up the page) and propeller speed to the right, the air would flow exactly along the (fixed) pitch of the blade.

Now, without immediately changing the speed of the plane you increase the RPM.  The blade moves to the right more quickly.  The angle of attack it makes in the air increases.

Put your hand out a car window.  Hold it at a fixed angle relative to the ground (say 15 degrees).  Now, holding the angle constant, move your hand downward rapidly.  The pitch has not changed, but the angle of attack has.  This is the equivalent of increasing RPM.


What is the minimum stopping distance for a 747-400? Im looking at purchasing  disused 747-400:

The aircraft will be stripped of all seats and other non essential items.
The aircraft will be then flown to my property in Victoria, Australia and perform a one off landing.
The landing distance would be between 1500-2300 feet.

Can this be done?

Interesting question. I think it could be done in the right conditions if you are willing to accept some damage and depending on the runway.

Is the runway level or uphill/downhill? If you could land uphill in a high wind that was almost all headwind, I believe you could either do it or would overrun the runway at a low speed. Airplanes slow down really quickly going uphill.

A stripped 747-400 probably weighs about the same as a 747-400 freighter, maybe a little less because the freighter has the equipment for loading and securing pallets. If you're using Firefox or Chrome (IE won't work), click on any aircraft at 747.terryliittschwager.com and you'll see the empty weight is around 362,000 lbs (you can change the weight unit to kilos there if you wish).

I used to land at Tel Aviv with only 15,000 lbs of fuel in 747-200s. The -400 burn rate may be a little higher, but let's use that. You would have to be comfortable with being totally committed to landing, with the possibility of only one quick go-around. Maybe you could even cut the 15,000 down to 10,000 with no go-around possibility?

Let's use 375,000. Vref for a -200 at 400,000 (lowest figure in the table I have) is 118 kts. In that area Vref decreases 3 kts for every 20,000 drop in weight, so we can take that 118 down to 115. I forget whether Vref is 1.2 or 1.3 times stall. Let's use 1.2, which would put the stall at 96. I would imagine a -400 is comparable insofar as these speeds are concerned, but I don't know.

How about one-time-use arresting gear of some sort on the last 400 meters of runway?

You could drop some more weight by taking out one engine. 747s regularly do 3-engine ferries. It's no big deal. However, I'm not sure if the weight loss would be worth losing the reverse thrust of the engine being taken out. There is a lateral imbalance consideration there, but 747s used to carry a spare engine in a pylon under the wing, so if you removed an inboard engine, I'm guessing you could live with the imbalance.

Speaking of reverse thrust, it used to be for the 747-200 that the published figures for minimum landing distance were brakes only, no use of reverse thrust. If that's the case for the chart shown in the previous answer, the figures there can be reduced, and you would want to use FULL reverse immediately. Don't worry about compressor stalls, after all, I'm guessing you're not going to use the engines again.

Also, I'm sure there are performance figures for the effect of runway slope and headwind.

It would be a challenging situation, one of those things some crazy Aussie might try!  lol


Why do aerobatic aircraft have separate tanks instead of a single acrotank? I've been reading about fuel tanks in aerobatic planes such as the Extra 330, Edge 540, XA41 and I don't really understand why in the Extra 330 there are separated center tanks and an acrotank instead of a single bigger acrotank like in the Edge and XA41.

I guess it's something related with balance and center of the aircraft but the Extra has two center tanks and their relative positions between them and with the acrotank are small.

A long time ago aerobatic aircraft had just a single tank with a flop tube inside it.

This works "OK", but when you're doing something complicated with a half-empty or mostly-empty tank, it's too easy for the flop tube to ingest air instead of fuel.  This leads to performance problems with both the engine and the pilot.

The solution is to have a very small "acrotank" with a flop tube and an arrangement of fuel line plumbing such that the tank remains 90% full, 90% of the time.

In my Laser, there's a very large tube (2" to 3" diameter) that feeds from the bottom of the main tank to a tube at the top of the acro tank (with that tube extending inside the tank to near the bottom).  There's a small vent/return line.

When I'm right-side-up, the acro tank fills from the main tank in seconds.  When I'm inverted, the tube arrangement prevents most of the fuel from returning to the main tank.  Some returns via the vent/return line, but I still have about 5 minutes of sustained inverted flight fuel.


What is the required cloud clearance in G airspace when there are clouds just above in class E airspace? This is my first question here and I am about to start my Private Pilot License (PPL).

Now I am flying in my Flight Simulator X. So, I am at KDKK airport. It has an elevation of 692 MSL . I got my ATIS report 2100 MSL scattered and 3000 MSL broken. The airport is in class G and the weather minimum here is clear of clouds.

At 1200 AGL we have class E airspace and 500 feet below clouds rule applies here. The question is: flying at 1800 I will still be flying in G airspace, but will not comply with 500 feet class e rule and will be closer than 500 feet to the clouds. Is it legal?

And the second question: still didn't find clear definition of "clear of clouds". Is that mean that there are no clouds at all? or it could be clouds, and I can fly very close to them, but I can't get through the clouds?

KDKK is under a Class E shelf starting at 700 ft AGL (Magenta vignette around the airport).  If the airport elevation starts at 692’ MSL, the Class E shelf will begin at 1392 ft MSL.  Therefore operation at 1800 ft MSL above KDKK will put you in Class E airspace and subject to the VFR weather minimums of 3 sm visibility and 1000 ft above / 500 ft below / 2000 ft lateral cloud clearances.

As the the second question, clear of clouds means remain in a position outside of clouds and a flightpath which will not place you in immediate risk of entering a cloud.


Why do airline toilets delay for a few seconds before flushing? I have noticed that when I push the toilet flush button in a regular passenger airliner, it takes several seconds (perhaps 2 or 3) before the toilet actually flushes.

What is the reason for this delay?

Most passenger aircraft of today (747, A380, widebodies of any kind) use vacuum-flush toilets.  It takes time for that system to go through. It also has to coordinate lids into the waste storage container that is usually fireproofed.

Those 2 seconds took decades to develop and keeps the odor and fires out!


