Errors during the hammerhead are most likely to occur during the pivot, which is the most unfamiliar segment of the maneuver for the aerobatic student, but they might also be committed during other stages. Let's look at them in sequence.

You are not paying attention to the wing chord line's angle with the horizon, and are establishing straightline flight too early.

The slipstream's buffeting of the fuselage is subtle and gentle. Watch for it carefully.

This error will cause the aircraft to go off proper alignement in the maneuver. If you have trouble with sequential control application, verbally call out each control in the proper order and apply each control as you say its name.

This error will cause the aircraft to be misaligned and off heading on the pullout. In some aircraft, at this point of the maneuver, forward stick and some rudder in either direction can cause an inadvertent inverted flat spin. Take great care to neutralize the controls.

This usually happens because the pilot is concerned with building up too much speed and is rushing the maneuver. Bear in mind that when you start heading down you are at zero airspeed. You have a reasonable amount of time to establish a vertical downline before you pull out.

Just as you should not pull out too soon, you also shouldn't let too much speed build up on the way down. Be ready to retard throttle, and be careful not to overstress the airplane on pull out.

Generally, four scenarios might threaten the safety of the flight if not properly recognized and handled:

-	An inadvertent tailslide at the top of the hammerhead
-	An inadvertent inverted spin because of a failure to neutralize controls while transitioning to the vertical downline
-	Excessive speed buildup on the vertical downline
-	Pulling excessive Gs on recovery

As described among common errors above, the inadvertent tailslide might occur if the pilot fails to initiate the pivot in time at the top of the hammerhead. It might threaten safety if not properly handled because the reversed airflow over the fuselage could cause the aircraft to enter a maneuver unfamiliar to the pilot with basic aerobatic skills, cause disorientation, and make it difficult to recover.

Completion of the inadvertent tailslide is really a benign maneuver and should be practiced while learning the hammerhead.

An inadvertent inverted flat spin situation can develop in certain aircraft, especially the Pitts, Eagla, and some other high-performance aircraft, if rudder and elevator are not properly neutralized during the transition to the vertical downline, or if excessive forward stick is applied during the pivot. The control input for the inverted spin is forward stick and rudder in the desired direction of rotation.

So, if you are pointing straight down and carry forward stick and right rudder because you neglected to neutralize them, you might find yourself in an inverted flat spin. It is not at all dangerous if you know how to recognize it and what to do. (For specific recovery techniques, see your flight manual.) Power off, rudder opposite to the direction of yaw, and neutral stick will put you back into a straightline dive.

If your aircraft is susceptible to inadvertent inverted spinning under the described conditions, include the appropriate recovery training in your hammerhead curriculum.

Excessive speed buildup on the downline should be a rare problem. You transition to the downline at practically zero airspeed; however, if speed does build excessively, retard the throttle immediately, and simultaneously start your pullout. Eye the G meter to stay within limits.

Excessive G buildup on pullout might be related to having built up excessive speed, or you might just be too enthusiastic about pulling out. There is really no cure, only prevention. Focus consciously on G load throughout the pullout and you will stay within limits. If you notice the problem too late, ease off the stick immediately.