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Home > Flying on the Boeing 777 > A Boeing 777 can’t fly this fast!
A Boeing 777 can’t fly this fast!
Flying - Flying on the Boeing 777
Monday, 07 November 2011 01:39

Another Boeing 777 flying across the Atlantic

Hi Capt Lim,

I recently took a BA 777 flight from Dulles to LHR... The Pilot announced that it would be a shorter journey time wise due to the prevailing wind.

1 hour into the flight and I heard some one say  "a 777 can't go this fast".

I know a little about the 777, but in no way professionally. But I looked at the GPS info and the ground speed was hitting 760 mph...

Shortly after, we reduced speed to around 650 mph, and then later it came down again, but considerably.

We were over London in 5.5hrs...

While impressed, I was left wondering what the hell is the maximum structural limit on both the aircraft and the wings.

Simon

Hi Simon,

Your question is similar to many others that I have received before, typically like ‘Does a strong tail wind cause the plane to fly supersonic?’

Flying in an easterly direction to London, one would usually expect to encounter strong tail winds due to the prevailing jet streams. Hence you would normally take a shorter time to get to London than to get back to Dulles.

Hence the common question from travelers when encountering a strong tail wind in a jet stream is whether the plane has actually broken the sound barrier?

Well the answer is no. The Boeing 777 that you flew on is a subsonic plane only. It means she is designed to fly below the speed of sound. Even if there is a strong tail wind pushing the plane forward and the ground speed may have exceeded the theoretical speed of sound, in reality, the plane has not gone supersonic.

The confusion arises between the understanding of ground speed and the plane’s speed. The ground speed is the speed at which an object travels relative to a fixed point on the Earth's surface. The difference between ground speed and airspeed is caused by the influence of winds on the overall speed of the aircraft.

This is analogous to you walking at 2 mph along a walkalater (travelator) that is moving at 2 mph. Your actual movement towards your plane at the gate is pretty fast at 4 mph (2 + 2) but as far as you are concerned, you are still walking at 2 mph!

So even if you were seeing a ground speed of 760 mph on the GPS, the Boeing 777 has never exceeded it structural limits of the plane or its wings it was designed for!

PS. To check for any latest updates or postings, you can follow my new Twitter at
@CaptKHLim


Jet Stream across the Atlantic

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Not the real speed per say
The plane had a tail wind. Its going a certain speed and the parcel of air it is in is going, in this case, with a velocity component going the same direction as the plane. Add the 2 speeds.
Hypothetically, ff you went for a swim in the sea and picked up a ocean current in the same direction, heck even riding a wave in to the shore, swimming as hard as you could you would go faster than the fastest speed you could swim at...
Andrew , 07 Nov, 2011
Ret. Chief, Aerospace Engineering
Using simple layman language, the ground speed (i.e. the aircraft speed relative to the ground) can be very high, say, 700 mph if there is a strong tail wind.
However, the airspeed (i.e. aircraft speed relative to its surrounding air such as a strong tail wind)in this case can be substantially below the speed of sound. As Captain Lin says, B777 is designed as a subsonic airliner, the structure is not certified to fly supersonic.
Another analogous example is one sitting in a row boat without paddling, but simply flows with a strong current downstream. The boat's speed is close to zero (if you put your hand in the water, you won't feel that the water is moving). However, you certainly can see well that you are moving downstream very quickly due to the strong current.
It should be also interesting to note that an observer on the sun, the little row boat on earth is flying at 67,000 mph (relative to the sun)!
Hope that helps or trigger more questions perhaps!
Peter
Peter To , 08 Jul, 2013
Ret. Chief, Aerospace Engineering (DAR)
Hi Simon,
I forgot to include the answer to your question on aircraft structural limits.
Almost all modern airliners are certified to FAA's FAR Part 25 (Airworthiness Standards for Transports.)
or near equivalent criteria such as CFAR and JFAR with Canadian and European respectively.
Specifically for loads on the wings and fuselage, the structures are required to withstand forces associated with landing, gust, manoeuver etc. under the various speeds, altitudes and aircraft weight conditions. Just to name one of the many, it should withstand 60 ft/sec guest (both up and down gust) up to various speeds. In other words, if the aircraft is designed to fly at Mach 0.9 (90% of sonic speed), the loads have to be determined accordingly. Once the type of aircraft is certified, they are limited to such aircraft weight, speed and altitude. The certification typically includes structural strength, durability and damage tolerance analyses and a long list of structural tests on aircraft built just to demonstrate compliance.
Hope that helps.
Peter
Peter To , 08 Jul, 2013
Congrats from Brazil!
hi captain! just passing by to congrat you for the awesome job! that's a really nice site and a good initiative, i've learned so much about the 777 since i found your site, thank you!
Jefferson , 24 Aug, 2013
...
Relativity well explained smilies/smiley.gif
Donny , 05 Nov, 2013
BA Flight 114 did go supersonic
Hi Captain Lim…

I'm gonna go out on a limb and respectfully disagree.

