How does the EPR reading behave in reverse, and in reheat modes? Will an engine pressure ratio indicator (EPR) show the same at full mil thrust as with full reversed thrust?
What does the EPR indicate at afterburner active? Will it just show the same as for max mil thrust?

What is EPR?

[It] is the total pressure ratio across a jet engine, measured as the ratio of the total pressure at the exit of the propelling nozzle divided by the total pressure at the entry to the compressor.

Placing an EPR probe in the A/B (afterburner) tube would not be a wise choice, and therefore in an A/B aircraft you wouldn't find an EPR reading. What is more useful is the NP (nozzle position). As that is a good indication of A/B level. N1 is still used of course.
In the civil world, GE and CFM don't have EPR sensors/readouts, unlike P&W and RR.
Not having an EPR avoids issues like EPR probe icing. But the downside is that it's harder to compute engine FADEC parameters.
More info here: What is the difference between EPR and rotor speed as thrust setting parameter?
Below are the F-14 engine instruments (analog tapes and dials are also present in the cockpit).
TBT is turbine blade temperature. EGT is also not used as it too would not survive the A/B stream.
EPR and Reverse
On the Boeing 747-400 with RR or P&W engine options, the EPR window is blanked when reverse is activated.
What the engine is redirecting is the bypass flow. So the EPR will still read as normal, which can be quite confusing to the crew especially in max reverse, hence the blanking.


Airspeed Calibration alternate static source, what is this for? I found a 'Airspeed Calibration Alternate Static Source' Figure in C172R POH.

There are three kinds of Figure,
1. Heater off, Vents and Windows Closed
2. Heater On, Vents Open and Windows Closed
3. Windows Open.

What are those figures for?

This table is included because of the fact that the Alternate Static Port on 172s is inside the cabin. Thus, changes in cabin pressure (even really small ones) can have an impact on the static pressure that the system is using and hence on the indicated airspeed.

The things the table mentions like windows and heat vents all influence the pressure in the cabin, and hence the airspeed. In reality, if you have to use the alt static port you aren't going to be pulling out the POH to correct the speed as it is likely very similar.


How to calculate Equivalent Airspeed immediately from Calibrated Airspeed? I want to calculate each step like IAS -> CAS -> EAS -> TAS
as a chain. Each chain step should depend on the previous step.

IAS is clear. This is a function of pressures. With the Bernoulli equation, IAS = f(p_total - p_static)

CAS is not clear. But I know that static error is taken into account, CAS = f(IAS,static error)

EAS is not clear. I know that EAS takes compressiblity effects into account.

TAS is again clear, dependent on density it is easy to calculate with TAS = f(EAS, rho, rho_0)

How to calculate EAS from CAS immediately
with EAS = f(CAS)?

I know that one can calculate EAS with
EAS = f(Mach, a_0), also by taking in account of
the compressibility of air.

Thank you very much!

I wanted to add to the very detailed and helpful answer above to clarify some things as hopefully it might help someone. Most pilot ground studies courses approach this from the viewpoint of how to go from IAS to CAS to EAS to TAS but skip a lot of detail, which I think is what is frustrating the OP. So here's a deep dive. Also, the previous answer ends with "Solving it for EAS = f(CAS) is left to the reader". Since that's sort of what the OP was asking I thought it might be worth a crack at it, although from a different angle.

IAS (Indicated Air Speed)

This is the speed that the air speed indicator in the cockpit displays. The best way to think of the ASI it that it's a pressure gauge that measures differential pressure (pitot minus static) only it's calibrated in knots (or whatever.) As you say in your question, IAS is a function of (p_total - p_static). Is to what that function is, we'll come to that in a moment.

CAS (Calibrated Air Speed)

What the instrument should have read if the instrument and the pitot/static system were perfect. In other words, the IAS after correcting for position errors (e.g. effect of flap setting on pitot/static system) and instrument errors. CAS can be derived from IAS by using look-up tables but these are aicraft-specific. In modern systems, this is done automatically by the Air Data Computer, so the ASI will directly read CAS. In other words, CAS = f(IAS, hardware errors, Mach, AoA, flap setting, ...) To find out this function, you have to do computational fluid dynamics, wind tunnel testing, flight testing or all three. So it's non-trivial. I can't just give you an equation for it. The good news is, in straight and level flight, the errors aren't usually very big.

EAS (Equivalent Air Speed)

This is where things get weird and annoying. Many aviation courses and textbooks treat EAS as a "stepping stone" to TAS but it's almost never used that way in reality and these sources never tell you how to calculate EAS from CAS. It's usually easier to go straight to TAS from CAS, e.g. using a CRP-5 flight computer (a kind of circular slide rule) or by let the aircraft work it out for you via the Air Data Computer. As a pilot, you very rarely need to actually calculate EAS. CAS, TAS and Mach number are usually far more useful.

EAS is supposed to be a version of CAS corrected for "compressibility errors". This might lead you to think EAS is necessary because the ASI uses some simple non-compressible flow assumptions, which need to be corrected at higher Mach numbers. However, at sea level, EAS is the same as CAS, regardless of airspeed, by definition Wikipedia's definition. I.e. you can be in a very compressible flow regime, like Mach 0.8, and EAS is still the same as CAS. This initially makes no sense until you realise that the definition of EAS has to make an assumption about how an air speed indicator actually works.

One might think the airspeed indicator is calibrated by simply reversing the well-known formula (from Bernoulli) for static pressure, namely:

$q = \frac{1}{2}\rho v^2 $,

like this:

$v_{IAS} = \sqrt{\frac{2q}{\rho}} $,

where $\rho$ is the air density and $q$ is the dynamic (pitot minus static) pressure.

This is not so. Here's where we get to "things they don't tell you in ATPL ground exam courses." It seems that the standard definitions of EAS makes the assumption that the airspeed indicator actually works by using isentropic flow relations. This would require it to know things like the static pressure (it only senses differential) and the local speed of sound (which requires knowledge of outside air temperature.) However, it is still possible to use this approach if you "cheat" and put in sea level values for all the things the instrument doesn't know. In other words, the ASI calculates IAS according to:

$v_{IAS} = a_0 * \sqrt{5\left[\left(\frac{q}{p_0}+1\right)^{\frac{2}{7}} - 1\right]} $

Where $a_0$ is the ISA (international standard atmosphere) sea level speed of sound (340 m/s), $q$ the dynamic pressure as before (aka impact pressure) and $p_0$ is the ISA sea level pressure (101325 Pa).

Note that the magic numbers 5 and 2/7 arise because of the value of $\gamma$ (gamma), the ratio of specific heats for air, which equals 1.4. The more general form is:

$v_{IAS} = a_0 * \sqrt{\frac{2}{\gamma-1}\left[\left(\frac{q}{p_0}+1\right)^{\frac{\gamma-1}{\gamma}} - 1\right]} $

This relation is a well known thing but I don't know how to derive it. Yet. (In any case, this answer is already long enough.)

It turns out that if you take a binomial approximation of the 2/7 power and neglect second order terms and higher, you get this:

$q \approx \frac{1}{2} 1.227 v^2$

The value 1.227 being very close to the ISA sea level density of 1.225, so it is equivalent to Bernoulli for low Mach numbers. Anyway, I digress.

Calculating EAS

From here on, we'll asssume IAS=CAS. Now let us assume you want to find the EAS given only the IAS/CAS and the altitude. Normally, as a pilot, perhaps tackling an ATPL exam question, you'd use a CRP-5, but let's assume we are proper nerds and we want to code this (e.g. in python) or get a better undersanding of how an ADC (Air Data Computer) or ADIRU (Air Data Inertial Reference Unit) works.

The EAS is a function of the CAS, the dynamic pressure and the static pressure.

Finding static pressure from altitude

The static pressure can be inferred from the altitude using the International Standard Atmosphere. For the troposphere (i.e. up to 11km or 36,080ft) the pressure is calculated by solving the hydrostatic equation for a constant lapse rate. Dealing with the tropopause and above is left as an exercise for the reader :-P

$p_s = p_0 \left( \frac{T_s}{T_0}\right)^{\frac{g}{L R}} $,

where $p_s$ is the static pressure at altitude, $p_0$ is the ISA sea level pressure (101325 Pa), $T_0$ is the ISA sea level temperature (288.15K, $g$ is the acceleration due to gravity (9.81m/s2), $L$ is the lapse rate (0.0065K/m), $R$ is the specific gas constant for air (287 J/K/Kg) and $T_s$ is the static temperature at altitude:

$T_s = T_0 - L h$,

were $h$ is the altitude in metres.

Finding the dynamic pressure from CAS

Note that the ASI does not tell you (the pilot) the dynamic pressure directly, it only tells you it in terms of the IAS/CAS. So we have to "reverse" the formula above:

$q = \left[\frac{\left(\frac{v_{CAS}}{a_0}\right)^2}{5} + 1\right]^\frac{7}{2} - 1$

Finding the Mach number

Next we need to find the Mach number. Note that this is a very similar equation to how the ASI calculates airspeed, but this time we use the real static pressure, not the sea-level value:

$M = \sqrt{5\left[\left(\frac{q}{p_s}+1\right)^{\frac{2}{7}} - 1\right]} $

The EAS is then defined as:

$v_{EAS} = a_0 M \sqrt{\frac{p_s}{p_0}} $

In other words, EAS uses the real Mach number but the sea level speed of sound to obtain a speed, and then multiplies this by the root of the pressure ratio. Putting this all together:

$v_{EAS} = a_0 \sqrt{5\left[\left(\frac{q}{p_s}+1\right)^{\frac{2}{7}} - 1\right]} \sqrt{\frac{p_s}{p_0}} $

Epic.

TAS

TAS can be obtained from EAS, which in effect gives another formal definition of EAS:

$v_{TAS} = v_{EAS} \sqrt{\frac{\rho_0}{\rho}} $,

where $\rho$ (rho) is the actual air density at altitude (which can be calculated from temperature and pressure using the ideal gas law ($\rho = \frac{p_s}{R T_s}$) and $\rho_0$ is the ISA sea level density (1.225 kg/m3.) In other words, TAS is just the EAS divided by the root of the density ratio.

However, it's actually easier just to calculate TAS directly, which saves knowing about densities:

$v_{TAS} = a M $

where $M$ is the Mach number as calcuated above and $a$ is the local speed of sound (not the sea level value like the in the EAS formula,) which can be calculated from $a = \sqrt{\gamma R T_s}$.