What does it take to maintain a private pilot certificate? What, if any, requirements are there to maintain a private pilot certificate? For example:

Do I have to renew my certificate after a period of time? E.g. annually, 5 years, 10 years
Do I have to receive a minimum number of continued training hours?
Do I have to fly a minimum number of hours?

For instance, say I had enough money/time to go out and start the process of getting a private pilot certificate tomorrow, but I didn't plan to make use of it in the foreseeable future, would it cost me more in time and money than if I waited until I had plans to use it?

This is assuming I live and fly in the USA.

Related Question:
How much, generally speaking, does it cost to learn how to fly?

Edit:
Note I understand the safety aspect of not practicing and "getting rusty", for lack of a better term on my part, and that practically a certain amount of retraining would be required (though it seems that it is not legally required). The retraining is not mandated and I would need to retrain until I felt proficient enough technically and practically. Retraining would include familiarizing myself with the controls, instruments, radio, physics, etc.

My main concern was that (a) at some point I my certificate would expire and I would have to go through the process again, making the initial effort practically useless, (b) that I may have to pay a significant amount of money to keep the certificate from expiring, or (c) that I would have to commit to a certain number of hours (instructed or solo) per year to keep the certificate.

A license, once granted, is good forever (barring some enforcement action).

But, you need more than just a license to fly.  To use your license, while flying solo, you also need a Medical Certificate (if required for the aircraft or flight rules) and a Current BFR (Biennial Flight Review).

Medical Certificates are good for 5 years until you're 40, then good for 2 years.
Its a quick trip to an FAA approved doctor to examine you for any obvious medical conditions.

A BFR is a review with an instructor every two years to make sure you're still proficient and safe.  If all goes well, it takes about 90 minutes.

However, if you want to fly passengers, you also need 3 landings within the previous 90 days.

If you want to fly passengers at night, you need 3 landings to a full stop, at night, within the previous 90 days.

If your 90-day landings have lapsed, you can quickly fix it with 3 trips around the pattern, taking off and landing each time (touch-n-go during the day, or full-stop at night).  Then taxi back to the ramp, pick up your passengers, and go on your way.


Did Lockheed test the TriStar in conditions similar to a century of operation? I have no source for this claim except a motivational book. I am unable to find anything on Google regarding this.
The claim is that they ran a test on s TriStar that put it through the stresses of one century's worth of operation, and it survived.
I cannot find the book at the moment. Can anyone confirm or deny this?

If a plane of the L-1011's range makes two flights per day for a century without any maintenance breaks, that would be 73,000 flights.
The answer is yes, more or less, literally, if you look at wing loads, or pressurization, respectively. If you account for AOG (aircraft on ground) time, then most certainly yes for both:

A total of 1,696,000 load cycles, simulating 84,000 flights, were applied to the fatigue test airframe. The first 52,000 flights included all loadings, including pressurization. The last 31,500 flights omitted fuselage pressurization, while wing loads were increased from 10 percent to 20 percent over the basic spectrum. This amounted to an equivalent of 115,000 flights. A specific number of load cycles applied by hydraulic jacks to the fatigue test airframe equates to the loads seen in one flight of the aircraft. [emphasis mine]

Since MSN 1000 (second airframe) reached the equivalent of 115,000 flights, it means it did not fail before that, but that doesn't mean that lessons weren't learned and applied to the production airframe, which is one of the purposes of such tests.

Upton, Jim. Lockheed L-1011 Tristar. Specialty Press Publishers and Wholesalers, 2001.


Are two people required in cockpit at all times on large commercial aircraft? It would seem reasonable on large commercial airlines to require that there be two people in the cockpit at all times. For example when the pilot goes to the toilet a member of the cabin crew must enter the cockpit until the pilot's return. Such a rule would greatly reduce the probability of pilots being locked out and reduce problems should they be locked out. Do any airlines enforce such a rule?

EDIT: please see at the bottom of the answer the update from EASA

Are two people required in cockpit at all times on large commercial aircraft?

No.
From EASA, pag. 101-102: (emphasis mine)

CAT.OP.MPA.210 Crew members at stations
Flight crew members

During take-off and landing each flight crew member required to be on duty in the flight crew compartment shall be at the assigned station.

During all other phases of flight each flight crew member required to be on duty in the flight crew compartment shall remain at the assigned station, unless absence is necessary for the performance of duties in connection with the operation or for physiological needs, provided at least one suitably qualified pilot remains at the controls of the aircraft at all times.

During all phases of flight each flight crew member required to be on duty in the flight crew compartment shall remain alert. If a lack of alertness is encountered, appropriate countermeasures shall be used. If unexpected fatigue is experienced, a controlled rest procedure, organised by the commander, may be used if workload permits. Controlled rest taken in this way shall not be considered to be part of a rest period for purposes of calculating flight time limitations nor used to justify
any extension of the duty period.

Do any airlines enforce such a rule?

As other answers mention, yes.

EDIT: On the 27th March 2015 EASA has issued the following Safety Information Bulletin :

operators are recommended to implement procedures requiring at least two persons authorised in accordance with CAT.GEN.MPA.135 to be in the flight crew compartment at all times, or other equivalent mitigating measures to address risks identified by the operator’s revised assessment.

This still does not mean that 2 people are required, but are recommended.


Where should the support points be for testing a model wing in a wind tunnel? When testing a certain wing profile inside a wind tunnel where should the model, small scale wing be supported? Should the support be under the center of mass of the wing or under the aerodynamic center? If so, why?

My understanding is that, for similarity reasons, the Reynolds and Mach numbers for the model wing tested inside the wind tunnel must be the same as the those for the actual aircraft...

Indeed, the Reynolds and Mach numbers should be the same, or at least similar to the actual aircraft, depending on what you want to measure. For instance, the maximum lift coefficient is very sensitive w.r.t. Re and if you add a Mach number above 0.3, compressibility needs to be taken into account as well (even though quite good correction models exist for this). On the other hand, if you want to measure for instance only the zero-lift angle of attack, the Re doesn't matter much.

