Who's Online

We have 1409 guests online

Live Traffic Feed

Life in the Skies

'A Local Bestseller!'

What Tony says
(See here)
and Book Launch video here
 
What others say

Les Posen
(See
here)

Yvonne Lee
(See
here)

Louisa Lim & Allan Koay
(See
here)

Aireen Omar, Asran & Bo Lingam
(See
here)

38 Readers' Comments
(See here)

Get an autographed copy here

AMAZON.COM  -  To Order, please Click here 

(eBook) Kindle edition - please Click here

Latest Comment

ETOPS and CRM querie
Hi, If the two engines Boeing 777 is just as good

SIA Plane on Fire
To be clear, the aircraft was evacuated, though it

Is it true that comp
The problem is a computer on any aircraft is only

A successful SIA Cad
Is there any deadline for application? If it is op

Should I proceed wit
I am reading some people's post, and to be honest,

A cadet pilot sharin
Before the test, the examiner said that they made

Thunderstorms and th
Hi Captain Lim, I would like to know if there ha

What happens if I ge
I have seen a comment from a Flight attendant that

What are the eyesigh
Captain, I am in class 11 in India and want to bec

What are the eyesigh
PLEASE stop asking about your eyes!!! Read the art

Paperback Version

 For Local Availability - Check Here
 
Home > Flying the Plane > Were we breaking the sound barrier or was this a false reading?
Were we breaking the sound barrier or was this a false reading?
Flying - Flying the Plane
Saturday, 17 December 2005 08:59


Hi Captain Lim,

Thanks for a great site.

I have a question about air speed versus ground speed. On a recent flight from LAX to ORD on a 737-800, the in-flight display showed an air speed of 520 miles per hour but a ground speed of 748 miles per hour!

Is that even possible? I think I remember hearing that the speed of sound is approximately 680 mph. Now I know that no commercial airliner (other than the retired Concorde) can travel faster than the speed of sound, so this doesn*t add up to me. The captain said that we had a very strong tail wind and we were in moderate turbulence for most of the flight.

Were we breaking the sound barrier or was this a false reading? Lastly, exactly what is the difference between ground speed and air speed?

Thank you!

Brian

Hi Brian,

The short answer to your question - whether you have broken the sound barrier, is no. Yes, it seems confusing to many others who asked me this similar question before. Thanks to the modern planes with all those in-flight displays that show how fast you are flying and what sorts of wind you are getting to push you home!

Many intelligent travelers (like you) have thought: "I am now flying at 748 mph but the speed of sound is around 680 mph. Therefore, the plane must have broken the sound barrier!" Well, this is understandably true to most people who have not grasped the difference between ground speed and Mach number or speed of sound (equals to the digit 1.0)

The ground speed is the speed at which an object travels relative to a fixed point on the Earth*s surface whereas the Mach number is measured with respect to the velocity of the object through the air or its airspeed. So, the main factor causing the difference between the ground speed and airspeed is the wind speed. Look at this formula:

Ground Speed = Air Speed (plus or minus) Wind Speed.

If you get a head wind, then you have to subtract the wind speed and if you have a tail wind, you add it to your airspeed. Thus, in your case, the air speed on the Boeing 737 was 520 mph, and your tail wind was 228 mph giving you a ground speed of 748 mph. A 228 mph tailwind is not impossible in a jet stream.

Remember, Mach number is independent of ground speed or wind speed. It depends on the plane*s speed relative to the local airflow. Yes, theoretically, the plane*s speed has exceeded the speed of sound relative to the ground but one cannot say that the Boeing 737 has exceeded Mach 1.0

Why? Well, the Boeing 737-800 was never designed to be a supersonic liner. Its shape and structure was not like the Concorde. If you were flying supersonic, the plane would vibrate severely as it hits the sonic boom. Further, damage to the wings is likely as it accelerates beyond Mach 1.0!

To be more exact, the speed of sound at 35,000 feet is 663 mph (1,067 km/h) based on the standard atmosphere. When you flew from Los Angeles (LAX) to Chicago (ORD) you noted your ground speed as 748 mph. That would be Mach 1.12 (748 divided by 663) but in reality, you were flying at Mach 0.78! (520 divided by 663)

The 748 mph registered on the in-flight display was not a false reading of the ground speed. It was a true reading! Further, your plane did not break the sound barrier but was merely flying at 78 per cent speed of sound. If it did, you may never have landed in one piece; but in your case, you have indeed reached your destination faster - not only that - safe and sound as well!

Trackback(0)

TrackBack URI for this entry

Comments (0)

Subscribe to this comment's feed

Write comment

smaller | bigger

busy
 

Paperback Version

 For Local Availability - Check Here

Recommended By..


LIFE IN THE SKIES

'A Local Bestseller!'

Recommended by

Patrick Smith
Boston USA
(See
here)

Capt Meryl Getline
ex-United Airlines USA
(See
here)

Capt Doug Morris
Canadian Airlines
(See
here)

Capt  Robert J Boser
ex-United Airlines USA
(See
here)

38 Readers' Comments
(See here)

Get an autographed copy here

AMAZON.COM  -  To Order, please Click here 

(eBook) Kindle edition - please Click here

View Book Launch video here

Follow me

@CaptKHLim

Like What You Read?


If you like what you read, more stories are found in my book LIFE IN THE SKIES (Preview here) and you can purchase a copy here. To check for any latest updates or postings, you can follow my Twitter at @CaptKHLim or Facebook here

MH 370 Interviews

Click here to View

10 Most Popular Posts



Disclaimer | Privacy
2004 - 2011 © AskCaptainLim.com | Site Concept by eQuilec.com