Reading cruise performance charts
The Cruise Performance chart (Aircraft Flight Manual) gives true airspeed and fuel flow for a power setting at an altitude.
Layout:
| Part | Content |
|---|---|
| Rows (left) | Pressure altitude |
| RPM column | Throttle setting producing each power setting at that altitude |
| 3 temperature columns | 20° below standard, standard, 20° above standard |
| Each temperature column | %BHP, KTAS, GPH |
Worked example (Fig. 2-20): 6,000 ft, Standard Temperature column, 2000 RPM = 45% BHP, 79 KT TAS. No wind: ground speed = TAS.
Temperature effect (same RPM and altitude): warmer than standard gives LOWER %BHP, TAS and fuel flow than colder. Standard falls between.
Altitude effect:
- Same %BHP needs MORE RPM as altitude rises.
- Above some altitude, setting unreachable at full throttle.
This chart, 75% BHP:
| Pressure altitude | RPM needed |
|---|---|
| 2,000 ft | 2400 RPM |
| 8,000 ft | 2550 RPM |
| 10,000 ft | Unreachable, any RPM shown |
| 12,000 ft | Unreachable, any RPM shown |
Reverse lookup: pick TAS or power setting on Cruise Performance chart, then take that power setting to Range chart for distance before refuelling.
Limits:
- Neither chart allows for wind, and in some cases not for non-standard atmosphere.
- Figures apply only under stated conditions, which differ chart to chart: check each chart's notes. This chart: fixed weight (1,670 lb), manufacturer's recommended lean mixture.
Trap: value from wrong temperature column or neighbouring RPM row.
References
- TP 1102 Ex.10, Fig. 2-20 (Cruise Performance chart)