Anyway, where you support the airfoil doesn't change the Reynolds or mach numbers. In fact, you can theoretically support it at any point. The lift and drag will always be the same for every point; what changes is the moment that you measure. It might be convenient to choose the aerodynamic center which typically lies at around 25% of the chord, since there the pitching moment doesn't change with angle of attack. But be aware that this location can still very slightly move with AOA in practice. For more information: https://en.wikipedia.org/wiki/Aerodynamic_center.


How do airports manage gate assignments for aircraft? Today, some companies like Delair, SITA and INDRA provide airport Resource Management Systems (RMS) for gate allocations, which show information as a Gantt Chart.

I know that air traffic controllers use a flight progress strip to track flights under their control, but how was parking managed in the past without today's RMS solutions? What do dispatchers use for gate management? If it's manual, what is the process?

Depends on the airport. Some airports assign blocks of gates to specific airlines, and they then assign them to flights (usually on an as-needed or first-come-first-serve basis).
Other airports assign them all themselves, again usually on an as-needed or first-come-first-serve basis.
Of course there are categories of gates as well, size and capacity determine what aircraft types can be handled by specific gates.
In theory at least, and no doubt small airports do it that way, it can be handled all with pen and paper. Larger airports of course will have some sort of computerized system for it, either purchased off the shelf or something they had custom built for them.


Does the Fairchild Metroliner have any unusual handling characteristics? I have been told by several people that the Fairchild Metroliner is a quite difficult to handle aircraft.
Aside from that, I have seen that it has suffered several accidents throughout its history.
Does it have any aerodynamic deficiencies in its design or something like that?

Your question is basically asking for opinions. That said, I'll give you mine plus a bit of objective information. I flew SA-226 and SA-227 Metroliners for about two years, commuter flying (4 to 8 legs a day), mostly in U.S. Pacific Northwest weather, two pilots, no autopilot.

The Metroliner was considered by many to be a high workload, unforgiving aircraft. See this for more on that, but I did not find it unreasonably so. However, anytime you have a high work load, fast aircraft, the potential for incidents/accidents gets a little greater than if either of those things is not true.

It was not an airplane that you could get consistently good landings in if you consider a good landing one that is smooth (whatever that means). Clunkers greatly outnumbered greasers, at least for me. The tires were small and hard, and in my opinion the long landing gear magnified clunkers. You could increase your chances of a greaser by carrying a lot of power onto the runway, but that used a lot of runway.

Nose wheel steering was hydraulically actuated via the rudder pedals. It was not uncommon for the system to be inoperative, and the aircraft could be dispatched without it, in which case you had a castoring nose wheel. New captains dreaded being dispatched with inoperative nose wheel steering. Once you got used to it though, it was no problem. In fact, if you had to do some really fancy maneuvering on the ramp, you could turn off the nose wheel steering and if need be pivot about a main gear.

Far from any aerodynamic deficiency, at least in my opinion, it was great in that respect in that it was fast, but had the ability to slow down quickly.
Controllers liked it (and took advantage of this) that it could fly at jetliner approach speeds from the outer marker inbound until short final. Controllers at SFO would often put us between jetliners going down final, or from a left downwind for 28L clear us from that downwind for an asap short final knowing that we could do it. We would touch down beyond where the jetliners did but still in time to make the turnoff to what was then the commuter ramp. When you dropped the gear and flaps and brought the props back to an almost flat pitch with no power, it quickly slowed and/or came down.


Why does aileron response diminish as speed becomes slower, from slow-flight up to the stall? (In a light trainer) Can someone phrase the explanation to this in simple words as you would explain to a student?

This is real easy, no explanation required:
Tell your student to stick their hand out the window of the car driving 25mph in the city and rotate it like an aileron.
Then have them do it on the freeway doing 60mph.
Feel the difference?  Did smaller movements produce larger effects at higher speeds?
Practical examples are typically more effective at driving a point home than a lot of words.


TIS-B anonymous address I could not understand what is the difference between ADS-R with anonymous address (CF=6, IMF=1) and TIS-B CF=5, which on ICAO Doc 9871 is described,

for Version 1 as:

  Fine TIS-B Message AA field contains a non-ICAO 24-bit address

and for Version 2 as:

  TIS-B messages that relay ADS-B Messages using anonymous 24-bit
  addresses

Any specific example when CF=5 is used?

Thanks!

My question was due to the confusing mention of ADS-B in what was supposed to be either TIS-B or ADS-R.

I've now confirmed that the text on ICAO Doc 9871 Version 2 I quoted was due to a error.

In fact DO-260B, and even Doc 9871 Version 2 at table C-37 specify that TIS-B CF=4 is "Fine TIS-B message, AA field contains a 24-bit anonymous aircraft address".

ICAO e-mailed me confirming that this has been fixed into their working document.


Can anti-radiation missiles be used to target stealth fighter aircraft? What I understand is that anti-radiation missiles are accurate and long range.
So, assume that a fighter aircraft is 100% invisible on radar. However, its radar is transmitting energy continuously which can be detected and targeted using anti-radiation missiles. As Wikipedia suggests there is Russian air-to-air anti radiation missile (Vympel R-27EP).
Wouldn't such a missile neutralize the stealth technology? And why don't we have any other examples of such weaponry other than Russian one?

There are many anti-radiation missiles in many arsenals, they are designed for air to ground missions, attacking surface to air missile radars from a long distance. I don't know whether it would be possible for an anti-radiation missile to hit an air target.
If we assume it is possible your idea has a significant flaw in that stealth aircraft turn off their radars when they are on a stealth mission. Running a radar is like holding a flare in the middle of a field on a dark night, which is why they don't do it, at least not using conventional radars.
Some stealth airplanes don't even have a radar, for instance the F117. Typically attacks were done using GPS for precision navigation to the target, which is then attacked using precision munitions like laser guided bombs or GPS guided bombs or missiles.
Nowadays here are advanced radar systems that spread signals over multiple frequencies which will blend in with other signals, and can be directed in specific directions to avoid being detected. If a radar beam isn't pointing towards a sensor then the radar won't be detected.


