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What does the crosshatched area on an altimeter tell you? It's only visible when below 10,000 ft indicated altitude. Can't be terrain awareness, a lot of terrain around here is over 10,000 feet.

This sample altimeter display from the FAA manual on flight instruments shows the crosshatched area:

enter image description here

Here's the accompanying text from the manual where it points out the crosshatched area:

Crosshatch flag: A crosshatched area appears on some altimeters when displaying an altitude below 10,000 feet MSL.

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    $\begingroup$ Related on modern PFD, here speed indicator with barber poles highlighting dangerous speed values. $\endgroup$ – mins Dec 24 '15 at 13:43
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It looks like the crosshatch (or barber pole) indicator doesn't have a consistent meaning (a similar marking is used on some airspeed indicators, by the way). I found several different examples of when it can appear:

The only source that gave any explanation is the last one:

When the altitude is below sea level the barber pole is no longer visible. This is provided to avoid the error of reading –1,000 Ft. as being +10,000 Ft.

After following a bunch of links and skimming some discussion forums, my conclusion is that @RalphJ is probably right: manufacturers added the flag to highlight that the aircraft is at a relatively low altitude, in a way that's much easier and quicker for the pilot than 'parsing' multiple hands on the altimeter face. And the fact that different sources give different altitudes highlights an important point: it's always a good idea to read the handbook or manual for all your instruments because you can't assume that everything works the same way in every aircraft, even something as 'simple' as an altimeter.

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It replaces to 10 thousands digit, evidently because X9000 has been found to be easier to read than 09000 or just blank space before the 9000. That crosshatched area would also scroll to display "NEG" if the altimeter is displaying an altitude below zero feet (i.e. Below sea level or below the reference datum plane).

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    $\begingroup$ If you find any research that supports the claim, please add a reference. I agree that is the most likely explanation, but I have yet to find any supportive research. $\endgroup$ – RockPaperLz- Mask it or Casket Dec 23 '15 at 23:43
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    $\begingroup$ @RockPaperLizard -- this requirement was written in blood, not research papers -- there have been several CFITs where the misreading of a three-pointer altimeter has been a contributing factor. $\endgroup$ – UnrecognizedFallingObject Dec 24 '15 at 6:43
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    $\begingroup$ I just reread the question and this answer, and I'm wondering if this answer is talking about a related, but different, altimeter design element. In the OP's image, the crosshatch is a discrete area, whereas in the answer, the crosshatch is referring to an 'X' that replaces the digit zero. $\endgroup$ – RockPaperLz- Mask it or Casket Dec 24 '15 at 22:07
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    $\begingroup$ Agreed @RockPaperLizard, I read the Q and A a few times and can't figure out how this relates to the question exactly. $\endgroup$ – zymhan Jan 29 at 21:15
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    $\begingroup$ @zymhan As best I recall, this answer came before the edit that added a photo to the question. On an electric altimeter with a single hand & scrolling digits, this answer is descriptive; for the present state of the question, it isn't responsive any longer. $\endgroup$ – Ralph J Jan 29 at 23:14
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Crosshatch also in some of pressurized aircafts predicts the caution to pressurize or de-pressurized while ascend or descend respectively. The indicator in some aircraft automatically pressurize the aircraft using electrical signals as precautionary to pilot and passenger safety to get into a partial or total hypoxia.

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  • $\begingroup$ Welcome to the StackExchange. Do you have a source for your answer? The reason I ask is that: 1. 10000 feet is a rather arbitrary number for pressurization purposes; 2. Pressurization is controlled by a separate device and guage that measures cabin pressure altitude instead of MSL; 3. Pressurization equipment is usually set to 8000 feet; 4. 10000 feet represents the altitude at which speed restrictions apply in the US NAS. $\endgroup$ – Dean F. Jan 29 at 23:19
  • $\begingroup$ This makes no sense; what value is there in a crosshatched warning flag that goes out of view above 10k when the need to pressurize is greatest??? Beyond that, in my experience this explanation is simply incorrect. Pressurization manually initiated at 10k? Never heard of such a setup. $\endgroup$ – Ralph J Jan 29 at 23:19
  • $\begingroup$ Also, this alert flag would not at all assist the pilot in case of a slow depressurization or equipment failure. $\endgroup$ – Dean F. Jan 29 at 23:29
  • $\begingroup$ You may be confusing this with the cabin pressure altitude gauge which measures the inside air pressure of the airplane in its equivalent feet of altitude at ISA. It would look similar. The crosshatched alert would work in the opposite manner as the one the original poster asked about. It would also not be located on the instrument panel in the same location as the six pack. It would either be with the engine gauges or a separate panel along with the pressurization controls. Since it works automatically, there would be no need to monitor it as closely as flight instruments. $\endgroup$ – Dean F. Jan 29 at 23:39

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