#!/usr/bin/env python3
#
# Calculate EAS and TAS, given CAS and a pressure altitude
# Warning: only works in troposphere, i.e. below 11km
#
# By Halzephron 2020
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
# THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

from math import pi, sqrt
import sys

ms_per_kt = 0.51444
feet_per_metre = 3.28
T_0C = 273.15 # 0 degrees C in Kelvin

# Some useful constants

g=9.81          # Acceleration due to gravity
R=287           # Specific gas constant for air
L=0.0065        # Lapse rate in K/m
T0 = 288.15     # ISA sea level temp in K
p0 = 101325     # ISA sea level pressure in Pa
k = 1.4         # k is a shorthand for Gamma, the ratio of specific heats for air
lss0 = sqrt(k*R*T0) # ISA sea level speed sound
rho0 = 1.225    # ISA sea level density in Kg/m3

# Return knots given m/s
def kt(m):
    return m/ms_per_kt

# Return pressure ratio given a Mach number and static pressure,
# assuming compressible flow
def compressible_pitot(M):
    return (M*M*(k-1)/2 + 1) ** (k/(k-1)) - 1

# Return Mach number, given a pressure ratio d=p_d/p_s
def pitot_to_Mach(d):
    return sqrt(((d+1)**((k-1)/k) - 1)*2/(k-1))

# Given an altitude h, return the temperature, assuming we're
# using the International Standard Atmosphere and are flying
# in the troposphere.
def temperature(h):
    return T0 - h*L

# Given an altitude h, return the local spead of sound, assuming
# we're using the International Standard Atmosphere and are flying
# in the troposphere.
def lss(h):
    return sqrt(k*R*temperature(h))

# Given an altitude h, return the pressure, assuming we're
# using the International Standard Atmosphere and are flying
# in the troposphere.
def pressure(h):
    return p0 * (temperature(h) / T0) ** (g / L / R)

# Given an altitude h, return the density, assuming we're
# using the International Standard Atmosphere and are flying
# in the troposphere.
def density(h):
    return pressure(h) / (R * temperature(h))

if len(sys.argv) < 2:
    print("usage: {} CAS ALT".format(sys.argv[0]))
    exit(0)

cas = float(sys.argv[1])*ms_per_kt # Convert kts to m/s
alt = float(sys.argv[2])/feet_per_metre  # Convert ft to m

ps = pressure(alt)
lss = lss(alt)
oat = temperature(alt)
rho = density(alt)

# First we need to "reverse" the air speed indicator to find the dynamic
# or "impact" pressure. It is tempting to assume that an airspeed
# indicator relates airspeed to dynamic pressure using Bernoulli's
# 0.5 * rho * v**2, but it that is not the case here. Instead,
# a sort of modified compressible flow equation is used. A "pseudo"
# mach number is found as a function of a pressure ratio, assuming the
# static pressure is equal to ISA conditions. The airspeed is then found
# by assuming the local speed of sound is that of ISA sea level.
pd = compressible_pitot(cas/lss0) * p0

# Find Mach number
M = pitot_to_Mach(pd / ps)

# Calculate EAS (equivalent air speed)

eas = lss0 * M * sqrt(ps/p0)

# Calculate TAS (true air speed)

tas = lss * M

# Or we could have used tas = eas / sqrt(rho/rho0)

print(f"Pressure Altitude:     {alt*feet_per_metre:5.0f}   ft")
print(f"Static Temperature:      {oat-T_0C:5.1f} C")
print(f"Density Ratio:            {rho/rho0:6.3f}")
print(f"Pressure Ratio:           {ps/p0:6.3f}")
print(f"Static Pressure:        {ps/1e2:6.1f} mb")
print(f"Dynamic Pressure:       {pd/1e2:6.1f} mb")
print(f"Total Pressure:         {(pd+ps)/1e2:6.1f} mb")
print(f"CAS:                    {kt(cas):6.1f} kt")
print(f"EAS:                    {kt(eas):6.1f} kt")
print(f"TAS:                    {kt(tas):6.1f} kt")
print(f"LSS:                    {kt(lss):6.1f} kt")
print(f"Mach:                      {M:4.2f}")


How is the pitot system drained? Since pitot static systems are open the outside, how does the system remain clear of water or other contaminants?

Though it's often omitted from diagrams, most pitot tubes have one or more drain holes connected to the pressure chamber, and are essentially "self-draining" (water doesn't make it into the lines):

In addition to drains on the pitot tube itself some aircraft have separate "drip chambers" and drains for the pitot (and occasionally static) system to remove condensation or rain that got past the drains (marked as 2 in the photo below):

Regarding other contaminants, at altitude the air is generally pretty clean.
The biggest problem you'll have with contaminants in the pitot tube is usually bugs (either building a nest in the opening, or smacking into it in flight). Pitot covers can be used on the ground to prevent this (and they're also used when washing or painting aircraft to prevent water or paint from getting into the system). Often some tape serves the same purpose - but you need to be sure to remove the tape or cover before flying.


How is the flight of a tricopter controlled? For a quadcopter, diagonally opposite rotors have the same speed and direction which causes it to hover, and adjusting the speed of the rotors give the three basic movements of a flying object, i.e. pitch, roll, yaw.

Similarly, what is the physics behind the tricopter flight control?

There are few examples of three-rotor helicopters. On of them is the Cierva W.1 Air Horse. According to Wikipedia, torque balance, and hence, yaw control
'was provided by slightly inclining each rotor axis to generate horizontal thrust components to provide anti-torque moments. The three rotor configuration was foreseen by Belgian helicopter experimenter Nicolas Florine in his patent of 1926 which presented the aforementioned means for balancing the reaction on the fuselage of two or more torque driven lifting rotors turning in the same direction.'

https://en.wikipedia.org/wiki/Cierva_Air_Horse

Pitch and roll control was obtained by regulating the lift of the rotors.

https://www.flightglobal.com/pdfarchive/view/1948/1948%20-%202048.html


How much responsibility does ATC have to alert a flight about being in a dangerous situation? There have been quite a number of accident reports referenced here at Aviation.SE about commercial (passenger or freight) flights executing controlled flight into terrain on approach for landing. The accident reports always seem to revolve around the pilots not knowing they are too low, failing to recognize or respond to the GPWS, etc.

While I understand that the pilot in command has the final say on what happens on board the aircraft, ATC is responsible for giving the PIC instructions to get him safely to the end of the runway.

In the case where ATC has given the PIC the approach route, whether VFR (flight following) or IFR, how much time, ability, and requirement does ATC then have to monitor that flight to ensure that it's on the expected course and at expected altitudes? In other words, how much responsibility does ATC have for failure to monitor the situation and allow these accidents to happen?

If necessary, differentiate between different CAAs, but I would think the answer would be fairly similar around the world.

There are a couple of nice answers, but they're not addressing (in my head, at least) the question I was attempting to ask, so it seems I didn't ask it well. I'll clarify further:

In a situation such as Eastern 401, the pilots had already contacted the tower to indicate that they had an issue - the nose gear lock light failed to illuminate. While attempting to determine what the cause of the problem was (bulb burnt out or gear not actually locked), they forgot to aviate and flew into the ground. In a situation like this, particularly where ATC has been notified that there was an issue with the flight, it seems to me that they (ATC) would have been more alert to the flight and notified the pilots that they were descending when they should have been holding a steady altitude.

This is the type of situation that I'm asking about (though not specifically EA401), and I'm wondering what role, if any, ATC should or does take in monitoring and alerting flight crews to altitude deviations, especially on approach.

This is potentially a very broad topic, but here's some information about the US.

As a general principle, safety is ATC's first priority so obviously they have a basic responsibility to provide whatever information and action are required to keep aircraft safe:

FAA ATC orders 2-1-1:

  The primary purpose of the ATC system is to prevent a collision
  between aircraft operating in the system and to provide a safe,
  orderly and expeditious flow of traffic, and to provide support for
  National Security and Homeland Defense.

The orders explicitly say that ensuring separation and issuing alerts take priority over everything else. Section 2-1-2, DUTY PRIORITY:

  Give first priority to separating aircraft and issuing safety alerts
  as required in this order. Good judgment must be used in prioritizing
  all other provisions of this order based on the requirements of the
  situation at hand.

Section 2-1-6, SAFETY ALERT defines what an alert is and when to issue one (this section is referenced throughout the orders):

  Issue a safety alert to an aircraft if you are aware the aircraft is
  in a position/altitude that, in your judgment, places it in unsafe
  proximity to terrain, obstructions, or other aircraft. Once the pilot
  informs you action is being taken to resolve the situation, you may
  discontinue the issuance of further alerts. Do not assume that because
  someone else has responsibility for the aircraft that the unsafe
  situation has been observed and the safety alert issued; inform the
  appropriate controller

It's difficult to say much more than that because there are so many possible scenarios. But you can search for "safety alert" in the orders and the AIM to get more information and there's some limited content in the orders 5-15 about radar displays and alerts.

If ATC fails to issue a safety alert and that leads to an accident, then it will be mentioned in the NTSB report, e.g. LAX04FA205:

  Contributing to the accident was the failure of both the Center and
  TRACON controllers to properly respond to the aural and visual Minimum
  Safe Altitude Warning System (MSAW) alert.

But actually acting on a safety alert is entirely the pilot's responsibility, of course (this is from the AIM 4-1-16):

  Once the alert is issued, it is solely the pilot’s prerogative to
  determine what course of action, if any, to take


How does an auto-pilot control altitude changes? When changing the set altitude with an autopilot with speed control engaged how exactly does the control system work...

I can envision one of three (four) methods.

Control throttle and adjust attitude to maintain speed at required descent rate, or
Change attitude to descent slope and adjust throttle to maintain speed, or
Change attitude to maintain descent rate and adjust throttle to maintain speed. or
Some combination of the above.

Follow up question....

When setting the descent rate alone, i.e. no target altitude, is there a difference in control method.

Short answer:  it depends on which mode your flight guidance system is in.  It can be either number 2 or 3, although sometimes even something else entirely.  Whether you're using a target altitude or descent rate, it still all comes down to what mode you're in.  If you're in FPA, the elevator is adjusted to maintain descent slope.  If you're in VS mode, the elevator is adjusted for descent rate.  If you're in FLC/IAS/SPD, the elevator is adjusted for airspeed.  You should reference your pilot's operating handbook as the exact names and engagement methods for these modes varies a little.