How do you compare private jet and commercial jet efficiency? I'm curious about commercial airplane efficiency vs private jet efficiency.

I was trying to compare something like a modern 737 flying a full commercial flight from NYC to LA vs an Embraer Phenom 300 at full capacity (8 people) flying the same route.

I know there are many variations of the 737, so can we just use the most common commercially flown 737?

If fuel costs were the same (I understand that big airlines get discounts) is there a simple way to compare the flights simply in terms of cost of the flight, or how much fuel each plane would need for the flight? I didn't want to get into profit, crew, etc. Simply the cost of flying both planes from NYC to LA.

Thanks for any help.

This has nothing to do with private vs. commercial. The size difference is what matters.

If you compare the absolute cost of moving the aircraft from NYC to LA, the 737 will be way more expensive, regardless of version. Since you asked only which one is more expensive, I don't see a need for detailed reasoning. Just use the data provided in this answer and it becomes obvious that fuel consumption will scale with the mass of the aircraft.

If all passengers have to share the cost, the cost per passenger will be much higher in case of the Phenom 300. Or almost any other Bizjet. The Phenom will even need to land somewhere enroute to refuel. The distance from NYC to LA is bigger than the maximum range with full payload.

But that kind of comparison does not make sense - both airplanes have been designed for different purposes, so their strengths are in their unique field of use. They do not transfer well to that of different aircraft types.


What is the benefit of upgrading Flight Management System to version UNS-1Ew? I am working in a private airlines. We have some Dash-8 Q400 aircraft in our fleet. Recently a SB has been issued telling us to upgrade the Flight Management System from  UNS-1E to UNS-1Ew.

Can anyone explain what are the advantages for upgrading the Flight Management System from UNS-1E to UNS-1Ew?

Here is my understanding of the update. As others I believe your primary source should be your OEM.

Main purpose: Ability to use SBAS procedures and navigation

The purpose of moving from UNS-1E to UNS-1Ew is getting the SBAS capability. From the UNS-1Ew manual:

  The new “W” series of Universal Avionics Flight Management systems
  incorporate the Global Navigation Satellite System (GNSS) Space Based
  Augmentation System (SBAS) known in North America as the Wide Area
  Augmentation System (WAAS). Other SBAS systems in the world will also
  be incorporated.

SBAS details

SBAS means using WAAS, EGNOS and other GNSS augmentation systems. SBAS in short provides for correction of GNSS radio signal propagation errors in a limited area. This increases pseudo-ranging accuracy and positioning precision.

SBAS (space-based) is opposed to GBAS (ground-based) where correction broadcasting stations are on the ground, like the DGPS, a surveying GBAS with a positioning precision up to 10 cm.

EGNOS measuring network (SBAS for Europe area):

Source: Wikipedia

Additional benefits of the update

From UASC documentation:

  What else you get with an SBAS-FMS upgrade:

  •  Performance-Based Navigation (PBN) including Precision-Area
  Navigation (P-RNAV) compliance and Required Navigation
  Performance/Area Navigation (RNP/RNAV) up to 0.3 NM approach accuracy

  • WAAS LPV / EGNOS APV, LNAV approaches

  • Mode S Enhanced Surveillance support

  •  Accurate SBAS-GPS information provided to the onboard Terrain
  Awareness and Warning System/Enhanced Ground Proximity Warning System
  (TAWS/EGPWS) and Traffic Collision Avoidance System (TCAS)

  •  Large database capacity for worldwide navigation databases

You can read "The Benefits of SBAS" on UASC site.

And if you need more information the UNS-1Ew manual is also available.


Why is fuel used as a hydraulic fluid? Why is fuel used in some servo-systems as a hydraulic fluid (fueldraulic systems) of modern civil and military aircraft such as the Boeing 777-200/-300 (Pratt & Whitney PW4084) or the Lockheed Martin F-35 Lightning II (Pratt & Whitney F135)?
Examples for fueldraulics are the actuation of

Fuel Valves, Start Bleed Valves, Cooling Valves,
Variable (Inlet) Guide Vanes (VGV/VIGV) / Variable Stator Vanes (VSV) for the compressor, and
Thrust Vectoring Exhaust Nozzles,

see for example Eder, M.: "Engine Fuel System Design Issues", Aviation Fuels with Improved Fire Safety, A Proceedings, 1997.

Here is a sketch of a fueldraulic system actuating the Bleed Valve and the VIGVs for a compressor, taken from "Rolls-Royce plc: The Jet Engine, 5th Edition 1996, Derby, England, ISBN 0902121235":

Are there specific reasons for not using designated hydraulic fluids such as Skydrol for all hydraulic applications?

The primary reason is availability -- the high pressure fuel pump is running whenever the engine is, while the airframe hydraulic systems may not be available in case of an accessory gearbox failure, hydraulic leak, or maintenance shutoff.

Also, this keeps hydraulic lines from needing to be plumbed deep within the bowels of the engine -- this has benefits in that the hydraulic system is confined to the accessory gearbox fire zone (where the engine driven pump lives), instead of having to run in several engine fire zones.


How does one differentiate a "fly by" from a "fly over" waypoint? I am referring more to Terminal Area Procedures (SID, STAR, Approach), rather than en route waypoints, airways etc. How do I know if such a waypoint is a "fly over" or not?
Especially for some RNAV procedures, where the turn to join the final approach course can be 90 degrees. I have already read the "Fly over or fly by question" but I did not find a reply to the above, although the example quoted and shown in that question was a very good example of what I am asking.

A flyover waypoint will have a circle drawn around it on a chart, whereas a fly-by waypoint will not. (Easy to remember: the circle looks like the O in Over)

The symbols for a fly-by and a flyover waypoint in aeronatutial maps and charts can be differenciated by the circle surrouning flyover waypoints.

https://www.skybrary.aero/index.php/Waypoint


What iPad for the cockpit? I am planning to pass my PPL and I am trying to buy everything I can before starting the flight lessons to spread the expenses. Most of the private pilots I have met told me that an iPad is almost essential for them now. But I am not sure which one should I take? An iPad Mini seems more than enough to me and the small size would help in small cockpit (like in a 152).
4G or Wifi-only, is 4G reliable at those altitudes?

