Rich Stowell Slow Flight Viewpoint! - Aviation Ideas and Discussion!

To: The Airman Certification Standards Working Group

Introduction

The following remarks pertain to requirements in the Private Pilot-Airplane Airman Certification Standards (ACS) regarding maneuvering during slow flight, specifically: PA.VII.A.S2 and PA.VII.A.S3.(1)

For some context, I have been a full time flight instructor since 1987. I am a nine-time Master Instructor, the 2014 National FAA Safety Team Representative of the Year, and the 2006 National Flight Instructor of the Year. I am a recognized subject matter expert on loss of control in general aviation with the following experience:

* 10,000 hours of total flight time

* 9,000 hours of flight instruction given

* 25,000 landings

* 34,000 spins in 235 general aviation aircraft

* 500 single-engine aircraft N-numbers in my logbook

* 380 aviation talks presented

* More than 75 aviation articles and three aviation textbooks published

At issue is wording in the ACS that requires applicants to demonstrate the following levels of skill while maneuvering during slow flight:

“Establish and maintain an airspeed, approximately 5-10 knots above the 1G stall speed, at which the airplane is capable of maintaining controlled flight without activating a stall warning.”

“Accomplish coordinated straight-and-level flight, turns, climbs, and descents with landing gear and flap configurations specified by the evaluator without activating a stall warning.”(2)

ACS Wording Versus Airworthiness Standards

Given FAA airworthiness standards concerning stall warning systems, the simultaneous requirements of “5–10 knots above the 1G stall speed” and “without activating a stall warning” are incompatible. Airworthiness standards in effect in 1993, for example, required the following:

“stall warning must begin at a speed exceeding the stalling speed by a margin of not less than 5 knots, but not more than the greater of 10 knots or 15 percent of the stalling speed…”(3)

Airworthiness standards since 1996, on the other hand, have required stall warning activation to begin “at a speed exceeding the stalling speed by a margin of not less than 5 knots…”(4) This standard does not specify an upper speed limit for activation of stall warning systems. As a result, while stall warning could be activated—indeed, should be activated per airworthiness standards—no less than 5 knots before the reference stall speed, it could activate with a significantly greater margin to the stall speed.

The ACS requirement to fly without activating stall warning clearly conflicts with the simultaneous requirement to establish and maintain an airspeed 5–10 knots above the reference stall speed. Moreover, design parameters that determine when artificial stall warning activates are beyond the control of the applicant—so much so that an applicant may be forced to transition out of slow flight to prevent stall warning from activating, defeating the purpose of this task altogether.

FAA Justification

The incompatibility between the ACS wording and airworthiness standards notwithstanding, the FAA has offered the following justifications:

“The guidance has always intended for there not [emphasis added] to be a stall warning—and that is consistent with slow flight guidance published in AC 120-111.”(5)

“Advocating maneuvering the airplane just below the critical angle of attack with the stall warning activated is neither desirable nor intended.”(6)

These assertions are demonstrably false. For at least several decades now, FAA guidance has been unambiguous about its intent to have stall warning activated while maneuvering during slow flight. For example, in the FAA’s General Aviation Pilot Stall Awareness Training Study conducted in 1975–76 (the FAA Study):

“the student slowed the aircraft to the speed at which the visual or aural stall warning indicator was continually activated [emphasis added]…. Turns were also made at 30° angle of bank with the stall warning indicator continually activated [emphasis added].”(7)

The objective during the FAA Study was for student-participants “to maintain desired heading and altitude at an airspeed and angle of attack which activated the stall warning device [emphasis added], but which did not cause the aircraft to stall.”(8) Two noteworthy results from this study:

“The most effective additional training was slow flight with realistic distractions, which exposed the subjects to situations where they are likely to experience inadvertent stalls.”(9)

“The extra stall and slow flight training was effective in preventing unintentional spins [emphasis added]”(10)

Training in slow flight with stall warning activated coupled with realistic distractions was effective in preventing unintentional spins. Read that again: Slow flight with stall warning activated coupled with realistic distractions was effective in preventing unintentional spins.