When talking about autothrottle and constant speed altitude changes there are two independent inputs to the system, pitch and thrust, and two outputs of the control system, which are vertical speed and airspeed (or flight path angle and airspeed). You either control vertical speed with throttles and airspeed with the elevator, or airspeed with the throttles and vertical speed with the elevator.  It's possible to mix both throttle and elevator for both airspeed and descent rate (as indicated by number 1 and 4 on your list) but every flight guidance system I know of keeps the two separate to avoid confusion.

Advanced flight guidance systems often have three basic altitude change modes: flight level change (FLC), also sometimes called IAS flight guidance, vertical speed mode, and flight path angle mode. There's more on the differences between VS and FLC here.  I believe assume you're talking about the authrottle in airspeed control with the flight guidance in vertical speed mode.

In vertical speed with autothrottle speed control, attitude control and thrust control work somewhat independently .  Your elevator will be set based on the vertical speed control law to target a certain vertical speed, while your throttle will be trimmed based on its own control law to maintain the target airspeed.  So if you need to decrease descent rate but increase airspeed, your throttle will generally increase while your airplane will pitch up.

You mention adjusting to the descent "slope", which would probably refer to flight path angle (FPA) mode. This mode basically tells the autopilot to adjust the elevator so that the flight path is at a certain angle (not the pitch, but the flight path).  This is a common alternative to vertical speed mode, and autothrottle will perform similarly for either mode.  If FPA is available the pilot can select it instead of VS on the guidance panel.  It's less used than VS, and you can read about why you would use one or the other here.

Flight level change (FLC), IAS, or even sometimes called SPD mode, behaves somewhat differently in that the elevator is used to determine your airspeed.  Thrust then determines your vertical speed and usually this is achieved by setting the throttles to climb or idle settings depending on the situation.

Of course, there's lots of what ifs that go into these systems like stall protection, overspeed protection, non-steady-state behavior, autopilot-authrottle interactions, and engine-out behavior.  I won't cover those details here.


Do Airbus aircraft have an "off" position for their landing gear? I know some Boeing aircraft feature an up, down, and "off" position for their landing gear lever. I know that some Boeing wide-body jets do not have a 3rd lever position, but still have an "automatic" "off" setting that takes effect after a time-delay (which begins when the gear comes up). Do airbus, bombardier, embraer, etc. have a 3rd gear lever position in any of their aircraft? If not, do they have an "automatic" off setting?

I was working for Airbus before, never seen it on A300, A320, A330, A340, A380, A350 or A400M. Only Gear Up or Gear Down position.

For confirmation on A320, here the FCOM section:

  NORMAL OPERATION
  The flight crew normally operates the landing gear by means of the lever on the center instrument panel. The LGCIUs control the sequencing of gear and doors electrically. One LGCIU controls one complete gear cycle, then switches over automatically to the other LGCIU at the completion of the retraction cycle. It also switches over in case of failure. The green hydraulic system actuates all gear and doors. When the aircraft is flying faster than 260 kt, a safety valve automatically cuts off hydraulic supply to the landing gear  system. Below 260 kt, the hydraulic supply remains cut off as long as the landing gear lever is up.


Why is there a difference between the procedural track heading and final approach course at Fort Hood (KGRK)? Looking at the NDB RWY 15 into Fort Hood/Killeen, why is the procedural track from OKEST showing a heading of 151 degrees when the final approach course heading is 150 degrees. Is that simply a typo? The best answer I can come up is that from OKEST to the LOM is 6.3NM on a heading of 151 degrees. Is the 151 heading to GRK VOR or to the LOM using the NDB? Any thoughts?

It's really only confusing because they are within one degree of each other, but this actually happens quite a bit with feeder routes.
The 151 Degree Course NW of STARN
The 151 degree course (and the OKEST intersection) is only used if you are using AGJ as the Initial Approach Fix (IAF). This is the course that you would fly from OKEST towards the GRK VOR (on the GRK 331 degree radial) in order to get to STARN.  After reaching STARN, you would fly the 150 degree bearing outbound from STARN and the rest of the approach using STARN to navigate.
The 150 Degree Bearing NW of STARN
If you were to use STARN as the IAF instead of AGJ, you would fly the procedure turn on the 150 degree bearing from it in order to get yourself established on the correct inbound course to fly the rest of the approach.  In this case, you would never use GRK at all.


Is it normal to use only one hand on the yoke while landing? In this video taken from this question, the video shows the pilot using one hand on the yoke and one hand on something else (perhaps the throttle? I'm not a pilot). Is it normal to do landings with one hand?

It is absolutely normal, in fact it's rare for a pilot to have more than one hand on the yoke at any one time as it isn't required. Movies will often show pilots manhandling yokes with two hands, but that's just Hollywood. The only time two hands is required is when extra strength is needed on the controls, for instance a hydraulic failure.
When landing a pilot will have one hand on the yoke (or stick) and one hand on the throttle, as throttle adjustments are needed to maintain the correct glide angle and initiate a go-around if required.


Why do we lean the mixture when the air become less dense? In a carbureted engine at higher altitudes, less dense air means there is less air entering the engine. Wouldn't we want the mixture to be full rich to allow the maximum air into the engine?

I understand that the engine will become rough if we don't lean the engine. I'm asking what is going on? It seems like a decrease in air means we would need to open the mixture not close it.

The decrease in air means less oxygen. If the fuel is burnt at "rich" there would not be enough oxygen in the air to burn some of the fuel, thus reducing efficiency, increasing the risk of carbon monoxide, and adding grime to the exhaust system. That is why "lean" is used for high altitudes, to burn efficiently and safely.


What part of the F-18's wing profile allows for high AoA flight? I'm trying to analyse the relationship between the angle of attack and lift coefficient of a wing profile I tested. However, I am finding that the data shows that the wing hasn't stalled by the time it reaches an angle of 25 degrees, whereas I am under the impression that a wing usually stalls around 15 degrees. I cannot recall where I have heard it, but I am under the impression that one of the reasons the F-18 is capable of high AoA flight is because of the "surfaces" around the cockpit:

Is that correct? Is high AoA associated with smaller wing width and high chord length?

Also, this is the data I've gathered. I tested the same wing at three different airspeeds. Sorry about the text being in Danish.

You're correct that the leading edge extension (LEX) plays an important part in increasing the maximum angle of attack of the Hornet/Super Hornet. The LEX basically forms a vortex above the main wing, which improves the high angle of attack capabilities by delaying separation. See this question about vortex lift and also another related question.

You can see the LEXs forming vortices over the wing in the following image.

An F/A-18E Super Hornet assigned to Strike Fighter Squadron (VFA) 81 maneuvers over the Nimitz-class aircraft carrier USS Carl Vinson (CVN 70) during an air power demonstration. (U.S. Navy photo by Mass Communication Specialist 2nd Class James R. Evans/Released)

  Is high AoA associated with smaller wing width and high chord length?

There is no simple way to answer this question because what matters is not the aspect ratio of the wing per se, but the way the lift is created. In case the wings have high sweepback, the method of lift generation is by vortex lift, which operates in higher angle of attack compared to the normal (moderately swept) wings.

In other cases where the wing sweepback is less, the lift generation will be by conventional means and consequently, the high angle of attack performance will suffer, though this can be remedied by the use of canards, thrust vectoring etc.


Why do the 737-100's JT8Ds, and no others, have suck-in doors? Of the various use cases of the Pratt & Whitney JT8D (the most-produced low-bypass turbofan in history, and the second-most-produced turbofan of any description1), those installed on the 737-100 appear to use a set of auxiliary suck-in doors, similar to those so familiar from the earlier JT3D:

(Image by Steve Fitzgerald at Airliners.net, via Fæ at Wikimedia Commons, modified by Fæ and Marc Lacoste at Wikimedia Commons.)
The always-useful 737 Technical Site confirms that these are, indeed, suck-in doors:

History
The original choice of powerplant was the Pratt & Whitney JT8D-1, but before the first order had been finalised the JT8D-7 was used for commonality with the current 727. The -7 was flat rated to develop the same thrust (14,000lb.st) at higher ambient temperatures than the -1 and became the standard powerplant for the -100. By the end of the -200 production the JT8D-17R was up to 17,400lb.st. thrust.
Auxiliary inlet doors were fitted to early JT8D's [sic] around the nose cowl. These were spring loaded and opened automatically whenever the pressure differential between inlet and external static pressures was high, ie slow speed, high thrust conditions (takeoff) to give additional engine air and closed again as airspeed increased causing inlet static pressure to rise.

In contrast, the JT8Ds installed on other types of aircraft, so far as I can tell, do not have suck-in doors:

DC-9

(Image by Cory W. Watts at flickr, via Josve05a at Wikimedia Commons.)

727

(Image by Pablo Andrés Ortega Chávez at flickr, via Wikimedia Commons.)

737-200

(Image by Andre Gustavo Stumpf Filho at flickr, via Wikimedia Commons.)

Caravelle 10

(Image by duch.seb at Wikimedia Commons.)

Mercure

(Image by Michel Gilliand at Airliners.net, via Fæ [again] at Wikimedia Commons, modified by Fæ at Wikimedia Commons.)

Why did the JT8D need suck-in doors on the 737-100, and not on any of the many other aircraft it was attached to?

1: Behind only the General Electric/SNECMA CFM56 high-bypass turbofan, used on (among many, many other aircraft) the 737-300 through -900.

Since the production of the 737-100 and -200 was concurrent, there were in fact -200s with blow-in doors, and more often than not, the inlet was replaced during the airframe's lifetime. An example is United's N9003U, line number 12, a 737-200:

Sources: jetphotos.com and airliners.net

The doors are spring-loaded and open by differential pressure at medium-to-high thrust settings and slow speeds, i.e. during takeoff and landing, i.e. near communities. The openings created in the inlet allows the inlet noise to escape, which is a major source of noise.
Therefore, and evidently, both manufacturers and customers have favored the sound-absorbing inlets.