Thanks !

In my experience, 4G is not reliable above 1,000 feet, and is frequently non-existent above 3,000 feet.

Many pilots have suggested the 4G model for two key reasons:

The 4G model has a GPS-chip in it, that the WiFi-only version does not.
Having a cellular connection means you can do on-the-ground operations in places that don't have WiFi. This includes things like: File Flight Plans, Check NOTAMS, and update maps and charts

Personally, my iPad is WiFi only, and I pair with an external GPS receiver for great accuracy.

I use the "regular" iPad (9.7"), but that is a little bulky in the cockpit of a 172.  The Mini may be preferable if space is at a premium.


Can Bell-lift distribution increase glider efficiency(L/D)? Can a Bell-lift distribution without a rudder has higher overall efficiency (L/D) then current gliders, if the wingspan is not limited?
(Is it really possible that most of aerospace engineers don't understand the essence of Al Bowers' PRANDTL project?
What is stopping the aero-industry from accepting his "new" design ideas? Is he wrong, or do they not want the risk of an unproven concept?)
AL Bowers say at 13:20 that Bell distribution increase L/D ratio by 12.5% ! ?

Try to fly fast and you will see who hasn't understood the concept.
Just one datapoint: The lack of a rudder might be acceptable in a glider (but Heinz Scheidhauer, the pilot with most experience on Horten gliders, clearly said that at high speed the Horten gliders would be directionally indifferent) but not in powered flight. The fatal crash of the Ho-IX V2 can be attributed to loss of directional control after one engine failed.
On the SB-13 the vertical was replaced by two winglets which gave sufficient directional stability. However, when operating the plane, we often wished it had a conventional tail. Flight testing showed no clear advantage over current standard class gliders but operational restrictions were many.
All flying wings are susceptible to flutter because of their low pitch damping. In some Horten designs, flutter would limit the speed range at both ends.
Next: The low pitch stability forces you to use airfoils with low pitching moments. Trailing edge flaps can only be used in a very limited way, nowhere near what is usual on airliners. This limits the speed range almost as much as flutter does already.
And concerning the bell lift distribution: That lesson has been learned since maybe 70 years. Just look at the actual planforms: They are not elliptic and have little lift towards the tips.


Why are the character keys on the Flight Management Computer in alphabetical order? Regardless of which aircraft type and manufacturer, the character keys on any Flight Management Computer are always in alphabetical order, see image:

From a human factors view, this is sub-optimal, so why? Where does this order come from?

From chats with pilots I learned that you get used to it easily and can type quite fast with this alphabetical layout, but still, we are all used to the QWERTY or QWERTZ layout...

"we are all used to the QWERTY or QWERTZ layout..." or AZERTY. But which one? Universally there is no one standard, so whichever of these 3 (and others) is chosen, it will be wrong for some users. So the only standard left is simple alphabetic.

Another thing to consider is that the QWERTY-types of layout were chosen to deliberately slow down typists on mechanical typewriters. This slow-down is not necessary on an electronic piece of kit, so the pure-alph format is quite fast once you're used to it.


Is it possible to maintain horizontal flight with the angle of attack above critical value by increasing speed? In this video a stall happens at 1:40 upon reaching the critical angle of attack. The air flow is now detached from the upper part of the wing, resulting in higher pressure above the upper part of the wing, thus eliminating the major factor of lift.

Let's imagine that we have extremely powerful engines and enough tail stabilizer authority to maintain horizontal flight with any speed.

Is it possible to maintain horizontal flight when:

wings have high angle of incidence (let's assume some experimental design)*
aircraft has zero pitch condition (straight and level flight)
permanent state of high angle of attack (as the consequence of the 1 and 2)

*high angle of incidence is an angle between wing chord and longitudinal axis of the plane (fuselage)

I assume, the lift that we will get in this case, will be generated mostly by Newton's law by deflecting air ONLY by the LOWER side of the wing.

That is kind of what the mechanism of lift of a flat plate does: the sharp leading edge destroys the upper flow pattern almost immediately. This question has more info on it.

The lower side of the wing does provide as much or more lift, even when stalled, at AoA of 45°, so lift would not be the problem. Drag would be:

Image source

On this scale, drag for the NACA 0012 profile is very close to zero until the aerofoil stalls, after which it jumps up and continues climbing. At AoA of 45° the C$_D$ is 1.1 instead of about 0.02 when not stalled - you need over 50 times the thrust to maintain the stalled condition you are describing!


Why are some legs of a SID considered transitions, even when there's only one? For example, consider the BAYPO8 departure from Tampa (KTPA). The CAMJO waypoint is the final waypoint on a rather linear route that does not contain other final waypoints. The second page of the FAA plate says that CAMJO is a transition.

But if you look at the ENDED8 departure (also from Tampa), there is no such designation on the LACEN waypoint. Although, coincidentally, it was considered a transition in previous AIRAC cycles.

After some digging and exploratory route planning in ForeFlight, it seems I've arrived at an answer that satisfies me: the last waypoint is listed as a transition only if the departure officially ends prior to it, and flying to the transition is optional. If there are other (more correct or plausible) answers, I'd still love to hear them.

The text description of each plate describes where the SID ends. In the case of BAYPO8, the last instruction is on-track to BAYPO. In the case of ENDED8, the text describes on-track to LACEN. So, it would seem that flying to LACEN is required when assigned the ENDED8 departure, however flying to CAMJO is optional for the BAYPO8, because you can drop off the procedure after BAYPO.

This is supported by the route planning features of ForeFlight. When I selected BAYPO8, and then "No Transition," my route of flight for the departure ended at BAYPO. I also had the option to choose the CAMJO transition. This is also supported by analysis of other similar departures that include only one final waypoint option.


