# Is the steep spiral maneuver criteria in the FAA Commercial ACS Flawed?

The Commercial ACS steep spiral maneuver calls requires a pilot to fly a descending spiral with power set to idle and maintain a constant radius around a point. The maneuver is supposed to be flown at best glide speed Vg. You are allowed to fly up to a bank of 60 degrees to maintain the radius around the point. You need to complete three 360 degree turns with no power and complete the maneuver at 1500 feet AGL

This maneuver is a piece of cake when flown with no wind; you just have to fly a constant bank and airspeed. However things get much harder when there is a wind. When you fly downwind your ground speed increases so you have to increase bank to maintain the radius and when you fly upwind you have to decrease bank to maintain your radius. The ACS specifies that the applicant has to maintain an airspeed of +- 10 knots, a rollout heading of +- 10 degrees and complete the maneuver at an altitude of no lower than 1500 feet AGL.

Best glide in my airplane is 80 MPH. The other day I had a 30 knot wind from the Southwest and tried flying this maneuver with the wind. On downwind I ended up having to bank the airplane to a 45 degree angle while trying to maintain best glide. When I did this the stall warning went off. When you do the math you can see why. Stall speed increases with bank angle. The calculation for the the new stall speed at a bank angle is the normal stall speed times the square-root of the load factor which for a 45 degree bank is 1.4. For my airplane the stall speed is 67 MPH so when you do the math, the new stall speed comes out to 79.27 MPH. So you can see why the stall warning went off.

Best glide is really related to an angle of attack and not airspeed. When you increase bank angle you need to increase angle of attack to prevent the airplane from going into a spiral dive. So shouldn't you increase airspeed when increasing bank angle to maintain the best glide angle of attack? If that is so, then you would violate the ACS standards and could fail your commercial check ride. I also feel that this maneuver is unsafe for high bank angles (high winds) as specified as it could cause a pilot to stall the airplane in a bank and risk a possible spin.

In my opinion, I believe the maneuver should specify to maintain a constant rate of descent, allowing for changes in airspeed to maintain the best glide angle of attack.

Looking for answers from pilots and aerodynamicist on this question.

• Stall speed only increases to the extent that you increase angle of attack. Your math works if you are doing a level turn, but the point here is to dump lift to lose altitude. Oct 27, 2022 at 14:25
• Also, if you read the general instructions & description for all ground reference maneuvers, somewhere, it says you should always enter the maneuver downwind. This ensures that at the beginning of the maneuver you will be establishing the highest bank angle that you will need in any part of the maneuver. But you are right that your "best glide speed" should really be a "best glide AOA", and, if you want that, you should establish it when entering the maneuver, (again, downwind). But I don't believe this maneuver calls for that. Nov 1, 2022 at 14:47

Your assumption that the steep spiral manoeuvre has to be flown at best glide speed is wrong.

Chapter 10 of the Airplane Flying Handbook (FAA-H-8083-3C) (pdf, ~6,7 MB) says on page 10-3:

The objective of the steep spiral is to provide a flight maneuver for rapidly dissipating substantial amounts of altitude while remaining over a selected spot.

If you want to get rid of altitude quickly flying at best glide speed is not really helpful. When you continue reading in above document it explains how you initiate the steep spiral on page 10-4:

In general, the throttle is closed to idle […] and gliding speed is established […] the steepest bank should not exceed 60°.

See how they only mention gliding speed and not best gliding speed? You want a lot of drag (= high bank angle) so you have to pitch down to keep your speed which results in the desired high sink rate.

You are totally right – flying high bank angles increases your stall speed and you did the math. The maximum acceptable bank angle in this exercise is 60°. You surely know that for a 60° bank turn while maintaining altitude the load factor is 2.

So to be on the safe side your worst case for this manoeuvre would be a 60° bank turn while maintaining altitude. Doing the math with a stall speed of 67 mph with wings level you get a stall speed of 95 mph for a 60° bank turn (67 * sqrt(2) = 95).

So selecting a glide speed of 95 mph for the steep spiral manoeuvre would be a safe first guess.

When you check the Commercial Pilot ‒ Airplane Airman Certification Standards (FAA-S-ACS-7A (with Change 1)) (pdf, ~1 MB) you'll find the requirements for the steep spiral manoeuvre on page 35.

Under Risk Management it says:

The applicant demonstrates the ability to identify, assess and mitigate risks, encompassing:

[…]

CA.V.B.R3 – Low altitude maneuvering including, stall, spin, or CFIT.

(my emphasis)

Selecting a safe glide speed that is higher than your best glide speed shows exactly that. You identified a risk and mitigated it.