(...) the noisiest test configuration had an inlet duct with simulated blow-in doors. When the blow-in-door inlet was installed, the A-weighted sound power levels were approximately two decibels greater than when the inlet with no blow-in doors was installed.[1]

An additional design feature for minimum noise is the use of a single opening inlet at all operating conditions. The use of devices such as blow-in doors to bring additional air into the engine at some engine powers has some aerodynamic advantages but increases engine noise. No blow-in door systems are in use on the DC-10.[2]

From a regulatory aspect, at least in the 747's case – which sported those inlets in its early days – the noise-aimed regulations of the 70s were the reason for their replacement:

After promulgation of FAR Part 36 and application to the 747, substantial changes were required for the engine installation. The blow-in-door inlet was replaced with a fixed lip inlet providing a large reduction of the fan source noise while the surface area of the acoustic lining contained in the engine and nacelle was more than doubled.[3]

1: Marsh, Alan H., Robert L. Chapkis, and Gary L. Blankenship. Study of Noise-Certification Standards for Aircraft Engines. Volume 3. Selection and Evaluation of Engine-Noise-Certification Concept. DYTEC ENGINEERING INC LONG BEACH CA, 1983.
2: McPike, A. L. "Community Noise Levels of the DC-10 Aircraft." Journal of Aircraft 9.8 (1972): 542-547.
3: Little, J. W., and R. E. Russell. "Status Report: Subsonic Aircraft Noise Reduction." Turbo Expo: Power for Land, Sea, and Air. Vol. 79757. American Society of Mechanical Engineers, 1976.


Why do cockpits with ACARS have printers? Most cockpits with ACARS have a thermal printer that is used for printing out information received.  Why is this information printed out rather than displayed on a scrolling computer screen?  Maybe I'm not used to ACARS or too used to 21st century technology where all messages and documents are on a screen, but it seems like the flight plan, maintenance, and weather information transmitted over ACARS could be saved and read off a screen as easily as a piece of paper.
What are the advantages of having a printer in the cockpit for ACARS and other systems, and is this technology being replaced at all in modern glass cockpits?

Edit:  As originally written this question didn't address other information that could be printed by advanced printers like charts and weather maps, but if you feel this is a relevant detail, you can include other systems other than ACARS in your answer.

Not all cockpits with ACARS have a printer for it -- it's an entirely optional part of the system.

The printer is there so that you don't have to copy by hand information that you want or need to have available.  For example, ATIS is desirable to have in hard-copy, so that when you need some piece of it immediately (say, the altimeter as you're descending through the transition level, or the identifier as you go to check in with Approach), it's there without going through the levels of menus to retrieve it from the ACARS system.

Also, on a Class II navigation route, if you get a significant reroute, your procedures may dictate that you have a hard-copy flight plan that shows the new route, and which you'll annotate with things like actual times & fuel onboard overhead various waypoints, actual winds, etc.  That may be required by company procedures or regulation, and the hard copy is there for those times when the computer decides to misbehave.  And all of THAT info is most certainly something you don't want to have to copy by hand -- it's pretty lengthy, and accuracy is really, really important.

Beyond that, it is "nice to have" a hard copy of things like a PDC clearance, and it is quicker to tap "PRINT" rather than copy it all down by hand.


What is this single-engine low-wing propeller plane? I found this picture in an article about constant speed propellers, but there was no reference to the type of aircraft. The Flickr account that first posted the image doesn't exist anymore. I did some research myself, I thought it could be a Cessna or a Beechcraft, but I don't know much about it and none of the models I found was a perfect match.

That’s a Piper PA-28R Arrow.  Most likely an PA-28R-201 Arrow III, by the looks of it.


How does an aircraft tailplane work? How does an aircraft tailplane keep the aircraft stable, and prevent it from tipping over? Also, how does the lift generated by a tailplane compare to that generated by the wing?

For conventional designs, the tail is composed of two parts: the horizontal tail and the vertical tail. They play a role in the trim and the manoeuvrability of the aircraft but at different levels. The horizontal tail is mainly used for longitudinal stability (and trim) while the vertical tails used for the lateral stability (and trim).

About Stability

It is possible to talk of stability only after having defined an equilibrium point around which the stability is studied. An aircraft is in equilibrium if the forces and moments it experiences are balanced. Using a simple model for the longitudinal analysis, it can be decomposed in three relations called the trim equations. In order to keep it simple, it will be assumed here that the angle of attack and the flight path angle are zero. (Note that the same reasoning can be achieved with non-zero values but the equations then become quite messy.)

Longitudinal Equilibrium

These three equations are:

$$L=mg$$
$$T=D$$
$$M=0$$

where $L$ is the total lift, $mg$ is the weight of the aircraft, $T$ is the thrust, $D$ is the drag and $M$ is the pitching moment around the centre of gravity of the aircraft. The second equation won't be studied further since it does not help to understand the role of the horizontal tail and its influence. Looking at the following picture, one can see that usually, the centre of gravity and the point where the lift applies (called the aerodynamic centre) are not the same. This means that the lift generated by the wing creates an induced moment around the centre of gravity that one should add to the already intrinsic pitching moment due to the main wing (usually a pitch down moment for conventional airfoils).

Knowing that, it is possible to rewrite the two equations of interest including the contributions from the main wing and from the horizontal tail.

$$W+L_t=L_w$$
$$M_0+bL_t=aL_w$$

From these equations and the figure, it appears that the horizontal tail is used to generate a lift which induces a moment helping to balance the moments equilibrium and thus prevent the aircraft to spin on itself (pitchwise).

Drawback and Solution

From both the figure and the equations it turns out that the lift contribution from the tail is usually negative, meaning that more lift from the main wing is needed to keep a trimmed (or balanced) aircraft. This drawback can be overcome by the use of a canard configuration instead.

Lateral Stability

The same can be done for the lateral equilibrium and stability but there it is the vertical tail that is used. It is symmetrical so that there is no yaw induced and if there is some side force experienced, it will create a moment in order to reduce the side-slip angle.

Comparison of Lift Created by the Tail and Main Wing

For a trimmed configuration, it is easy to see that the lift created by the main wing is more or less the one created by the tail plus the total weight of the aircraft, which gives an idea of the difference between the two forces.


Why are some cardiac conditions disqualifying even if asymptomatic and corrected? The FAA's website, in its list of medical conditions disqualifying for a would-be pilot, includes a number of cardiac conditions and procedures that would not intuitively seem, on their own, to warrant keeping someone on the ground, and, yet, automatically disqualify the applicant (even if they are completely asymptomatic or the condition has been corrected) from receiving a standard-issue medical certificate, requiring them to beg the FAA for a special-issuance certificate ([italicised brackets] indicate my additions):

[...]

Cardiac valve replacement [even if far in the past, with no history of symptoms or complications, and even if it completely and permanently corrected the precipitating condition]

Coronary heart disease that has been treated [even if completely resolved by said treatment] ...

...

Heart replacement [even with no history of complications]

Myocardial infarction [even if far in the past and devoid of complications or any recurrence]

Permanent cardiac pacemaker [even if causing no complications and completely resolving the underlying pacemaker-necessitating condition]

[...]

Even if some of these conditions have the potential to be associated with other, potentially-impairing conditions (such as cardiac arrhythmiae, heart failure, or heart transplant rejection), those associated conditions, if present, would be perfectly visible in their own right, without the need for using the conditions in the aboveexcerpted list as proxies.  In addition, some of the above procedures (such as valve replacement, pacemaker implantation, and some types of coronary-artery-disease interventions) are curative, or functionally-curative, procedures, and, yet, automatically disqualify their recipients from receiving a standard-issue medical certificate.
Why is this?

Basics :  Why did the event happen? Will it happen again?

Curative care is fine, but what about the possibility of the same, or similar, events happening tomorrow/next month/ad astra?

The FAA medical standards are for the protection of everyone other than the pilot.

I am the proud owner of 6 cardiac stents. But it has become impossible to maintain a Special Issuance Medical as I aged.


Why do you think Pakistani LACM and SLCM have different ranges? https://en.wikipedia.org/wiki/Babur_(cruise_missile)

The following is the picture of Pakistan's LACM Babur-II. Its billed range is 750 km.

The following is the picture of Pakistan's SLCM Babur-III. Its billed range is 450 km.

Why could the range of Babur-III possibly be smaller than that of Babur-II?

Probably, the changes needed to make the missile sea-launchable affected the range by either decreasing the size (fuel capacity) or increasing the weight of the missile, or both. Launching an air-breathing missile by first shooting it through the water is significantly different than launching it straight into the atmosphere from the land-mobile launcher, so it isn't surprising that other changes had to be made to the missile itself -- changes that reduce its range.


Is responding to an internet forum post looking for a scenic flight "holding out"? Suppose I'm a private pilot and see a post in a closed group on Facebook where someone is looking for a scenic flight around my area. If I offer them a flight with me, sharing costs equally, am I holding out? Does it matter if I know the person vs. if I don't? Thanks!

This is almost certainly holding out: someone asks for a flight and you agree to provide it. You weren't going to make the flight until the person asked, and you're receiving compensation for it. This is from the Bobertz interpretation:

  Absent a bona fide common purpose for their travel, reimbursement for the pro rata share of operating expenses constitutes compensation and the flights would be considered a commercial operation for which a part 119 certificate is required.

The Haberkorn interpretation that acpilot mentioned is somewhat similar to your scenario in that it talks about using Facebook but with the difference that the pilot advertises for passengers, rather than passengers asking for pilots. You can read the whole thing and although the FAA said they can only comment on specific cases, not general examples, they did make clear that they're concerned about situations where:

  there is an interest in carrying passengers with whom there is no previous personal relationship and that the offer to accept passengers is being made to the general public

But even if you already know the individual, it's probably still a commercial operation.


Why/How is the B-52 still in service? The B-52 was introduced in 1955, and is still used by the US Airforce, and is expected to remain in use till the 2040s.

On the other hand, the A-4 Skyhawk was introduced in 1956 and retired in 1998/2003.

Now granted these are completely different aircraft. What I'm wondering, is why one aircraft can remain so useful for so long, while others quickly become obsolete.

Three reasons:

The B-52 stayed useful by taking on "easier" roles as it aged.

