Science Questions

Does take-off weight vary with latitude?

Sun, 27th Jun 2010

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Thomas Gass asked:

I know that the earth isnít totally spherical. If mass stays the same but the gravity differs when moving from the midlatitudes towards the equator or polar region, how will it show for example when you take a Boeing 747 - how many percent can the measured weight differ compared when you take the mass from the pole to the equator?


Andrew -  The strength of gravity on Earth is measured in terms of how fast it accelerates things and roughly speaking, itís about 10 metres per second every second.  Thatís how fast it accelerates things.  And you're absolutely right that it differs slightly depending on whether you're at the equator or at the poles, and there are basically two effects that are contributing to that.  The first is actually the rotation of the Earth itself.  We were saying earlier on that if you're standing on the poles, that point is fixed so you're not moving at all whereas if you're at the equator, you're moving very fast around it in a circle, and that reduces the gravity that you feel by whatís actually 3.4 centimetres per second per second.  So thatís about a 0.3% effect.  It reduces the weight by 0.3%.  You also said correctly that the Earth actually bulges around the equator and I think if I'm getting this right, thatís going to make it up into about a 0.5% effect.  So itís going to have a 0.5% effect on the weight thatís measured of your Boeing 747 which is not really a huge amount in terms of the uncertainties involved in aviation.

Chris -   [With aviation] they've also taken into account extra things in terms of safety factors before those kind of take offs anyway.  So itís already been taken account of.


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Thomas Gass asked the Naked Scientists: Question - If mass stays same but gravity differs when moving from the mid-latitudes† towards the equator, how will you ensure for example maximum certified Take-off weight of an aircraft like a Boing 747 will work at all latitudes? How many percent would the weight or gravity increase when you compare a specific mass at mid latitudes compared to the same mass at the equator? What do you think? Tomcat, Thu, 8th Apr 2010

Latitude has almost no effect on take off mass, the biggest factors are altitude and temperature. If you are high up, your maxinum take off mass is limited because the air ia less dense. A hot day is also a limit, as air is less dense when it is hot as well. Most difficult airport is Mexico City, as it is at a very high altitude, and is also very hot during the day. Being in a mountain bowl does not help either, as your maximum rate of climb is also dependent on air density and engine power, both being reduced due to altitude and temperature.

That is why helicopters in the mountains cannot go above a certain level, as the air is not dense enough to allow the craft to lift it's own mass. SeanB, Thu, 8th Apr 2010

Latitude has no effect on mass, but it does affect weight. However the effect is very small. Altitude is more likely to have a significant effect. Bored chemist, Fri, 9th Apr 2010

Latitude has no significant effect on aircraft, as SeanB and BC have said, however, it is a factor in the launching of space rockets.

Exo-atmospheric rockets that are launched from locations near to the equator need less fuel than ones launched further away from it because of the higher linear velocity of the Earth at the equator.  This is only a factor once the rocket has left most of the atmosphere behind, of course; a rocket that doesn't leave the atmosphere (or at least most of it) will gain no advantage from an equatorial launch site.

Many ocean-going, floating launch pad schemes have been mooted over the years to take advantage of an equatorial launch location and the reduced fuel requirements they bring. LeeE, Fri, 9th Apr 2010

As SeanB pointed out, temperature can be quite significant. The airport in Phoenix has to suspend takeoffs sometimes because the air temperature is too high. I think they pull the plug at around 115 Fahrenheit.

I'm not sure if they suspend landings at the same temperature or not. Geezer, Fri, 9th Apr 2010

Heh  - landings can't really be suspended for aircraft (especially if there's no suitable diversion).  Landing is normally less of an issue though, because landing takes place at the end of a flight, generally after a lot of the fuel has been burned and the aircraft is lighter, and instead of needing power to generate lift against gravity when taking off, gravity assists in maintaining speed and lift as it descends down the glideslope. LeeE, Sat, 10th Apr 2010

Interesting, didn't know the difference could be that big, and never thought of the centrifugal force lessening gravity either, but I expect it to be correct :)

But one of them won't work as soon as the plane became airborne will it?
or?? Naah :) yor_on, Sat, 3rd Jul 2010

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