February 22, 2024

How unusual is it for aircraft to exceed the speed of sound?

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The media this week was full of stories talking about aircraft going faster than the speed of sound.

Freak, supersonic wind speeds, some exceeding 800 miles per hour.


For reference, the speed of sound around cruise level (roughly 35,000 ft) is around 664 mph.  Some eastbound, north Atlantic flights arrived 30 to 45 minutes early, one over an hour early!

So what happened this week over the North Atlantic and how unusual was it?  All will be revealed here.

First, a bit about planes going "supersonic".   To be truly supersonic, the aircraft has to move faster than the speed of sound (roughly 664 mph a cruise altitude) relative to the air.   

They never do.  

When the media talks about "supersonic" they are talking about the speed with respect to the ground, which is NOT the same thing.  Aircraft can go very fast relative to the ground if the aircraft has a large tailwind.....as was experienced earlier this week.


Since the average jet aircraft flies around 560 mph at altitude, a tailwind of over 110 mph would result in it moving faster than the speed of sound relative to the surface.  

The winds aloft were truly screaming along the U.S. East Coast on Sunday and Monday of this week.  Here is a short-term forecast of the winds at a pressure of 200 hPa (about 38,000 ft) for Sunday morning.  Wow.   

Over the mid-Atlantic states and offshore, some winds were forecast to reach 215 knots (247 mph).  That is cranking.  And since these winds were from the west, aircraft heading to Europe were going over 800 mph to the east.


So how unusual were the winds, say over Washington, DC during this event?    

There is a radiosonde (balloon-launched weather station) at Sterling, Virginia (Dulles Airport), just to the west of Washington, DC.     On Sunday morning, the ballow measured a speed of  218 knots from the west.

Impressive.  But how unusual?  Below is a plot  (red line) of the daily 250-hPa (about 33,000 ft) strongest winds at that location for a period from 1950 to today.  The star indicates the Dulles winds on Sunday.  

That morning, there were the second strongest winds at that level in over 70 years! That is very, very impressive.

Monday evening the strong winds moved offshore, still giving European-bound aircraft a mighty boost eastward (see wind speeds below)


Although these winds were exceptional over Dulles and clearly stronger than normal, they are not as exceptional when considered on a hemispheric or global scale.

To illustrate, below is a plot of the winds aloft (250 hPa level, about 33,000 ft) at the same time for the entire hemisphere.  Several strong-wind hot spots are evident, with several showing winds exceeding 160 mph.


A forecast of the winds for next Wednesday at the same level shows very strong winds over the SE US, with plenty of "supersonic" potential around the world.


The southeast U.S. has done very well for strong winds during this period,  but typically the strongest jet stream is found over southern Japan and out into the Pacific.  

The strongest jet stream ever observed by a radiosonde?  259 mph near Yanongo Japan in 2004.


9 comments:

  1. The USAF discovered the phenomenon known as the jet stream when the bombs they were dropping over Japan during WW2 kept falling fall short of their targets. They were dropping them from very high altitudes at the time, due to the use of the Superfortress bombers. They then transitioned to dropping bombs from extremely low altitudes, with devastating effects.

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  2. I once flew from Seattle to London Heathrow in 6 hr, 45 minutes (Sept. 2017) because the jet stream was so strong. I think our speed hit 775 on the seatback monitor- the captain noted that this would be ours, and likely his, SEA-LHR flight ever.

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  3. This post makes me think of one of Albert Einstein's thought experiments. More generally, the concept of relativity.

    Speaking of wind:
    I was on I-90 east of Ellensburg yesterday (2/22) at Noon and again at 5pm. There are numerous wind towers that can be seen on the Ryegrass crest. At Noon there was thick fog but the blades I could see were not moving. At 5 pm, I could see better. A few blades were slowly moving, most were stationary.
    The BPA chart of load and supply confirms this entire week to have been a poor producer of wind energy. This morning, the airport (KELN) just on the north edge of Ellensburg reports zero mph wind.

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    Replies
    1. John, I've downloaded the BPA's SCADA data for 2022, 2023, and January of 2024 into an Excel spreadsheet.

      Within the BPA's area of load balancing authority, the 2833 Mw of installed wind nameplate has been running at a capacity factor of approximately 24% for the last two years.

      The installed solar nameplate of 138 Mw is also running at about 24%, a figure higher than I thought it would be. (I've got to recheck my calculations to be sure the solar CF figure is accurate.)

      In contrast, the Columbia Generating Station with a nameplate of ~1200 Mw nuclear has a capacity factor of 87% over the last two years.

      In addition to refueling outages which last a month or so, there are periods where the BPA instructs the station to generate at a partial capacity of 60% or less.

      Within the coming decade, as the shortage of electricty in the US Northwest begins to become acute, we should expect to see the Columbia Generating Station's capacity factor rise into the 92% to 95% range.

      When it isn't offline for a refueling outage every other year, it will be humming along at near 100% capacity 24/7/365.

      Delete
  4. You are talking about ground speed, not airspeed. These airplane's airspeed is not supersonic.
    The two are competently different measurements!

    ReplyDelete
    Replies
    1. I guess you didn't read his blog post

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    2. Which Cliff made abundantly clear in the post.

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    3. . . . Restating the point of Cliff's post, as if he didn't just make EXACTLY THE SAME point himself, is not a great look.

      Delete
  5. Most aircraft cannot go supersonic because their airframe is not strong enough to withstand a supersonic shockwave. The airframe would have to be heavier to have the necessary strength and while you a small fighter jet with powerful engines can have a heavy airframe it would be hard to do that with a big commercial jet. The Concord had to consume an enormous amount of fuel to fly faster than sound and Concord tickets were alot more expensive than the typical airline ticket. What matters is the speed at which the plane is traveling through the air the plane may have a ground speed of 800 mph but if there is a 200 mph tailwind the plane is only moving through the air at 600 mph.

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Please make sure your comments are civil. Name calling and personal attacks are not appropriate.

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