It actually started life as a high-performance penetrator, relying on speed and altitude to stay safe. Think Early Cold War, shortly after WWII.
When ground-launched missiles improved, the B-52 was forced into flying low-altitude penetration underneath radars (a role for which it was NOT designed and rapidly wore out the airframe) accompanied by a host of other aircraft to protect it and help it penetrate defenses. Despite their escorts, we lost low-dozens of B-52s in Vietnam.
Later in the Cold War, the B-52 was completely outmatched by air defenses (fighters, SAMs, etc), and had little hope of penetrating defenses (at either low or high altitude) to drop nuclear bombs. Instead, it was relegated to flying in friendly/neutral airspace, lobbing long-range nuclear cruise missiles into the Soviet heartland (a tactic they feared more, as it turned out, than manned low-level penetrators). It was basically a missile bus.
In the Gulf War, the B-52 only flew after air defenses had already been defeated by other aircraft (F-16s, F-15s, F-111s, etc). It was basically a bomb truck.
During ALLIED FORCE (1999), "B-1Bs and other conventional (non-stealthy) aircraft could not survive in the face of double digit SAMs, even when employing advanced decoys and other countermeasures."
The B-1 program, btw, went through a similar progession, starting out as a high-performance penetrator,  then forced into even lower-altitude penetration, then also relegated to bomb/missile trucking, pushed back by AAA, MANPADS, SAMs, look-down shoot-down radar, improved Soviet interceptors, and Soviet AWACS. In fact, the B-1 and B-52 fly largely* the same missions today.

*There are some important differences. The B-1 is no longer nuclear capable (START compliance), leaving that mission to the 20 B-2s and ~40+ ALCM-armed B-52s. The B-1 can also reposition more quickly, lending it to CAS role: it dropped 37% of the then-total ordnance on ISIS (2x that of the A-10)(see source) in OIR before they returning stateside for upgrades and were relieved by B-52s. (And, yes, the B-1 does CAS.)
Overall, the B-1s fly ~330-450 hours/yr, the B-2 ~230-290 hours/yr, and the B-52s ~200-220 hours/yr (2008-2012). The B-1 and B-52 cost roughly the same per hour (the B-1 has higher fixed costs but lower operational CPFH).

In 40 years, the B-52 went from being the US' primary strategic tool against another superpower to being outclassed by small nations.
Bomb/missile trucking is an "easy" job, but far from ideal. You can't bomb your targets until the air defenses have already been defeated---after the "first week" of a war, the most critical phase, when you need them the most. In other words, the B-52 (and B-1) are relatively easy to shoot down, so they're not nearly as useful as they used to be. You can't fly them unless many of the toughest enemies are already dead. So you can't use them as often as you want.
@MSalters phrased it quite nicely: the reason the B-52 (and B-1) hasn't changed is because other aircraft are doing the hard work for them (tearing down enemy defenses), and such aircraft have advanced immensely. But that amount of support is quite resource intensive, so if you wanted to fly B-52s in the "first week" when enemy defenses are still effective, you'll need a lot of help: (Image source: Beyond the "Bomber": The New Long-Range Sensor-Shooter Aircraft and United States National Security - Lieutenant General David A. Deptula, USAF (Ret.), 2015.) The same was true with strike packages during Desert Storm, eg only a third of the aircraft dropped bombs while the rest provided support (jamming, anti-radiation, top cover, etc).

We didn't build enough B-1s and B-2s to replace it, so we kept some B-52s to fill in the gaps (we don't have any other suitable heavy bombers in inventory, this is the only choice).

The 100 B-1Bs were just a stopgap until the B-2 showed up, of which we meant to build 132 (cut to 21 airframes). Had the Cold War continued, the B-2 would have comprised the bulk of today's bomber force.

We were busy buying new fighters and cargo planes.

The official, stated goal is to recapitalize the entire bomber fleet (less the B-2) because it's increasingly obsolete. That's worth repeating: the B-1 and B-52 are near obsolete. They're not nearly capable enough. The next bomber isn't another B-1, B-52, or modded 747 because we don't want to maintain the same capabilities. We want more capability--much more--than what we currently have. We've only waited this long because replacing the airlift and fighter fleets took priority.
The LRS-B (new stealth bomber; formerly NGLRS) will replace the B-52 starting around next decade. The contract is expected to be awarded very soon. The Pentagon briefed a select group earlier this month. Looks very promising---well managed, on budget, very capable, and surprisingly mature.

UPDATE: The B-21 (ex-LRS-B, ex-NGLRS) was indeed awarded 27 Oct 2015, a month and a half after this post was originally written. Very little is known, but it is indeed a stealthy bomber (the official render closely resembles the outer mold line of early B-2 iterations before the redesign). And it is indeed on time and under budget by ~9% (APUC = \$511M vs. the \$550M requirement, FY10\$).

In short, we still fly the B-52 today because we don't have a choice, not because we want to.

Parting thoughts:

I want to emphasize that the B-52 did not remain effective in its intended role (penetrating the USSR on Day 1), upgrades notwithstanding.

Yes, the B-52 survived multiple replacements (eg XB-70, B-1A), but not because it was more capable (it wasn't; the B-70/1A went higher/faster/lower/with more payload), but because those potential replacements couldn't outpace Soviet defenses either and so were cancelled in favor of ballistic missiles and cruise missiles... except the B-2, which could outpace Soviet defenses but was curtailed when those Soviet defenses disappeared. The B-52 survived basically because of luck: against the XB-70 and B-1A, it was the incumbent, already bought and paid for; against the B-2, history smiled.

Also, the B-52 did not "survive" the B-1B. The B-1B was never meant to replace all B-52s in the first place. The B-1B was meant to fill an urgent requirement as a stopgap from ~1985-1995 until the B-2 could enter service. Unlike previous B-52 would-be replacements, B-1B production was not cut short: 100 were ordered, and all 100 were delivered, essentially on time and on budget (though that was actually partly the problem).

Thankfully the B-52's range and payload kept it productive during its long service.

Some background.

The USAF operates ~160 bombers:

76 B-52s
63 B-1s
20 B-2s


What is a counseling session with the FSDO/FAA? I just flew into Bravo unintentionally 48 hours ago. My error: I thought I was 1,500 feet above the ceiling, but I was 1,000 feet below it.

Furthermore, I had an incorrect frequency for Approach, and while I eventually found the proper frequency (I'd planned to request flight following) I had crossed over the outer ring.

When I contacted Approach, the Approach Controller explained I was in Bravo, and to turn West and exit. I did say, "Am I not ABOVE Bravo airspace?" He said "No, you're in it" and to fly West and exit.

From that moment, I followed approach control's directions to a T, but could not understand why he directed me West (remaining in Class B for 13 more miles) instead of having me climb 1,500 feet or angle back North or North-West to exit Bravo more quickly.

I was given the dreaded phone number, made the call after landing, had a friendly enough discussion, and he said it would probably lead to a counseling session. My question is what is that?

I'm familiar with the remedial training option the FAA may offer, but what did he mean by "counseling session"? The controller implied it would be handled by phone.

I'm not freaking yet, just concerned. I told everything that happened on that phone call, and then filed an ASRS report.

A "Counseling Session" is the lightest slap on the wrist the FAA can give you (aside from doing nothing).

Basically it means "you screwed up, you know you screwed up, and we (the FAA) know you know you screwed up" -- they just want to sit you down with someone from the FSDO (probably a FAA Safety Team representative) and have a conversation to make sure everyone is clear on what happened, how the violation happened and that you know how you can avoid this sort of thing happening in the future.

My understanding is that counseling sessions are "informal" (they're not an official "administrative action", so I'm pretty sure they don't go on your airman record) - the FSDO will note that they talked to you and bury that tidbit in their internal file, to be generally forgotten unless you decide to make a habit of busting airspace.

(The closest "official documentation" I can find on counseling sessions is in the FAA Safety Team's Representative Manual -- Page 12 if you're interested.)

Some things to bear in mind:

You're the kind of pilot the FAA isn't out to beat up on.
Yes, you screwed up - but you know that and you clearly want to make sure you don't do it again, which is really all they want too.
Make sure you're clear in your mind what happened & how to avoid similar incidents in the future.
Did you misread the chart? The altimeter? Get distracted looking up the frequency? Something else?
Don't admit to any other violations during the counseling session.
(Presumably you didn't commit any, you got a proper flight briefing, etc.)
Hang on to your ASRS strip.
You won't need it for a counseling session, though mentioning you've filed an ASRS report won't hurt anything. If the FAA opts for a more formal action that strip is going to be important.
If you have AOPA pilot protection services now would be a good time to chat with them too, just to cover all your bases.


What are the differences between hand and hold luggage screening? I have googled a little about the type of security screening done to hold and hand luggage.  From what I have seen the hold luggage is clearly subject to more rigorous screening, including tomography should it raise suspicion.

However, in light of the recent announcement for passengers to travel with charged mobile devices and be able to switch them on, I have become slightly confused as to why an explosive device would not cause an issue in the hold (I presume passengers won't be asked to remove all electrical items from their hold luggage and switch it on!).

Is the feeling that a small explosive device could do more damage in the main cabin, whereas in the hold the aeroplane's structure would absorb it?  Or is there really a "foolproof" way for the hold luggage scanners to detect whatever is being searched for ?

As ratchet freak pointed out you do have some more flexibility in ensuring that a small cellphone-sized explosive device is positioned to cause maximum damage if it's in your carry-on baggage as opposed to if it's in checked baggage/cargo. As such there is an inherently higher risk here.

Counterbalancing that risk is the fact that we already have good screening tools to detect explosives or other anomalous things (like say knives or tear gas) concealed in what purports to be an electronic device:

Electronic devices are subject to X-Ray inspection.
Trained operators can spot quite a few anomalies using these "classic" tools (though rechargeable batteries can often look like explosives).
Explosive Trace Detection machines are already a part of security screening.
There are also swab machines which operate on a similar principle. They are not foolproof (they have false positive results occasionally, and may miss well-packed explosives), but they're a good tool.
Airport security has already asks to see suspect devices power on.
I had an old Compaq laptop with a completely useless battery, and security would let me plug it in if the battery was dead.

These same techniques (and others) are available for use on checked baggage.
Checked baggage may also be opened for inspection, which could conceivably include powering on any electronic devices in checked baggage (but note that the TSA recommends you not pack your electronics in checked baggage).

It's worth pointing out that, at least from a technical standpoint, faking a "power-on" is trivial with today's technology. Unless they're making folks do a full functional demonstration of the device in question a motivated individual with malicious intent would have no problem making a device that's mostly filled with explosives but can still "power on" and display a home screen, which makes the additional security benefit here dubious, at least in my estimation.


What does N mean after a runway visual range (in a METAR)? This morning's METAR for Cambridge looked like this:

EGSC 110720Z 11001KT 0150 R23/0200N R05/0175N FZFG BKN001 M02/M02 Q1028

Note the bold runway visual range (RVR) indications. What does the N mean?

RVR often has a letter at the end to indicate the visibility trend, so you can tell if it's going to improve (and maybe you should circle overhead for a while) or worsen (and you should consider going to your alternate).