Why are flight recorders generally located in the tail? Is there a particular reason why FDRs and CVRs and associated similar bits of equipment are kept in the tail section of a plane?

When an aircraft is crashed, the entire front of the aircraft is expected to act as a crush zone. By placing FDRs and CVRs near the tail, the shock wave that reaches FDRs and CVRs is reduced significantly.

The probability of survival based on seating is shown in this picture:

(Image Source)

Hence one can see that the shock wave reduces as it reaches the rear of the plane. That is the reason FDRs and CVRs are installed in aircraft's empennage section.

(Image Source)

Please note that the precise location of the recorders depends on the individual plane. Sometimes they are located in the ceiling of the galley, in the aft cargo hold or in the tail cone that covers the rear of the aircraft.


Can coaxial rotors be mounted on a piston engine like the Lycoming O-320? What are the drawbacks of this compared to coaxial configuration with a turboshaft engine?

Stan Hiller's earliest helo designs were coaxial and used relatively small 4-cylinder aircraft engines to drive the coax transmission, so it is indeed possible to power a small coaxial helicopter with a relatively small piston engine.
Hiller also found that autorotating a coax main rotor blade setup was difficult and so his later designs used a more conventional single main rotor blade and a tail rotor.


What's the minimum number of sensors for a hobby GPS waypoint-following UAV? I would like to develop my own guidance, navigation and control (GN&C) flight software on a Raspberry Pi, for a small hobby fixed-wing UAV. From a physical standpoint, what is the minimum set of sensors/inputs needed to implement plain old GPS waypoint-following?

I understand I have a lot of math to learn, but what is the minimum set of inputs for that math? I have experience with embedded C++ for vehicle entertainment systems, but no knowledge in the aviation realm.

The absolute minimum for a generalized vehicle that needs to know its position and attitude (orientation) in space is one per degree of freedom. This can be reduced if we have information about the natural modes of the system and their stability.

For simplicity, let's assume a vehicle moving in 3 dimensions, that means a total of 6 DoF:

3 coordinates in space to know the position
3 angles (or similar) to know the attitude

The simplest way to meet these requirements are accelerometers for the XYZ coordinates and gyroscopes for the angles, and these are often packaged together into an IMU. Technically you don't even need an actual GPS as long as you know the coordinates of the starting location, as you can just integrate from it to know your position. This is known as dead reckoning via inertial navigation and it works this way (let's use coordinate $x$ as an example):

You need to know the initial value $x$ and its rate of change: $x_0, \dot{x}_0$
Listen to the values of the $\ddot{x}$ accelerometer.
Integrate the acceleration to obtain the speed: $\dot{x}=\int{\ddot{x}\,dx}+\dot{x}_0$
Integrate the speed to obtain the position: $x=\int{\dot{x}\,dx}+x_0$

Of course this has multiple shortcomings. Real-world accelerometers have noise, gyroscopes have drift, your starting location is probably inaccurate, your vehicle likely needs more data than that to operate and the whole systems I just described had no way of finding itself if it resets in-mission.

For a realistic barebones project like you described you will need:

3 accelerometers (in an IMU)
3 gyroscopes (in an IMU)
3 magnetometers (usually come with the IMU and help keep the previous 2 in line)
GPS (can be bundled with the IMU and you specifically wanted it)

Further data sources are desirable but greatly complicate the FCS architecture, since you need to properly weight the data, after all, you don't want the aircraft to trust the magnetometer more than the gyros and show you a little trick it learned when passing near a magnetic anomaly.

In particular, a Pitot probe is good to have know your airspeed as opposed to your ground speed. An alpha vane is a bit of a luxury and not needed unless you are pushing the flight envelope.

Any ground-watching sensor with sufficient update rate, range and accuracy (IR, laser, LiDAR, acoustic, etc) will allow you to smooth your landings, and which one you pick should depend on your design and budget; they all have pros and cons.

From the coding standpoint, if you are serious about doing this by yourself from scratch, you should look into real time systems, Kalman filters and the vast field of sensor fusion. Also learn some aerodynamics while at it, so that the airframe itself is not a black box to you. You can run an FCS on an Arduino, even for quadcopters, if you are content with just a basic SAS.

There is a large community of hobby UAV builders online that can provide you with an almost off the shelf solution for your FCS (Ardupilot), if you prefer that approach. I personally often find their documentation lacking, so I suggest you at least learn the basics behind what you are doing, to help you navigate the inevitable gaps in the manual. Also, since it is an open source project, you can then fill in those gaps.


How does the VOR receiver pick up the signal and how are the needles controlled? Knowing the basic principle of a VOR and how it works, I am interested to know how the receiver works.

What exactly makes the needle deflect? And how does this deflecting mechanism know how much to deflect the needle?

So here is what I am thinking so far: since the signals are sine waves (I guess) and there are two of them, if you lay them over each other there should be a unique voltage for every direction from the VOR. To keep on guessing, this voltage is probably amplified and put onto a electric magnet that will then control the needle's deflection.

Your basic understanding of the underlying electronic principles is correct: The VOR receiver detects a phase offset between two signals, which tells the receiver what radial the aircraft is currently on. Wikipedia has a decent explanation, and this article on Digital Flight Instructor has a nice widget you can play with to see the frequency offset.

  The underlying electronic principles the receiver uses are generally not useful for pilots: if your VOR receiver dies you're not going to hook up an amplifier to an oscilloscope and try to sort out the phase offset yourself, if for no other reason than your head would be buried in the cockpit staring at the oscilloscope as opposed to looking out the window for traffic or watching your flight instruments.
  In addition, the "strobe and sweep" explanation is generally easier for students to understand: Imagine the VOR station has two lights, a white light that rotates, and a red strobe that fires ever time the white light passes the 0 degree (North) radial. By counting the time between when you see the red strobe and when the white light passes you its possible to determine what radial you're on.

The mechanism by which the needle is deflected depends on the type of indicator, but it basically boils down to "The receiver does some magic and converts it into a number of dots of deflection on the indicator". The deflection can displayed by moving needle attached to a meter (applying a specific positive or negative voltage), blinking some LEDs, or showing a graphical representation of the CDI (e.g. on a GPS display) with a mark over the appropriate dot.
How this works for each specific indicator is generally only of interest to an avionics technician, and the information would be contained in the service manual for the radio and indicator.


