At what point would speed make a difference?

Posted 1 September 2021

At what point would speed make a difference?

Plane #1 is flying at 25,000 feet at 250 miles per hour

Plane #2 is flying at 35,000 feet at 220 miles per hour.

Will plane #2 flying at 35,000 feet outrun plane #1 at 25,000 feet because plane #2 is flying at a higher altitude even though it is flying at a slower speed?

Introducing a few terms used in aviation

IAS- Indicated Air Speed

CAS – Calibrated Air Speed

TAS – True Air Speed

GS – Ground Speed

IAS is what the pilot will be able to see on the instruments. The instruments will have a small error margin (just as in your car), when you take this out you have the CAS. TAS is the speed when you correct for density. What mostly matters in the end, when you talk about aircraft speed and distance traveled, is the GS.

At what point would speed make a difference – who is winning #1 or #2?

You can use this free tool to calculate other examples.

#1 Has a TAS of 317 mph

#2 Has a TAS of 333 mph

So purely from a TAS point of view, the plane at 35.000 ft keeping 220 mph flies faster than the plane flying at 25.000 ft flying 250 mph.

However, the winning factor is likely to be the GS. If the plane at 35.000 has a 150 mph headwind from a jet-stream and the aircraft at 25.000 ft would have an 80 mph headwind – then it looks like this:

#1 Has a GS of 317- 80 = 237 mph

#2 Has a GS of 333- 150 = 183 mph

So a lot of the time, the GS will be the speed you need to assess. There are other factors like terrain, weather, and fuel consumption that might restrict you from flying at lower levels. The optimum cruising altitude for commercial jet planes is likely to be around 35.000- 41.000 ft. A few planes can go a little higher than that but they are the exception. The optimum cruising altitude is likely to be a factor of the airplane weight and weather and on a longer flight, the optimum cruising altitude is likely to change over time. In order to achieve an operational efficient flight pilots usually have to change cruising altitudes multiple times after having analyzed the circumstances. Of course, other traffic or weather can be a factor that prevents the pilot from getting an optimum level and this is why pilots generally carry reserves (contingency fuel) for all flights.