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The recent eruption of the Hunga Tonga–Hunga Ha'apai sent a shockwave so massive that it got picked all over the world by weather stations.

This is the pressure detected one in Italy (source), where multiple spikes were detected:

enter image description here


What I'm wondering is, could such a "sudden" pressure change significantly disrupt flying?

I'm both thinking at high altitude (standard pressure), which might start ascent/descent manouvers (what about RVSM?), as well as low altitude (where one might find themselves in the wrong spot for, say, an approach; or violate MSA, etc.).

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    $\begingroup$ Interesting graphics show an initial pressure wave moving West to East, followed by the long path no one around the world East to West. WaPo article also shows records moving across FL and also reports of audible blast heard in Alaska! $\endgroup$
    – Ian W
    Jan 17 at 12:01

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I will convert to Freedom Units as that is what I am familiar with:

1028 millibars (the top of the spike) is 30.36 inHg.
1025 millibars (the minimum) is 30.27 inHg.

As a rule of thumb, at sea level, one hundredth of an inch of mercury corresponds to about ten feet of pressure altitude. Thus this difference would correspond to a difference of about 90 feet, if a pilot happened to set the altimeter at 2100 (the spike's maximum) and did not reset it by 2130 (the minimum).

This is a very noticeable difference, and would certainly trigger a "pressure rising/falling rapidly" remark on a weather report. But it really wouldn't have any detriment to most aircraft. Separation standards (both between two aircraft and between an aircraft and the ground) are quite conservative, and can handle a discrepancy of 90 feet.

It would be most dangerous for an aircraft on a non-precision approach, as there is no vertical guidance along the approach path and the pilot must instead make sure to not descend lower than a specified altitude at one or more specified points. An altimeter setting higher than it should be will cause the aircraft to be lower than expected ("high to low, look out below"). But again, procedures are designed with a margin of safety—a difference of 90 feet is probably not enough to cause an accident.

In the flight levels there would be no noticeable effect. Pilots and autopilots would silently adjust their altitudes to maintain the same pressure altitude, which would indeed result in a difference to their true altitudes, but because everyone uses the same altimeter setting everyone would change at once and separation would never be compromised.

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  • $\begingroup$ Here's pressure chart from Washington/Reagan Natcl Airport of the larger, second East to West pressure wave $\endgroup$
    – Ian W
    Jan 17 at 12:19
  • $\begingroup$ @Ian: larger but not by much, perhaps 00.12 inHg, corresponding to ~120' difference in pressure altitude peak-to-trough over ~30 minutes. Again, noticeable but not immediately dangerous. Certainly nothing that would "significantly disrupt flying" per OP's question. $\endgroup$
    – randomhead
    Jan 17 at 12:27
  • $\begingroup$ Sticking with scientific units ;) the change in hectoPascals between 1025 and 1028 equates to about 15 meters pressure altitude which is on the limit of whether or not it would trigger an auto-pilot adjustment. It would not affect a landing as internally measured pressure altitude is not used on final. $\endgroup$
    – Paul Smith
    Jan 17 at 18:39
  • $\begingroup$ @Paul, thanks for the SI units! I'm curious about your last statement—what reference does the pilot use when on a non-precision approach, such as a GPS or VOR approach, if not the "internally [or externally] measured pressure altitude"? I was under the impression they did use that alone, which of course is the reason non-precision approaches have higher minima. $\endgroup$
    – randomhead
    Jan 17 at 19:23
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    $\begingroup$ Have never before heard the expression "Freedom Units"-- is that what they use in the recipe for "Freedom Fries"? $\endgroup$ Jan 17 at 20:51

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