U means an "upward" trend: visibility is improving. D means a "downward" trend: visibility is worsening. N means no change.


How do airlines ensure each passenger has a life jacket before a flight? It's very easy for a passenger to walk away with a life jacket. What do airlines do in general to ensure that there is a life jacket available for each passenger? Do they check it manually as part of pre-flight check up?

It's essentially a manual check for many airlines, performed as part of the pre-flight checks.

However, the airline I have worked for recently upgraded from manual checks to RFID tags on each lifejacket (a sticky tag kind of like this). The RFID tags can be read from the aisle by a handheld reader. So the person who does the checks simply walks down the aisle with the reader and is able to tell if a seat is missing a jacket and also if the jacket needs to be replaced due to the expiry date fast approaching.

In addition to a tag on the actual life jacket there are also security RFID tags across the life jacket holster under the seat. So the scanner is able to tell if the life jacket has been tampered with as the security tag would have been broken.

As for the frequency of checks, different regions have different requirements. Usually life jackets are only needed at all if flying over a large body of water. I believe the USA requires a check before any flight which passes over the USA (as well as water). Europe requires less frequent checks.


How often is a passenger jet aircraft painted? I just read an article (published in 2012) telling the special livery for British airways' A320 to celebrate the Olympics game is made to last one year. I assume this mean those aircraft are painted once per year because painting is not free and thus it is reasonnable to change the livery only when painting is already schedule. But because this is a special livery, it may last less than other painting. As this is only assumptions, my question is the following:

How often is an Airbus or a Boeing painted (even without changing the livery)? Does it significally change from on airline to another? Does a special livery change this frequency?

As always, it depends. From what I've seen and read, it's usually redone every 5-10 years. This can vary on various factors from paintshop availability, to new color scheme rollout, to deterioration of the previous one due to bad painting, to quality of the paint used. Sometimes, special liveries are just an aviation specific sticker that's put onto an aircraft, other times it's fully painted onto the aircraft, just depending on the airline, the planned length of time for it to be special, and any other considerations.


Could Mach 1.4 be a better design point for SST? There's been a number of recent proposals for design work on low-supersonic bizjets that would cruise at Mach 1.1-1.6, the common theme being avoiding a sonic boom.

The conventional wisdom is to avoid this range, and either stay subsonic or go all the way to Mach 2+, because you sacrifice some efficiency the moment you cross Mach 1:

Sharp nose and supersonic airfoils result in lower L/D across the envelope.
Thin airfoils add weight and require more fuselage space for fuel tanks.

On the other hand, staying at low Mach avoids a few other penalties:

The 12:1 slenderness of modern widebodies translates well to a low-Mach SST, allowing for a similar cabin layout. This is critical.
Skin temperature allows the plane to be built with the same resins as subsonic airliners.
Less COL shift, higher AR wings, reasonable bypass engines make it easier to meet TO lift and thrust requirements.
Speculatively, cruise altitude might be kept at FL450 to avoid stricter pressurization rules.

What I'm not sure is which set of factors matters more. It's difficult to glean from existing designs, as they're not really comparable between one another.

The Aerion's wing should behave well on landings, and it claims only a 22% fuel penalty for going supersonic. But its slender nose and long tail leave just 18% of its length for the cabin, so it can't be used as an airliner template.

This is probably caused by its role as a status symbol and regulatory weight limits; actual SST managed a proper length cabin, and the 2707 was even planned as a widebody. Thin wings have are also featured in some subsonic proposals, so they're not a total efficiency killer.

I'd imagine a Mach 1.x airliner to be a 779-sized twin, with 788-like seating capacity, area-shaped akin to the Sonic Cruiser, an ogive nose (F-16 style), Aerion-like trapezoid SNLF wings, nacelle engines.

The question is, could a low-Mach SST, if sonic boom was publicly accepted, be closer in efficiency to subsonics than to historic high-Mach designs?

To narrow down the scope, let's explicitly exclude:

Whether there is any market for such an aircraft.
Whether Mach 1.x is fast enough to make a difference.
Engines, as it's a major subject in itself. Just assume a rubber engine matching a loose trend line of TSFC~=0.4*Mach.max.

Leaving in scope the technical subject of the efficiency loss imposed by the aerodynamics of supersonic flight. It comes down to how good a L/D can high-tech airfoils like SNLF and area-ruled widebody provide, with how little compromise on capacity, in the range of Mach 1.1 to 2. Fuel per seat-mile being the figure of merit.

To help set a baseline, I've plotted a mix of historic and calculated numbers for known aircraft:

To put the question in more formal terms, I'm looking to estimate the seat-miles/kg function's behavior for an optimal design in the low-supersonic range.

The red line assumes the worst-case scenario and is based on just stuffing seats into the AS2. The green line represents a best-case scenario based on claims with the Sonic Cruiser as a baseline. The truth is probably in between.

Only for a business jet.

The high end of the business jets has been in a race for higher cruise Mach numbers which has resulted in a top speed of Mach 0.935 so far (Gulfstream 650, Cessna Citation X). So there seems to be a market value for the top spot in cruise Mach. To improve this further, and substantially, the jump to supersonic speed is unavoidable. So far, the expense of developing such an aircraft for a rather small market has been too high.

On the other hand, a Mach 1.4 design will be much easier than one flying at Mach 2.0. The reasons are:

Intake complexity: At Mach 1.4 a simple pitot intake with a single straight shock has acceptable losses, while Mach 2 demands a complex, adjustable intake with ramps producing a cascade of oblique shocks.
Wing aspect ratio: In order to keep the wing's leading edge subsonic, a Mach 1.4 design can get away with 45° sweep while a Mach 2 design needs at least 60° sweep. The Mach 1.4 wing will be much better for take-off and landing, and for the subsonic legs of the trip.

On the other hand, transport performance at Mach 1.4 will only be ⅔ of that at Mach 2 for comparable L/D, so the Mach 2 design will win hands down if airliner economics are used to evaluate a design. As you said, if you venture into supersonic territory, you might as well go all the way to Mach 2 in order to optimize the profitability of the design. Mach 2 still allows to use a fixed-geometry wing and aluminium, so the overall complexity of the resulting design is still manageable. Beyond that, you need to buy more speed with increasingly more system complexity.

Dietrich Küchemann proposed an empirical formula for maximum supersonic L/D: $$\frac{L}{D}_{max} = \frac{4\cdot(M_{\infty}+3)}{M_{\infty}}$$
Using this as a yardstick will result in 12.5 at Mach 1.4 and 10 at Mach 2. Combine this with the Mach number in order to have something close to transport efficiency, and the Mach 2 design enjoys a hard-to-beat 14% advantage in economic value.

So the only sensible reason to design for Mach 1.4 is to capture the high end of the business jet market, where airliner economics don't apply but bragging rights at the golf course are more important.


Can the airport name be omitted when contacting ATC if the frequency is not used by any other nearby airports? I was thinking of contacting Clearance/Ground/Tower/approach and omitting the airport name to shorten the transmission. If the freqs are not used by any other nearby airports, I figure there would be no confusion which airport I'm talking to.  Is there any other concerns that make this not a good idea?

When every transmission to the tower begins with "Executive Tower...." that is the keyword that makes the controller's ears perk up and start taking notes.

If you don't say the magic words, your controller may be a step or two behind as you rattle off your request.

Also, you think you're on the right frequency, and you think you know who you're talking to, but what if you mis-tuned the frequency knob by a digit or two, or forgot to hit the COM1/COM2 button?  You could be talking to the wrong receiver and not know it.  That is how airplanes sometimes land at the wrong airport.

Every initial transmission should start with the intended receiver; that is part of the rules and just common sense.  Don't try to shorten transmissions and end up causing more work for everyone.


Am I correct about what AC/DC BAT BUS does? The purpose of the AC/DC BAT BUS is to distribute either A/C electrical power and D/C electrical to the things in the aircraft that need them. For example, I think he whole ECAM system and displays use A/C electrical power. There are 2 of each BAT BUS (AC and DC). Do they work together so AC BAT BUS 1 and 2 work together and both distribute randomly or for example does AC BAT BUS 1 supply certain systems and AC BAT BUS 2 supplies the others. Do they have assignments of which systems to power, or do they work together?

You can't easily link the two AC buses together, you'd need to synchronize the phases first. And then a short on one side could cause a complete failure so you'd have to isolate them again. Way to complicated to get that failure tolerant. Instead they just use two independent AC systems. Generally each system exists twice and each one gets a different power source.

In the A320 you have two main AC buses (AC 1 and AC 2) in flight these are powered by engine 1 and 2 respectively. And there is a AC essentials bus, which is either connected to AC bus 1 or if that bus is unavailable it is connected to AC bus 2. The AC essential bus can also get power from the DC essential bus via essential transformer. So you can run the captain PFD and ND and the upper ECAM on battery power (only available above 80kts or sth. or in the air - for emergency only)

Check out the A320 "electrical system descriptions" and "indicating and recording" http://www.smartcockpit.com/plane/AIRBUS/A320.html

At the bottom of the indicating/recording systems you have a bus equipment list telling you exactly which consumer uses which bus. This is true for all other sections as well, so if you are interested in a particular system you can quickly find out where it is powered from.

According to A320 FCOM, 1.31.75 Indication/Recording Systems, Electrical Supply, Bus Equipment List

  AC ESS bus powers the captain PFD and (via AC ESS SHED) the captain ND, the upper ECAM screen, the display management computer (DMC) 1 and
  3 (if selected and AC1 failed), flight warning computer (FWC) 1 and SDAC1. (and many many other things ...)
  AC 2 powers F/O PFD, ND, lower ECAM, DMC 2, FWC 2 and SDAC2 (...)
  AC 1 powers the DMC 3 normally (...)


If turning off electronic gadgets is so important, why are there no detectors used to find them? It's supposed to be that electronic gadgets' emission are dangerous for airplane systems. If this is true, why don't they use any detectors to locate such emissions?

Three reasons I can think of:

It's easy to detect the general presence of a specific transmitting device (a WiFi antenna, say), but very hard to precisely localize it. They could tell if there's one on the plane, but someone with a very sensitive piece of equipment would have to walk up and down the aisles pointing it at people to actually find it in somebody's pocket. And cellphones (when not in use) are generally burst transmitters: they don't broadcast continuously, they just "ping" the tower every so often. This means it could take a long time and a lot of aisle-pacing.
Many "electronic devices" which are banned are entirely passive and have no traceable[*] emissions. Think cameras, handheld GPS receivers, iPods. (On the other hand this passiveness also means they are extremely unlikely to harmfully interfere with the operation of the airplane, but right or wrong, such devices are still banned in some cases.)
It's not that important. There are a few incidents where electronic interference is suspected as a possible factor, but no conclusive evidence. As soon as there is actually a fatal accident where an electronic device can be conclusively blamed as the cause (and I doubt this will happen), the airlines will be a lot more willing to spend the kind of money that would be required to implement this.