I think this British Airways Flight 114 did in fact exceed Mach 1. The reason for that is Mach is not dependent on relative WIND speed. Mach speed is only dependent on temperature and pressure.

At 35,000 feet, Mach 1 is approximately 660 mph (574 knots). That speed is relative to a fixed point on the ground on earth. BAW114 exceeded that speed by quite a bit and was likely going around Mach 1.2.

Boeing aircraft only measures relative Mach wind speed based on what the plane's wings and pitot airspeed tubes feel as aerodynamic forces. It's not actual Mach speed relative to ground. For example, if you were to theoretically put a rocket engine on a 777 and take it into space and fly it at 17,000 mph, the speed tape (IAS would likely measure 0 and the Mach would also be 0. But that doesn't mean the plane is not traveling at Mach 28! The wings only care about how much aerodynamic load is exerted on it. Boeing did not design their planes to be flown supersonic with 200 mph tailwinds in mind.

So although the IAS speed tape on BAW114 was only showing Mach .85, this plane was actually traveling at Mach 1.2 with the tailwind and did in fact break the sound barrier and did emit a sonic boom. Think of the space analogy I just gave..the speed tape (IAS) in a 777, if mounted in the Space Shuttle, would likely read Mach 0 when it was traveling at 17,000 mph in space orbit. But that doesn't mean the space shuttle is not exceeding Mach 1!

Again, I cannot find a single physics equation that shows Mach speed is dependent on relative wind. Relative wind has nothing to do with measuring Mach speed.

If you were to be picked up by a super-strong Janka tornado with winds of 800 knots, you would in fact break the sound barrier even if your RELATIVE speed was 0 mph within that 'jet stream' (which is what the cockpit IAS measures in a 777 - RELATIVE wind speed and RELATIVE Mach speed). Not actual Mach.

So Boeing speed tape measures RELATIVE Mach, not ACTUAL Mach. And BAW114 was going about Mach 1.2
TERMINATOR , 12 Jan, 2015
speed difference
Hi captain Lim,
My question is, will a boeing 777 and a boeing 737, flying both at 230 knots be at the same distance or will it be a relative difference in distance between the two because one is more powerful than the other. Please note that both have the same velocity and no acceleration, will they be side a side or relatively different?
I hope you understand my question
milan salgado , 24 Mar, 2015
speed difference
Hi captain Lim,
My question is, will a boeing 777 and a boeing 737, flying both at 230 knots be at the same distance or will it be a relative difference in distance between the two because one is more powerful than the other. Please note that both have the same velocity and no acceleration, will they be side a side or relatively different?
I hope you understand my question
milan salgado , 24 Mar, 2015
No, Lim is right.
Terminator, No, Capt. Lim is correct. The Mach number is *always* relative to the local wind speed. You can't find anything specifically mentioning wind in the relevant equations because the equations are defined in terms of the frame of reference of the medium in which the aircraft is traveling, which already inherently accounts for the wind. Ground speed is irrelevant for aerodynamics, including for whether a shock wave (what you hear as a 'sonic boom') will be generated or not. It is not accurate to describe an aircraft flying at Mach 0.85 as supersonic, regardless of whether its actual speed over ground is above or below what the local Mach 1 speed would be in still air. The aircraft is *not* moving faster than sound would propagate in the same medium, nor is it generating a shock wave (well, ignoring that part of the aircraft probably actually is experiencing supersonic flow at speeds above Mach 0.8 or so... look up critical Mach number.) Conversely, it's entirely possible to an aircraft to be supersonic while its ground speed would be subsonic. We do it all the time in wind tunnels where, not only is the test article not moving faster than Mach 1 with respect to the ground, it *isn't moving at all* with respect to the ground.
vbscript2 , 13 Apr, 2015

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