Cessna C172 Skyhawk SP Apparent Anomaly
 

A student of mine pointed out an apparent anomaly in the Cessna Flight Manual regarding Short Field Take-Off. It recommnends a climb airspeed of "56 KIAS (Until all obstacles are cleared)". Research gives a 'Best angle of climb' speed of 62 KIAS at sea level. Why the difference?

A second question involves the <Leaning of the Mixture for Ground Running> (Flight Manual 4 - 26). Some fellow instructors tell me this is no longer done at many flight schools. FIrst question here is: are they correct? If so, then what has been the change in thinking?

The Cessna Flight Manual to which I refer is dated 2007.

The POH states "For a short field, 10° wing flaps and an obstacle clearance speed of 56 KIAS should be used." Take-off distances are always provided for a nominal obstacle height of 50 ft and they have determined that 56 is the optimum nominally. In that text on page 4-32 of "If an obstruction dictates the use of a steep climb angle, the best angle of climb speed should be used with flaps UP and MCP. This speed is 62 KIAS at sea level, increasing to 67 KIAS at 10,000 feet." the scenario is beyond the typical obstacle near the end of the runway and the pilot would normally accelerate to the speeds as appropriate mentioned on that page 4-32.

A large flight school here using that type leans on the ground per the POH. What you have been told by other instructors - I suggest that you seek a reference to the SOPs or is it just individual opinions? Maybe there have been incidents of students attempting take-off with the mixture leaned and they have decided to accept the risk of plug fouling instead (better to lean more aggresively so the student gets an obvious message from the engine when increasing power).

Vx is a published and very specific number based on flight testing. That testing assumes certain conditions are also maintained. Things like ground effect(or lack of), acceleration, flap settings, and weight, all play into how steeply a plane actually climbs. If the manufacturer publishes a climb speed for takeoff that is different than the basic Vx, they have likely taken those other factors into account.

No anomaly at all. One speed is with 10 degrees of flap, the other is clean.

Calculation
 

I have just been able to show that, regardless of flap position, the very early stage of the climb using 56 KIAS will place the aircraft at a higher height than using 62 KIAS. This is simply because the aircraft leaves the ground earlier.

However, the point about the Vx using a flap clean config versus the 56 KIAS Flap 10 speed is a clincher.

Thanks to all for a swift response.

Have you noticed the stupid doors yet, when you shut it and push the lever flush in the armrest, you cannot open the door from the outside as it is locked, all well and good until someone needs to get in to rescue you.