I have looked everywhere to find an answer why during cruise if you have a TW your Ground distance will be more than your Air distance. And vice versa!

 

Can anyone explain why this is?

 

Thanks.

Never heard the term "Ground distance V Air Distance"?

Do you mean Ground Speed, and Indicated (or true) airspeed? If so, with a tailwind (TW I assume), the tailwind component is added to your true airspeed, resulting in a ground speed greater than the true airspeed.

Eugene

On my OFP you have air distance and ground distance (PFPX) i know about the effects of TW and HW with TAS and GS.

Because your distance through the air is based on your speed through the air. And if you have a tailwind, that means the air mass is moving along with you, decreasing the actual distance through air you have to travel to get to your point B on earth. It is your true airspeed times your time to go. A tailwind effectively decreases your air distance while a headwind increases it.

So for the air distance its about the acft reading the speed its moving through the air mass which a HW or a TW will have an affect on?

 

So how come a HW or a TW affects GS? Is it due to the air mass pushing the acft?

It's not due so much to the aircraft being "pushed" by the air mass, but while the aircraft in the air, it moves within the air mass. It's a lot like a boat crossing a river that has a current. Say an aircraft is moving at 100 KTAS; this means it's moving at 100 knots through the air. If the air is moving at 20 knots in the same direction, you will have a 20 knot tailwind, and your groundspeed will be 120 Knots. For an hour flight, your air miles will be 100 nm (TAS) and your ground miles will be 120 (GS).

Headwinds or tailwinds affect the groundspeed because the airmass is pushing the plane. The air miles value disregards the distance actually travelled over the ground to recalculate a imaginary distance value based on true airspeed through the air and time spent flying. It is an 'as if' value for distance, as if there was no wind. A tailwind decreases the air miles since you don't spend as much time to get there at your airspeed. It is as if you didn't have to fly as far, since the air mass carried you along as well.

Lets try break this down. So TAS will always be the same for the planned flight and only GS is affected with HW or TW?

Lets try break this down. So TAS will always be the same for the planned flight and only GS is affected with HW or TW?

 

Yes, TAS is used for the purposes of flight planning, but subject to variation caused by head- or tail-winds. It simplifies journey and fuel planning by reducing the variables. it's not absolutely accurate to do so, but it does mean a reasonable approximation for TTG, and with it fuel uplift considerations. 

Lets try break this down. So TAS will always be the same for the planned flight and only GS is affected with HW or TW?

 

Hi Vernon,

 

TAS will be the same for a specific flight level with specific temperature AND pressure altitude. Jets will be flown in high altitude with Mach instead of IAS/TAS. The Mach number depends only on temperature. Just for info

 

there are a five "speeds" for an aircraft. I = Indicated AS, C = Calibrated AS, E = Equivalent AS, T = True AS and G = Ground Speed in short reminder ICE -T GS . Three of them are most necessary for flying your indicate AS is to keep you in the air the TAS is for your flight planning and you GS is keeping track of your flight plan.

 

TAS changes with altitude cause of the decreasing air pressure. The lower you fly the TAS comes close to the IAS. Therefore in VFR flight planning is simplified despite to high alt IFR flight planning. TAS is the speed you fly within still air. Now if you take your wind component into account you will have the GS. 

Yes, TAS is used for the purposes of flight planning, but subject to variation caused by head- or tail-winds. It simplifies journey and fuel planning by reducing the variables. it's not absolutely accurate to do so, but it does mean a reasonable approximation for TTG, and with it fuel uplift considerations. 

 

Hi Louis,

 

guess you ment the GS which is subject to wind variations. TAS depends only on temperature and pressure ALT.

Mickey, 

 

Correct: TAS is not actually fixed, but varies accordng to temperature and pressure.. But the TAS mentioned in most flight manuals and checklists calculates on the basis of ISA, to create the constant. With TAS as a `fixed` point, the variables are then calculated from wind. As I said, not truly accurate as one is ignoring a calculation for the TAS, but it does give a reasanbly accurate result in the absence of extremes of temp and pressure. In the real world, the variation in wind and en route weather reporting means you can ignore TAS discrepancy.

Thanks alot guys. Just trying to put all this together! So if you fly at the same TAS in a HW or a TW, how come your time to the dest changes with a TW or HW?

Thanks alot guys. Just trying to put all this together! So if you fly at the same TAS in a HW or a TW, how come your time to the dest changes with a TW or HW?

 

Should be obvious, as we've just supplied information, analogy and description: 

TAS + Tailwind  = faster GS = shorter TTG

TAS + Headwind = slower GS = longer TTG

 

Coming back to your original post, distance doesn't change, speed does. But as the air mass itself is moving, the effective result is to travel a shorter distance. Same as sitting with engine off in a boat on  fast flowing river. You may not travel relative to the water you are in, but the water itself is moving. 

Should be obvious, as we've just supplied information, analogy and description: 

TAS + Tailwind  = faster GS = shorter TTG

TAS + Headwind = slower GS = longer TTG

+1

What i'm struggling to grasp is, if you are flying at a fixed TAS of 500 mph to your dest and you have a TW of 112kts your GS will be like 611 or something, and your TTG will be shorter? Is it because of the moving air mass thats making the plane go faster?