Speed and Time — In Depth
This lesson goes deeper into the relationship between speed and time in the cockpit, beyond the simple D = v × t formula.
Speed terms
| Term | Symbol | Definition | Source |
|---|---|---|---|
| IAS | indicated airspeed | what the ASI shows | pitot-static |
| CAS | calibrated airspeed | IAS corrected for instrument/position error | POH |
| EAS | equivalent airspeed | CAS corrected for compressibility (>200 kt) | rarely relevant for PPL |
| TAS | true airspeed | CAS corrected for density; actual speed through air | POH or E6B |
| GS | ground speed | TAS modified by wind component | from wind or GPS |
| Mach | M | TAS / speed of sound | not relevant for PPL |
TAS from IAS — rule of thumb
Rule: TAS ≈ CAS + (CAS × 2 % per 1000 ft pressure altitude).
Example: CAS = 100 kt, pressure altitude FL080 → TAS ≈ 100 + (100 × 8 × 0.02) = 100 + 16 = 116 kt.
More precise via POH tables or E6B (accounts for OAT and pressure).
Time encoding
- UTC (Zulu): standard in aviation. Munich (summer): UTC = LT − 2; (winter): UTC = LT − 1.
- Local time (LT) only for ground service coordination.
- Cycles: AIRAC cycles (28 days), daylight-saving changes (last Sunday in March, last Sunday in October in EU).
Speed-time relation in practice
- C172 cruise: 110 KTAS at 75 % power.
- Climb speed Vy (best rate of climb): typ. 75 KIAS.
- Climb angle speed Vx (best angle of climb): typ. 60 KIAS.
- Approach speed Vapp: typ. 65 KIAS (on final).
- Landing speed Vland: typ. 55 KIAS (flare).
These must be read from the POH and combined with time/distance rules of thumb:
- Climb at Vy 75 KIAS with rate 700 fpm: in 10 min you gain 7000 ft.
- Cruise at 110 KTAS: in 30 min 55 NM.
- Descent from cruise to pattern (1000 ft AGL) at 500 fpm: losing 3000 ft takes 6 min and (at 110 kt) covers 11 NM.
Time-distance for pattern and approach
- Downwind leg: 1 NM abeam runway, 1000 ft AGL.
- Base leg: 90° perpendicular to landing runway.
- Final: typ. 3 NM final at approach sink rate of 5 × GS (e.g. 500 fpm at 100 kt).
Exercise: descent profile to the field
Task: cruise FL080 (8000 ft AMSL). Destination elevation 1500 ft AMSL. Pattern at 2500 ft AMSL (1000 ft AAL). Sink rate 500 fpm at 110 KIAS.
Altitude to lose: 8000 − 2500 = 5500 ft. Time: 5500 / 500 = 11 min. Distance (at 110 kt TAS, no wind = 110 GS): 11/60 × 110 = 20.2 NM.
→ Begin descent 20 NM out from the field.
Bonus: 3-to-1 rule
For every 1000 ft of altitude loss, you need 3 NM distance for a typical 3° descent profile.
- Descent from 8000 to 1500 ft = 6500 ft = 6.5 × 3 = 19.5 NM descent distance.
(Matches the exercise above closely.)