For a full, evidence-based discussion of the actual risks, I refer you to this question on Skeptics Stack Exchange: Are personal electronics a risk to commercial aviation?

[*] No traceable emissions. All electronic devices generate some very, very tiny incidental signals. But these are so weak that they will be virtually impossible to detect on a crowded plane filled with its own (approved) electronics. Such emissions are measured in laboratories with extremely sensitive antennas in special, heavily-shielded rooms under carefully controlled conditions.


What are the different types of procedures for starting a fuel injected engine? Can someone explain the various types of procedures to start a fuel injected piston engine? What is a cold start, hot start, etc.?

I'm referring to a GA aircraft.

The key thing that makes FI engines different is how you prime them.  You are using the pressurized fuel injection system itself to do the priming instead of pumping fuel into the intake ports using a syringe pump primer.  You do this by turning the boost pump on to pressurize the system and setting the mixture to rich, which on a crude-ish mechanical FI system like the continuous flow systems used on airplanes, just starts spraying fuel in the injectors as long as the mixture is in, whether the engine is turning or not.

Two main differences from a carb:

The priming procedure is one of timing using mixture, not counting pump strokes.
You start the engine with the mixture in idle cutoff (because with the system pressurized and the mixture not at idle cutoff, it'll be spraying fuel in continuously and you'll soon have a little pond of fuel under the cowl).  You don't advance the mixture until the engine catches.

As far as hot/cold starting goes, the basics are the same as a carbureted engine, more prime when cold, less prime when warm, no prime when hot.  So for a cold engine that uses 6 strokes of a manual primer, with the FI engine it's pump on and mixture rich for some period like 5 seconds, then back to idle cutoff before you crank it.  On a warm engine that uses 2 strokes, the FI version might be 1-2 seconds.  You're doing the same thing, just with a different process.

Because it requires careful timing with prime and then keeping the mixture at idle cutoff until start, it's easier to screw it up and flood the engine by messing up the timing, or to be late with the mixture after it starts and it goes and dies on you.  So they can be a little bit fussy until you get the hang of it.   If you're bush flying on floats in something like an injected C-185, where you need to be able to know you can start right now once you push off from the dock, getting it right really becomes important and hilarity will often ensue (drifting toward rocks or moored boats in the breeze as you scream and curse) if you don't.

At the other extreme, because the injector distribution system is normally on top of the engine where all the heat collects, when an FI engine is hot on a hot day it can cause vapour lock in the fuel distributor and lines (even with the very low Reid Vapour Pressure of avgas) and they just don't want to start at all until the cowling cools down a bit.  Not realizing this, you may decide that it needs priming, so you start priming when it doesn't need it and things have cooled down enough for fuel to start to flow again, and now it still won't start (the raw fuel smell with be a clue).  They can be frustrating beasts.


Why do police helicopters always fly in circles? Why do police helicopters always fly in circles while news helicopter hover in one spot? Is it to avoid criminals opening fire at them, or is it because there is a difference between the two types of helicopters?

Police helicopters don't always fly in circles. I've often seen my local police helicopter hovering. There are a few reasons you might have seen police helicopters flying in circles:-

Not all types of helicopter can hover out of ground effect. It might be that the police helicopters in your area can't hover at the heights they work at, with the weight of their usual equipment.
Helicopters are safer if they're moving forwards than hovering. In the event of an engine failure, they can autorotate to land safely, but they need to have some airspeed to be able to do this. The required airspeed depends on height (see the chart in the linked question), so if your news helicopters are usually operated higher up than police helicopters, they may be able to hover safely. (This is quite likely, as police aircraft are exempt from the 500 ft rule and the 1000 ft rule, while news helicopters are not.)
Police helicopters often circle to search an area for a fugitive or casualty using thermal imaging cameras. News helicopters tend to either follow a vehicle or hover to get a steady camera shot of a news event.


What causes the ball to not be centered in a slip/skid? I've reviewed quite a few questions on this forum including
Forces in a slipping turn
is-this-vector-diagram-of-the-forces-at-play-in-turning-flight-correct
what-does-the-balance-ball-actually-indicate
but I don't believe they have answered this specific question.

It is the same thing that makes you slide sideways in your seat going around a corner in your car.
Or the same thing that would cause you to slide sideways the other way if you were driving straight ahead on a side hill.
Don't overthink it... the ball simply responds to the same forces you feel.
ADDENDUM:
“I agree the comparisons are spot on if you are only considering a coordinated turn, but I'm asking about a steady level turn in which the aircraft is slipping/skidding; which I don't think the comparison can easily be extended to?”
I actually think it is the other way around.  Consider the following:
•   CAR - Unless it is travelling on a banked racetrack, at precisely the speed the banked turn was designed for, a car will never be able to make a “coordinated turn”.  There will generally always be a centrifugal sideforce.
•   PLANE - Once adverse yaw at the turn initiation has been countered, and when established in a constant angle of bank turn, most (positively stable) aircraft will default to a coordinated turn.
In case it helps further, let’s take the aircraft “uncoordinated turn” to a couple logical extremes:

Wings Level, unbanked turn, using rudder only.  Think about this as it compares to turning a car on level ground.  This is an exaggerated skidding turn, with all the centrifugal force pulling you (and the ball!) horizontally, or sideways.

Straight ahead, level flight in a side slip.  The aircraft nose is tracking straight ahead and not turning, while the wings are banked.  This is an exaggerated slipping turn, (minus actually turning...) and is just like the example of driving your car on a side hill.  In this case, gravity is pulling you (and the ball!) vertically straight down, which from the aircraft's frame of reference, is a bit sideways.

The ball is a "dumb" instrument.  Nothing more than ball in a tube reacting to centrifugal force and gravity.  You could accomplish the same thing by holding a full glass of water between your knees:  If the inside knee gets wet it is a slipping turn, if the outside knee gets wet it is a skidding turn.
If these explanations are inadequate the only thing left to suggest is that you find a local flight school that offers orientation flights and ask the pilot to demonstrate for you.
If that suggestion isn't practical or affordable, try this simple experiment:  Get a round mixing bowl and a marble.  The round bowl simulates the curve of the glass tube.  Put the marble in the bowl and hold it at arms length and begin turning in place.  (if you think you might get dizzy and fall against the furniture, wear a helmet!)  Your goal is to adjust the angle of the bowl to keep the marble in the center.  Note that if you tip the bowl too much either way the marble will start to roll up the sides.
It isn't a perfect comparison because it doesn't simulate yaw, but can you appreciate the correlation?  Please let me know if any further explanation is needed...


Why is ground effect greater over smooth hard surfaces? Helicopter Flying Handbook: Ground effect is the increased efficiency of the rotor system caused
  by interference of the airflow when near the ground. The air pressure
  or density is increased, which acts to decrease the downward velocity
  of air. Ground effect permits relative wind to be more horizontal,
  lift vector to be more vertical, and induced drag to be reduced. These
  conditions allow the rotor system to be more efficient. Maximum ground
  effect is achieved when hovering over smooth hard surfaces. When
  hovering over surfaces as tall grass, trees, brushes, rough terrain,
  and water, maximum ground effect is reduced. Rotor efficiency is
  increased by ground effect to a height of about one rotor diameter
  (measured from the ground to the rotor disk) for most helicopters.
  Since the induced flow vertices are decreased, the AOA is increased,
  which requires a reduced blade pitch angle and a reduction in induced
  drag. This reduces the power required to hover IGE

I understand ground effect and In Ground Effect versus Out of Ground Effect. Air pressure is increased below the helicopter, reducing induced flow, increasing AOA and decreasing drag.

But why is ground effect greater over smooth hard surfaces than over tall grass, trees, bushes, rough terrain, and water?

I've searched Aviation Stack Exchange, textbooks, and the front page of Google, but haven't found an explanation.

At first I would think friction from surfaces like tall grass would hold air below the helicopter, increasing ground effect. But we know this isn't true, since this would mean there's less ground effect over a runway.

Another guess would be the terrain disrupts the air from bouncing back toward the helicopter. Does the air reflect sideways instead, meaning the force of air bouncing back toward the helicopter is what increases lift? The Helicopter Flying Handbook contradicts this explanation with the following as the core source of ground effect:

  "The air pressure or density is increased"

Don't be afraid to include complicated aerodynamics if necessary. :)

From a conservation of energy perspective, the rotor is transferring energy to the air, which then moves down and is deflected by the ground. A soft or drag-inducing surface will absorb more of that energy, whereas a smooth hard surface will deflect it.

Think of it like running: If you run on a hard surface, your down force creates an equal force up. If you run on sand, some of the down force is lost by moving the sand, and you are not propelled up/forward as much. This is why it is harder to run on sand than a solid surface.


Are you required to record flight hours of serialized parts? Is it a requirement to record/maintain flight hours for serialized A/C parts? I have searched the FAA website with no luck.

Short answer: only for the airframe, engines, propellors and rotors (as applicable).

This legal interpretation is a response to someone who asked which records are required to identify when life-limited parts need to be replaced. It says that you're only required to keep flight time records of the airframe, engine(s), propellor(s) and rotor(s), as appropriate:

  Existing regulations, specifically 14 C.F.R. § 91.417(a)(2)(i), require
  each owner or operator to keep records containing the total time in
  service of the airframe, each engine, each propeller, and each rotor.
  This is accomplished by the owner or operator recording and tracking
  in some form and manner the time in service of the airframe,
  engines(s), propeller(s), and rotor(s) from the moment the aircraft
  leaves the surface of the earth until it touches it at the next point
  of landing, as referenced in 14 C.F.R. § 1.1. In addition, §
  91.417(a)(2)(ii), and similar provisions in 14 C.F.R. §§ 121.380(a)(2)(iii) and 135.439(a)(2)(ii), require owners or operators (certificate holders) to keep records that show the current status of
  life-limited parts of each airframe, engine, propeller, rotor, and
  appliance.

