I’m fairly new to this and have started trying various custom VFR flights with all nav aids turned off, just relying on heading and leg times in the Nav Log. Is there a way to adjust the performance, hence time calculation, for the plane you are using? Currently, flying a Cessna C152, the timings equate to a cruising speed of 120 kts which is way above what the plane is capable of.
It’s going to depend on your altitude and/or any winds aloft.
I wrote a breakdown of how different airspeeds work, which may be worth checking out.
In summary, the indicated (IAS) speed is a direct measurement of dynamic pressure at the pilot tube. At cruise IAS is effectively the same as Calibrated airspeed (CAS) for most aircraft.
True airspeed (TAS) is the actual speed through the medium of the air and diverges from CAS with any changes in density altitude (which is pressure altitude and/or non-standard temps for a given altitude). TAS will increase by roughly 2% per 1,000 above sea level.
Headwind or tailwind components are subtracted or added, respectively, to your TAS to give you groundspeed (GS).
For navigational purposes, we use whatever TAS we derive from aircraft performance charts, using the combination of power setting and altitude. We then apply the wind correction to that to get our ground speed, which is then used to derive timing for the leg. This can and often does differ on each leg, especially if there are heading changes, or long distances with shifting winds, etc, so it’s tedious to figure this out in preflight planning. Luckily we have tools that can do it for us (but I still like doing it manually, kinda fun).
1 Like
Thanks for the comprehensive response. I’ve taken a look at the linked post and will probably read it a few more times before it all sinks in.
1 Like
You’re welcome!
The long and short of it is the speed indicated by the airspeed indicator isn’t necessarily the speed we’re flying over the ground. It’s only that when we’re at sea level in standard atmospheric conditions (which is 29.92” hg/1013.25mb and 59°F/15°C) and no wind.
The higher we go, and to a lesser extent, the hotter it is, the faster we’re actually going, and vice-versa, despite what the gauge says. Then add the effect of wind, which usually gets stronger the higher we go, but can have a net positive, neutral, or negative effect, depending.
If you were somehow able to get the 152 to cruise up at 30,000’, and it was the standard -44°C up there, your 100 knots indicated would be 163 knots true, which would also be the groundspeed in a no-wind situation.
If you did this over central Wyoming right now and went ENE-bound, you’d add 100knots of tailwind to that, giving you 263 knots groundspeed. Going the opposite direction would back it down to 63 knots groundspeed.