The results of this landmark study have driven FAA stall/spin training policy ever since, starting with the introduction of realistic distractions in 1980, followed by the shift from “stall avoidance training” to “stall and spin awareness training” in 1991.(11,12)

Derived from the FAA study, the series of Advisory Circulars (ACs) entitled, Stall and Spin Awareness Training has offered “guidance to flight instructors who provide that training.”(13) The following wording appears in AC 61-67B published in May 1991 through AC 61-67C (Change 2) published in January 2016. All of these ACs recommend the following in Chapter 2, “Stall Avoidance Practice at Slow Airspeeds”:

“(1) Assign a heading and an altitude. Have the student reduce power and slow to an airspeed just above the stall speed…”

“(2) Have the student maintain heading and altitude with the stall warning device activated [emphasis added].”(14,15)

FAA guidance for at least a quarter century has been crystal clear, and for good reason: Training in slow flight with stall warning activated, coupled with realistic distractions, is effective in preventing unintentional spins. “Maneuvering an airplane just below the critical angle of attack with the stall warning activated” not only has been intended, but also is desirable if preventing unintentional spins remains a safety priority with FAA.

Regarding the reference to AC 120-111, slow flight is described therein as “flight just above the stall speed.”(16) This specialized flight training element is intended to expose pilots to “how to maneuver the airplane…without stalling.”(17) The status of the stall warning system during slow flight is not mentioned in this AC. However, the AC does list “manually controlled slow flight” under the heading “Extended Envelope Training.”(18) Revising the long-understood meaning of slow flight as a condition “with the stall warning system activated” now to one “without activation” is incongruous with, and a move away from, the whole concept of “Extended Envelope Training” mandated by CFR §121.423.

In reality, the treatment of slow flight in AC 120-111 is consistent with recommendations made by the International Civil Aviation Organization (ICAO). ICAO describes this specialized training element as follows:

“Slow flight exposes the trainee to flight right above the stall speed of the aeroplane and to manoeuvring [sic] the aeroplane at this speed without stalling. The purpose is to reinforce the basic stall characteristics learned in academics and allow the pilot to obtain handling experience and motion sensations when operating the aeroplane at slow speeds during the entire approach-to-stall regime in various aeroplane attitudes, configurations and bank angles.”(19)

The “approach-to-stall regime” referenced by ICAO is defined as “Flight conditions bordered by stall warning and aerodynamic stall.”(20) Activation of the stall warning system during slow flight is an obvious and integral part of ICAO’s Upset Prevention and Recovery Training (UPRT) framework—the very same framework that informed AC 120-111.(21)

The assertion that no activation of stall warning is somehow “consistent with guidance on slow flight published in AC 120-111” is unsubstantiated at best, disingenuous at worst.

Further Rationalization

The August 2016 issue of DPE Tips offers further justification for the ACS wording: “The FAA does not advocate disregarding a stall warning while maneuvering an airplane.”(22)

It does not follow that having a student learn to maneuver in slow flight with stall warning activated advocates “intentional disregard” for stall warning. I am not aware of any studies that show a correlation between exposure to stall warning and increased inoculation to it. Recall the FAA Study found that training in slow flight with stall warning activated coupled with realistic distractions was effective in preventing unintentional spins.

Consistent with longstanding FAA guidance on stall and spin awareness training, pilots should be taught to integrate sight, sound, and feel while maneuvering in slow flight. They should also be taught to acknowledge stall warning and understand its ramifications. The ability to fly the airplane precisely while stall warning is activated can be a confidence building exercise as well as a way to incorporate angle of attack (AOA) and G-load awareness in real time. While many permutations are possible, following is an example of dialogue that might occur between an instructor (CFI) and student (STU) while practicing slow flight with stall warning activated:

CFI: Do you hear the stall warning?

STU: Yes.

CFI: From now on, I want you at least to verbally acknowledge it every time you hear it.

CFI: We are hearing stall warning in this particular configuration, but when else might we hear it?

STU: At any speed, in any attitude, at any power setting.

CFI: Is mechanical stall warning 100 percent reliable?

STU: No.

CFI: What other indications of reduced margin to the stall might we expect?

STU: Reduced control effectiveness and more pronounced engine effects.

CFI: What conditions could cause you to miss hearing the stall warning?

STU: High workload in the traffic pattern, distractions, stress, lack of proficiency.

CFI: I dropped my pencil, please pick it up for me.

STU: Not now, I’m busy aviating!

CFI: What does stall warning mean?

STU: We are operating at high angle of attack, close to the critical angle.

CFI: With regard to your control inputs, what else does stall warning mean?

STU: Do not pull the elevator control any farther aft.

CFI: Are we in a stall?

STU: No, it’s just stall warning.

CFI: What will happen if you apply additional back elevator pressure now?

STU: We’ll stall the airplane.

CFI: What could happen if we encountered a vertical gust right now?

STU: We could stall the airplane.

CFI: What will happen if we increase the G-load by trying to execute a steep turn now?

STU: We’ll probably stall the airplane.

CFI: What should you do if we encounter the stall?