Pilots: what is the purpose and function of the "barber pole" on an altimeter?

Question

I searched high and low through my Jep training manual for an answer to this but couldn't find anything, so I googled it and got somewhat different answers from two different sites. Apparently it's something that computes the maximum allowable airspeed for a given density altitude and temperature....but what does this mean exactly? Is it V-NO or V-NE?

Plus, how do I read it? On some of the cockpit illustrations I've studied, the needle is a red and white pole (like a candy cane) which points directly to V-NO/V-NE (whichever it is) but in the plane which I'm flying now (a Cherokee) it's black and white (like a zebra) and looks like a fixed card that doesn't move.

References:
http://webpages.charter.net/flyian/757-767/capt.htm
http://fsinsider.com/About/Learning-Center-Support/In-the-Cockpit/Cockpit-Basics.htm

Answer 1

it marks v-ne,

and its on the airspeed indicator, not the altimiter

The reason it is stationary in the cherokee, ( and most light aircraft ) is because the V-NE never changes,

heavier aircraft have different V- speeds depending on how they are loaded.

It wont be on your written test.

Answer 2

The barber pole functions as a limiting speed calculator. It essentially shows you your max speed expressed in % mach (Maximum Mach or Mach Crit: Mmo). That number changes with altitude/pressure/temperature so the needle must be able to move. It is painted as a barber pole rather than red to avoid confusion with Vmo (V maximum operating speed) which is a fixed value that is typically more limiting at lower altitudes.

I have no idea why your cherokee would have a barber pole. It most likely acts in the same fashion, though you probably are not having mach issues in that plane.

Answer 3

It's pretty much Vne, the speed at which the fuselage and wings part ways.

To reference the old Hawkers and Jetstars I used to maintain, it's actually a decimalled Mach number. When dealing with Mach limited aircraft, the speed of sound slows at higher altitudes because sound travels more rapidly through denser mediums.

0.82 mach at 1000' ASL is going to be a much higher indicated airspeed than it will at 30,000'.

The red and white barber pole, which I've never seen in an aircraft incapable of achieving sonic speeds except in a prolonged and deliberatly suicidal dive, is on the ASI. In such aircraft, Vne is typically a redmark on the ASI.

The B&W zebra is on the altimeter, an entirely different instrument, (still a part of your pitot static system,) and likely indicates the maximum certified altitude for the airplane.

Unless you really wish to finalize your life in a smoking crater, I don't recommend mistaking one instrument for the other; in which case, might I suggest practicing over a largely unpopulated area? Please, I insist.

Answer 4

There is a very small barber pole on drum type altimeters that tells you you are below 10,000 feet, replaced by the number 1 when passing this altitude. (#9,990 - 10,000)

Barber poles are found on the air-speed indicator to let you know when you are approaching Vne.

Answer 5

It is Vne but it doesnt change on smaller planes, it does change on larger planes, not based on weight, but altitude, as they climb, it marks the Vne mach speed. It will stay at the knot Vne until the mach Vne is less...

Answer 6

Never seen one on a Cherokee or one that is B&W, but it is VNE I'm 99% sure.

Answer 7

theres one on the cessna 172 its just there to give you a quick perspective that you are below 10,000 ft once you pass 10,000 ft it goes away.