What are the merits of an inlet cone/spike/centerbody vs. a 2D inlet? And which is better for a scramjet?

Is a 2D inlet basically a "2D cone"? Similar to how an aerospike engine is made "2D" in the linear aerospike?

What's inside the cone/spike? Or is it just hollow?

Both are ways to create compression shocks in order to compress and decelerate supersonic air.
Spike intakes
The central spike is characterized by:

undisturbed flow,
light weight and compactness, and
easy adjustability to speed and mass flow at high flight speeds by translating the spike.

Its disadvantages are:

large loss of efficiency with angle of attack and sideslip,
long internal ducting,
limited space for radar installation, and
limited mass flow at low speed.

Typical central spike intakes are those of the MiG-21 or the SR-71, as shown below. Note the variation in cone angle which helps to create additional, steeper shocks such that the flow is gradually decelerated over a sequence of shocks. The inside is filled with a radar system or hollow in case of the SR-71. At the height of the intake lip the SR-71 spike has a perforated wall to suck away the spike's boundary layer, and aft of that it contains the translation mechanism which shifts it back and forth to adjust the intake cross section and to adjust the shock position.

A variant of the central spike is a side intake with a compression half-cone, as used on the Mirage III, the F-104 Starfighter or the TSR-2. Their location at the fuselage side helps to reduce the length of internal ducting and to reduce flow distortion due to sideslip. It also frees up space at the fuselage tip for a larger radar antenna. However, now the fuselage boundary layer has to be split off by a splitter plate to ensure equal flow energy over the whole intake cross section.
The picture below shows the English Electric Lightning with its central spike intake feeding two Rolls-Royce Avon engines, and the side intake of the TSR-2. Note the additional intake doors aft of the intake lip of the TSR-2 which were needed to increase mass flow at low speed.

The F-111 has a quarter-circle spike intake. Here, the wing root also helps to direct the flow, so angle-of-attack variations produce less intake flow distortion. To suck away the intake boundary layer, the spike is perforated at the lengthwise position of the intake lips.

Ramp intakes
What you call 2D-intakes are normally rectangular with an oblique shape. The forward edge is used to create an oblique shock which can be positioned to hit the rearward edge on the opposite side by shifting one or both edges up and down. This also changes the cross section of the intake, so adjustments for mass flow can be made over a wide range. An adjustable ramp aft of the forward edge allows to create more shocks with decreasing obliqueness, such that the flow is decelerated by a sequence of shocks. Typical ramp intakes are those of the F-14, the F-15 or the Concorde. Their advantages are:

Adaptability over a wide range of flow conditions, angles of attack and mass flows, and
tailoring of the shock system for optimum efficiency over a wide range of supersonic speeds.

Their disadvantage is a consequence of the wide adaptability:

High weight and complexity.

Note that the pressure in the intake is a multiple of outside pressure at supersonic speed, so the intake is actually a pressurized duct. A rectangular intake will need massive structural reinforcements to stay in shape while the cross section transitions to a round shape downstream.
The drawing below shows the intake of the F-14 at different flow conditions. Note that the sharp intake lips require them to be exactly aligned with the local flow direction to avoid flow separation.

While a central spike intake is the best choice for flying at the design speed, the rectangular intake works better over a wide range of speeds. The shift in air combat doctrine from flying as fast as possible (1950s) to flying mostly subsonic with brief supersonic dashes (since the 1970s) is reflected in the choice of intake type.
Hypersonic intakes
When flying at Mach 5 and above, the obliqueness of the shock waves will need a very long intake, and effective designs use the lower side of the forward fuselage as part of the intake to create the necessary sequence of shocks for pressure recovery. Now the fuselage nose becomes the forward lip of the intake, and the body is shaped to create ever steeper shocks. Alternatively, the lower forward fuselage is shaped to work like a section of a spike. What looks like the actual intake is actually the rear intake lip, and the divergent section of the intake aft of the rear lip is reduced since the flow in the combustion area is still supersonic in order to keep temperatures manageable, making the engine a scramjet. This answer has more information on supersonic combustion.
Note that this type of intake is optimized for one speed, and very little adjustment is possible. The pictures below show a NASA CFD study of the X-43A (left, source) and the Russian Ajax project (right, source). A scramjet can use both types of intake, and the fuselage design will determine which type it will be.

The rear fuselage now becomes the nozzle of the engine, because most of the expansion will happen after the flow leaves the enclosed part of the engine.


Why do some airfields not have manned ATC towers? I was wondering why some airfields, for example Tatenhill Airfield (EGBM), don't have anybody in their ATC towers?

One of the biggest reasons to not man a tower is (not unsurprisingly) costs.

Many remote airports only have a few movements per day, and a permanent staff in the tower makes it even more difficult keep the operation economically viable.

For this reason, remotely staffed towers are being developed now, and for instance Örnsköldsvik airport (ESNO) in Sweden has just received approval from the Swedish Transport Agency to operate with a remote tower:

  The system’s sensors are installed at Ornskoldsvik Airport but traffic
  is remotely controlled from Sundsvall, Sweden.


How do you lean mixture using only the instruments on a C172 Skyhawk? POH manuals always tells to use the voice of the engine as a reference. (Which complicates things a little bit since carbs and fuel injected planes have different "reaction times" for each cylinder)

But how do you lean the mixture without using your ears? How do you lean mixture with only the instruments as a guide?

Lets assume we have a brand new C172 Skyhawk

The procedure varies depending on the engine instruments in the panel. Here’s an good explanation using the EGT probe on a Cessna 172.

And here are the manual pages from a Cessna 172SP that explains it.

