Three questions

[I-FICO]

Hello everybody...
This is my first post...

1)Is there any difference between Vref and Vth.Where can I find definitions?

2)Why it is said that the airplane stalls at the same Ias regardless of the altitude?
I've found some explanations but never really understood.

3) Is there any difference between "Mayday Mayday Mayday" and"Emergency"?

Thank you for your collaboration :ok:

[kwokwinguk]

1.Vth/Vat is the threshold speed
Vref is the reference speed
To me, they are both the same. The target threshold speed when coming to land.

2. Pressure difference between the upper and lower wing doesn't change with altitude.

3. Not too sure, but it tend to be spoken on rt's calls.

[alpha.charlie]

In order to declare an "emergency" the RT call has to be prefixed with "mayday mayday mayday" for the avoidance of any doubt.

[welliewanger]

1. Vref is 1.3 times the stall speed in the approach configuration. It is a calculated minimum safe speed for the approach. Many operators will approach at speeds higher than this, eg Vref+10
Vth (I'm assuming you're talking about what I know as Vat - speed At Threshold) This speed is not so much of an "official" speed. However if the approach is not going to be flown at Vref, then the Vat speed may be discussed, for example "I'm going to fly the approach at Vref+10 and close the throttles early to slow reduce my Vat closer to Vref".

2. Actually, it stalls at the same EAS (Equivalent Air Speed) To explain this would take many pages. If you've got plenty of time on your hands then I suggest you research:
IAS (Indicated Air Speed) - Shown on the instrument
CAS (Calibrated Air Speed) - Corrected for position error
EAS (Equivalent Air Speed) - Corrected for position and Compressibility
TAS (True Air Speed) - Corrected for position, compressibility and density

3. An aircraft may be having an emergency, but not declare a "Mayday". Mayday means something very specific. It means "We require urgent priority to land"

[Aviast]

Assuming you mean Vat here's what my Jepp has to say:
Vat is the indicated airspeed at the threshold which is equal to the stalling speed Vso multiplied by 1.3 or the stalling speed Vs1g multiplied by 1.23. Both Vso and Vs1g apply to aircraft in the landing configuration at maximum certificated landing weight. If both Vso and Vs1g are available for an aircraft, the higher resulting Vat must be used.Vat is used to categorise aircraft to determine which variation of an instrument approach is used. It seems that the difference is that Vat is calculated at max. landing weight whereas Vref varies with Vso (which varies with weight).
I just answered this question the other day - there must be something in the water! My answer (http://www.pprune.org/flight-testing/412815-stall-speed-altitude-sea-level.html) is being ripped to shreds in the flight testing group :}
"mayday mayday mayday" is the proper lingo to use when declaring an emergency.

[phillpot]

A bit more info on Q2, If you think of the ASI as a dynamic pressure gauge rather than a speed indicator that might help, the needle points at a speed because thats useful to us pilots however its actually measuring dynamic pressure i.e the amount of air molecules passing over the wings, the wings are only interested in the density of airflow passing over them not speed, at altitude you have to go much faster in reality to achieve the same volume or density of air passing over the wings therefore the same dynamic pressure as the ASI reads dynamic pressure the aircraft will always stall at the same indicated speed or pressure to be more correct.