91.417 does indeed say - among other things - that owners and operators have to record the time in service and status for the airframe, engines, props and rotors, but only the status for life-limited "appliances":

  (2) Records containing the following information:

  (i) The total time in service of the airframe, each engine, each
  propeller, and each rotor.

  (ii) The current status of life-limited parts of each airframe,
  engine, propeller, rotor, and appliance.

1.1. defines "time in service":

  Time in service, with respect to maintenance time records, means the
  time from the moment an aircraft leaves the surface of the earth until
  it touches it at the next point of landing.

And 43.10 defines "life-limited parts":

  Life-limited part means any part for which a mandatory replacement
  limit is specified in the type design, the Instructions for Continued
  Airworthiness, or the maintenance manual.

To summarize:

You only need to record service time for the airframe, engine, props and rotors
For any other life-limited parts, you only need to record their status, i.e. you need to be able to say when it's time to replace them


What does it mean when your destination airport is currently "below IFR minimums"? Does it mean that absolutely nobody can land there until conditions improve? Can ATC still give you an IFR clearance to your destination airport in this case?

It would be very helpful to know where you read or heard the phrase, to get some context. But, it likely means that weather conditions at the airport are below the IFR takeoff minimums in 14 CFR 91.175:

  (f) Civil airport takeoff minimums. This paragraph applies to persons
  operating an aircraft under part 121, 125, 129, or 135 of this
  chapter.

  (1) Unless otherwise authorized by the FAA, no pilot may takeoff from
  a civil airport under IFR unless the weather conditions at time of
  takeoff are at or above the weather minimums for IFR takeoff
  prescribed for that airport under part 97 of this chapter.

  (2) If takeoff weather minimums are not prescribed under part 97 of
  this chapter for a particular airport, the following weather minimums
  apply to takeoffs under IFR:

  (i) For aircraft, other than helicopters, having two engines or less—1
  statute mile visibility.

  (ii) For aircraft having more than two engines— 1⁄2 statute mile
  visibility.

  (iii) For helicopters— 1⁄2 statute mile visibility.

Landing minimums are per-approach so it wouldn't make sense to say that the whole airport is below IFR minimums based on landing criteria. However, without reading or hearing the phrase in context, I may be wrong.

As for landing there anyway, even in zero visibility a private pilot operating under part 91 is free to take off, or to start an instrument approach. You can't legally land if visibility is below the approach requirements, but you can still fly the approach to 'take a look'. It might not always be smart, but it's legal.

On the other hand, commercial flights under part 135 or 121 can't even start an approach unless conditions are above certain minimums (e.g. see 135.225(a)). Those minimums are usually just the ones required for the particular approach, but they can also have operator-specific requirements. Of course, many commercial aircraft have crew and technical capabilities that light aircraft don't (like shooting a CAT III ILS), so they might still have a better chance of landing in marginal conditions.


Does currency in one C172 model apply to other models? If I am current on one model of a Cessna 172, am I also legally meeting the currency requirements for a different version of the same aircraft model? For example, if I have many hours in a Cessna 172M, but have lately been flying a Cessna 172S, would I also be able to take passengers in the 172M?

This could be the case for a flying club that has many different versions of the same aircraft model. Perhaps on a nice day of flying the aircraft that you have been flying more recently has already been booked. You would still like to perform the flight with a different aircraft also offered by the club. However, it has been more than 3 months since you have last flown that specific aircraft.

@RyanGriffith is correct on the legal standpoint however its worth elaborating on your other point a bit

  This could be the case for a flying club that has many different
  versions of the same aircraft model. Perhaps on a nice day of flying
  the aircraft that you have been flying more recently has already been
  booked. You would still like to perform the flight with a different
  aircraft also offered by the club. However, it has been more than 3
  months since you have last flown that specific aircraft.

Most flying clubs, FBO's and anywhere else you may rent an aircraft from can impose other currency requirements and in many cases their insurance may require it. So while you may be legal to fly an aircraft that does not mean you always can, and in some cases if you do it may not mean the insurance will cover it should something go wrong. For example the flight school I fly out of has both fuel injected and carbureted 172's if you are checked out in the fuel injected one you are allowed to fly the carb'ed one but not the other way around. They also have similar requirements for their Cherokee's where you can fly anything with less HP than what you have been checked out in, for example I am checked out in their archer so I can fly that as well as their 161 warrior.

So in the specific case you mention for a flight club you should further confirm what their operating rules are.


Will an airliner with a quieter cabin have a quieter cargo hold? I am aware that this question is on the border of travel and aviation but nobody on the travel side of the fence seems to be able to answer it. Apologies if it is regarded as off-topic for this site.

The question and answers on Are cargo holds pressurised these days? provide some good info on pressure and temperature management in airliner cargo holds but none on noise management.

As part of the process of emigrating my wife and I will be taking our cats with us on an 11 hour flight. I have travelled the route regularly on both A380 and Boeing 747 services. My experience is that the passenger cabin of the A380 is significantly quieter.

Will the experience for the animals in the hold be similarly quieter, or is noise in the passenger cabin separately and specifically managed?

The cargo hold will probably be quieter if the cabin is quieter, but there are multiple variables that can affect this. As Peter Kämpf commented, the noise source will depend on the position in the airplane. The two main sources of noise are the air moving past the airplane, and the engines. The two main noise concerns of manufacturers and operators are cabin noise and community noise.

Reducing the noise at the source is one option. Engines are the main sources of noise and receive special treatment to reduce noise as much as possible. This can reduce cabin noise and/or community noise, and should help both the passenger and cargo decks. Newer aircraft like the A380 will tend to have quieter engine installations than older aircraft like the 747-400.

In the case of the cabin, noise can also be controlled with insulation. Insulation in the cabin will help with both noise and temperature control. Both are less important in the cargo hold, but do have some importance since it is only separated from the passenger area by the floor. Cargo hold insulation will provide protection for structure from fire or accidental damage, and some degree of temperature protection. So it is possible that the cabin receives added protection from these features that the cargo area does not benefit from.


What is the difference between RAIL and SFL lights? What is the difference between Runway Alignment Indicator Lights (RAIL) and Sequenced Flashing Lights (SFL)?

Both of them are same in terms of flashing light.

SFL start at the decision bar. RAILs may not.  The key differentiating factor is that RAILs extend beyond the centerline lights.


Do modern jet engines need igniters? I've read that modern aircraft don't need spark plugs like conventional aircraft do, as the jet fuel autoignites once it's mixed with the hot air exiting from the compressors. Then why do aircraft like the Airbus A320 still have 2 igniters in the combustion chamber, to ignite the air fuel mixture?

Jet fuel will not self-ignite when starting a modern turbine engine.
This article from the WingMag Aviation Magazine says:

As the temperature isn’t quite sufficient to initiate self-ignition (the autoignition temperature of aviation fuel is around 220 degrees Celsius), spark plugs are arranged around the combustion chamber. They generate a spark that ignites the air-fuel mixture and the turbine now drives the fan and compressor through a shaft, as described above. The exhaust gas temperatures begin to rise and the engine will now keep “running” as long as there is a supply of aviation fuel. The igniters are switched off by the FADEC once the exhaust gas reaches a certain temperature because they are no longer needed.

Once the engine is running at a sufficiently high speed, the autoignition temperature can be sustained (same article):

[The fuel] is injected into the combustion chamber through several “fuel nozzles” where it can self-ignite and continue to run if the temperatures are sufficiently high.

See also How is combustion flame maintained in the combustion chamber after igniters are switched off?
Igniters are not only needed for a cold start. They are also used in case of an engine flameout. From the Boeing 747-8 FCOM (7.20.8 - Engine System Description, emphasis mine):

Auto-Relight
An auto-relight capability is provided for flameout and rollback protection.
Whenever the EEC detects an engine flamout or rollback, both igniters are
activated. A flameout is detected when a rapid decrease in N2 occurs. A rollback
is detected when N2 falls below idle RPM or compressor pressure falls below idle
minimums.

Note that this is for a modern jet engine, the General Electric GEnx.


Can I convert an FAA medical certificate to a Canadian one without visiting an AME? I currently hold a first class FAA medical certificate. Can I transfer to a Canadian medical certificate without meeting with a Canadian AME?

  (2) The applicant may obtain a TCCA Medical Certificate as follows:

  (i) The applicant may submit either the original or certified copies
  of FAA medical examination reports directly to the Civil Aviation
  Medicine Branch of Transport Canada, Headquarters in Ottawa for
  processing and assessment;

or

  (ii) Applicants may complete an aviation medical examination by a
  person who is considered to be a medical examiner with the authority
  to conduct aviation medical examinations pursuant to CAR 404.16

You cannot convert an FAA medical certificate directly into a Transport Canada medical certificate. However, you can obtain the Transport Canada medical certificate on the basis of the original or certified copies of FAA medical examination reports by submitting them directly to the Civil Aviation Medicine Branch of Transport Canada, Headquarters in Ottawa for processing and assessment. If they are deemed acceptable, the certificate would be issued in Canada without having to start the process with a Canadian AME. For more information, see Transport Canada Advisory Circular (AC) No. 401-001, "Application Guidelines for Aeroplane FAA to TCCA Licence Conversion Agreement".


From how far strobe lights are visible? At what distance would you begin to see the sequenced flashing strobe on an approach light system at night? 10 NM? 20 NM? Visibility plays a role, but consider CAVU.

Also, strobe lights on radio and cell towers, what is their visibility distance at night?

And at those distances, are they initially faint as they fade into view?

For cell towers the FAA wants them to be visible for pilots in time for them to avoid them.

  Pilots of aircraft travelling at 165 knots (190 mph/306kph) or less
  should be able to see obstruction lights in sufficient time to avoid
  the structure by at least 2,000 feet (610m) horizontally under all
  conditions of operation, provided the pilot is operating in accordance
  with FAR Part 91. Pilots operating between 165 knots (190 mph/303
  km/h) and 250 knots (288 mph/463 kph) should be able to see the
  obstruction lights unless the weather deteriorates to 3 statute miles
  (4.8 kilometers) visibility at night, during which time period 2,000
  candelas would be required to see the lights at 1.2 statute miles
  (1.9km). A higher intensity, with 3 statute miles (4.8 kilometers)
  visibility at night, could generate a residential annoyance factor. In
  addition, aircraft in these speed ranges can normally be expected to
  operate under instrument flight rules (IFR) at night when the
  visibility is 1 statute mile (1.6 kilometers).

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2070%207460-1K.pdf (page 63)