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What are some drawbacks to having a wingtip propeller on an aircraft? I was reading about aircraft concepts that involved wingtip propeller design and was wondering what the drawbacks of such a technology would be. I could not access the full paper but came across this: https://arc.aiaa.org/doi/pdf/10.2514/3.44076

For me, one of my concerns is that if the propeller fails, feathering would be very difficult on the wingtip prop. I am also trying to decide on optimal placement for aircraft propellers in general. Would the drawbacks from having a wingtip propeller outweigh the benefits in high-lift generation and drag reduction?

Feathering isn't really an issue, if you can feather a prop on an engine further inboard you can do it on the tip too. There are 3 major drawbacks that come to mind:

The wing structure has to be stronger: engines are heavy, the further out they are the beefier the structure has to be to hold them. Stronger wings mean more weight and possibly a thicker cross section. Neither are good traits in a wing
Decreased roll rate: the farther the engines are the greater the moment arm and the slower your roll rate will be. Think about skaters spinning around, the farther their arms from their body the slower they spin. The same principle is at work here, so you need bigger ailerons to give you maneuverability, so more weight, cost and complexity
Safety in a single engine failure scenario: engines on the wingtips will cause more yaw in a single engine failure than engines further in board, so you'll need a bigger rudder to counteract it. A bigger rudder means more weight and cost, and there are also limits - eventually you will get to the point you can't counteract the force effectively and an engine failure will cause a loss of control. Yaw onset will be faster as well, giving a pilot less time to react, and there's nothing you can do about that; a bigger rudder doesn't help with reaction time. Mechanical cross-connections could be used to share power across the wing in the case of a single engine failure, like the V-22 Osprey, Chinook helicopter, however these increase weight, cost and complexity. Also, these systems aren't perfect, a single engine failure is still a possibility


What are these "patches" on an Airbus A321 wing? While on a WizzAir flight I noticed on the wing the patching marked in this image:

I'm just curious, not implying lack of safety or whatever else. Why the wing is patched that way? I mean why the two small patches, the apparently missing a bit of the cover, and...why the leftmost patch is between other parts?

I am not familiar with that particular aircraft but I would hazard to guess it is not a patch.  I say this as I have never... in 20 years of aircraft maintenance... seen a patch that incorporated screws.  If this were a patch for a crack in the sheet metal it would be rivets all the way around.

AC 43-13 standard practices is the general reference for skin structural repair although manufacturers will publish their own Structural Repair Manual with their own preferred methods.  And certainly I have seen design Engineers come up with odd things to deal with AD's etc.  But I have never seen screws used as a method for installing a patch.

So... I would say it is likely a designed in strengthening piece. It is also a single piece if you look at it carefully it contains a Joggle which makes it look like two separate pieces.  A Joggle being a step in the metal to account for two different surface heights.


Is it illegal to partially block the aisle on a passenger airliner? After reading all the commotion about the dog that died after its carrier was placed in the overhead bin I wondered- is it against any FAA regulations to allow a flight to take off with a partially blocked aisle?

Cargo cannot block the aisle -
FAR Part 121.285, governing U.S. Air Carrier Operations, states in pertinent part:
FAR 121.285-
(emphasis mine)

(c) Cargo may be carried aft of a bulkhead or divider in any passenger compartment provided the cargo is restrained to the load factors in §25.561(b)(3) and is loaded as follows:
(4) Its location does not restrict access to or use of any required emergency or regular exit, or of the aisle in the passenger compartment.


Is there a list of what information different airlines track about their flights? It has been suggested in the media and in some answers here that airlines vary in the information they track about the status of their flights. Is there a publicly available resource that lists what information different airlines have about the location and status of their flights? For example, could a British Airways flight over the Atlantic or the middle of the Pacific have "vanished" in the same way that the Malaysian flight over the Gulf of Thailand?

There is no particular list that provided by airlines company about their flights because it can be misused, but several statuses are provided:

(from http://www.flightstats.com/go/AirportTracker/airportTracker.do#1)

  Geographic Coverage - FlightStats provides definitive information for approximately 99.5% of U.S. flights, and better than 86% of flights worldwide.

  Completeness - FlightStats queries multiple sources to create a record for each flight enabling us to offer a broader range of information (for example, gate information).

  Accuracy - We have invested heavily in the areas of parsing, interpretation and error checking and developed the logic that enables handling of difficult issues such as cancellations, diversions and changing schedules.

  Codeshare Support - Our codeshare logic enables us to deliver flight information for both the operating and the marketing carriers, filling what is often a major gap in coverage.

  Real-time data sources include:

  FAA ASDI Data Feed
  European Data Feed
  GDS (Sabre, Amadeus, Apollo, Galileo)
  Direct Airport / Airline Data Feeds

  Batch data sources include:

  Innovata Schedules
  TSA Security Wait Times
  Security Information
  Health Information
  Consular Information


How to compare turboprop engines and with which values? I have found this website that lists a lot of turboprop engines. I would like to compare them with as few values as possible. As an example, there are tons of different values that describe a jet engine, but the most important/significant values are thrust and bypass ratio. Out of the many different values for a turboprop engine, which would be the few most important/significant to include?
My goal with the data is to approximate (very roughly) comparative fuel usage.

EDIT: To clarify the question, what are the most important engine values/specifications that can be used to estimate the general fuel economy of the turboprop engine?

This database covers turboshaft and turboprop engines. Typically, the key factors one would use to compare such engines are:

SFC: Specific fuel consumption. This is how many kg/s (or lb/s) of fuel the engine consumes to produce 1 kW (or 1 hp). To calculate the SFC, divide the fuel flow at a throttle position by the amount of shaft power produced at that throttle setting. SFC decreases as overall pressure ratio increases. A low SFC means a more fuel efficient engine.
Specific power. This is, how much shaft power (kW) is produced per kg of air ingested by the engine. To calculate this, divide the shaft power by the air mass flow. A high specific power means you can get the power you need from a smaller, and hence lighter engine. A higher figure occurs when the engine has a high turbine temperature, and aerodynamically efficient compressors and turbines.
Power to weight ratio. To calculate this, divide the shaft power produced by the engine weight. As above, a higher figure occurs when the engine has a high turbine temperature, and aerodynamically efficient compressors and turbines.