The Advantages of Angle of Attack Indicators in General Aviation Aircraft THESIS Presented in Partial Fulfillment of the Requirements for Honors Research Distinction in the College of Engineering of The Ohio State University By Justin Frank Abrams The Ohio State University 2015 Dissertation Committee: Dr. Seth Young, Adviser Dr. Morton O’Kelly, Adviser
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The Advantages of Angle of Attack Indicators in General Aviation Aircraft
THESIS
Presented in Partial Fulfillment of the Requirements for Honors Research Distinction in the College of Engineering of The Ohio State University
By
Justin Frank Abrams
The Ohio State University
2015
Dissertation Committee:
Dr. Seth Young, Adviser
Dr. Morton O’Kelly, Adviser
Copyright by
Justin Frank Abrams
2015
ii
Abstract
The General Aviation Joint Steering Committee (GAJSC), a government and
aviation industry representative, conducted a detailed analysis of fatal general aviation
accidents for the period 2001–2010 and found that 50% indicated loss of control as a
contributing factor. A loss of control occurs when an aircraft stalls and the control surface
movements made by the pilot no longer control the airplane until the stall recovery is
initiated. As an aircraft flies, the smooth airflow overtop the wing creates a low pressure
area, and the airflow underneath a high pressure area. This pressure differential generates
lift. When this airflow is disrupted, the aircraft loses lift very suddenly and a stall is said
to occur. Looking deeper into stalls involves a look at what is called the Angle of Attack.
This is the angle between the chord line of the wing (a line connecting the leading most
point on the wing with the trailing most point) and the flight path of the aircraft. The
Angle of Attack changes throughout different phases of flight, but a stall will always
occur when the Critical Angle of Attack is exceeded. Though the aerodynamics behind
and recovery procedures for stalls are taught extensively in private pilot training, there is
no instrument in standard general aviation cockpits that indicate the aircraft’s angle of
attack. Instead, student pilots are taught cues related to an impending stall which they can
gather from standard instruments in the cockpit.
Though not a standard piece of equipment, Angle of Attack indicators are
available for purchase through several manufacturers. These indicators vary in their
iii
presentation, but all convey the same information. However, that pilot would find that
there is a relatively small amount of information available to them on how to operate
their aircraft more safely with the equipment. This study will address the overall benefit
of an Angle of Attack Indicator and its intuitiveness. By comparing the performance of
pilots using an Angle of Attack Indicator who have received training on its operation and
function with those who receive no formal training, it will become clear if education is
necessary for pilots to achieve better operational performance and safety.
The research findings showed little variance in the performance between pilots
with training on the Angle of Attack Indicator system and without, but much can still be
gained from the results. Future experiments can isolate other variables at play in this
experiment and attempt to explain why the Angle of Attack Indicators did not seem to
offer any distinct advantage in this research project.
iv
Dedication
This document is dedicated to the aviation students at the Ohio State University
v
Acknowledgments
I would like to acknowledge Seth Young, Shawn Pruchnicki, Marshall Pomeroy, and the
research staff at Purdue University and Florida Institute of Technology for their
contributions to the project, without which I would not have been able to write this thesis.
vi
Vita
June 2011 .......................................................Suffield High School
2015................................................................B.S. Aviation, The Ohio State University
Fields of Study
Major Field: Aviation
vii
Table of Contents
Abstract ............................................................................................................................... ii
Appendix A: Legacy Model Lighting Display Overview ............................................... 255
Appendix B: Sample Group 1 Participant Data Calculation ............................................ 26
Appendix C: Sample Group 2 Participant Data Calculation ............................................ 37
Appendix D: Sample Group 3 Participant Data Calculation ............................................ 44
Appendix E: Sample Group 4 Participant Data Calculation ............................................. 53
viii
List of Tables
Table 1. Participant Groups ............................................................................................ 16
Table 2. Cumulative Data Findings ................................................................................ 21
ix
List of Figures
Figure 1. Percent Increase in Stall Speed versus Increase in Load Factor ....................... 11
Figure 2. Alpha Systems Legacy Model Cockpit Indicator ............................................. 14
Figure 3. Alpha Systems Legacy Model Angle of Attack Indicator Probe ..................... 14
Figure 4. CAPACG Flight Data Recorder ....................................................................... 15
10
Chapter 1: Introduction
The General Aviation Joint Steering Committee (GAJSC), an organization which works
to improve general aviation safety through data-driven risk reduction efforts, completed a study
which found that 50% of fatal general aviation accidents were attributed to a loss of control. A
loss of control in this circumstance means a stall, where the flow of air around a wing results in a
rapid reduction in lift, or a spin, where one wing stalls more than the other and results in a
descending helical path. This illuminates a need for change when it comes to loss of control
recovery. In order to reduce fatalities and decrease the loss of life, the ability of general aviation
pilots to perceive an impending stall must be addressed. In recent years, there has been much talk
about the Angle of Attack Indicators, which provide much more direct information to the pilot
regarding how much excess lift the aircraft wings can produce before the critical angle of attack
is reached and a stall occurs. These indicators produce a display based on the aircraft’s angle of
attack by measuring the pressure difference between the air flowing underneath the wing and the
static air around the aircraft.
An angle of attack is defined as the angle between the chord line of the wing (a line
connecting the leading most point on the wing with the trailing most point) and the flight path of
the aircraft. The Angle of Attack changes throughout different phases of flight. The Critical
Angle of Attack is reached when “smooth airflow over the airplane’s wing is disrupted, and the
lift degenerates rapidly” (Airplane Flying Handbook). Though most general aviation aircraft are
manufactured without such a device due to the added cost (roughly $2500 for the system utilized
in this experiment), their potential safety benefits may make the extra cost worthwhile (Alpha
Systems
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11
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12
exactly how much higher the stall speed will be for a given increase in wing loading. Though not
a fundamental instrument used to train pilots, the information Angle of Attack Indicators
displays to the pilot may be significant enough to reduce the number of loss of control accidents,
as its lighting configurations will alert the pilot when the wing angle of attack is getting close to
the Critical Angle of Attack. This research study, incorporating the use of two flight training
aircraft and approximately forty pilots, will answer the question of the indicator’s impact on
airspeed control during final descent to landing. The proper angle of attack on final approach to
landing coincides with a specific airspeed of the aircraft. Using an indicator which indicates to
the pilot when he or she is holding that proper angle of attack should allow the pilot to maintain
the approach airspeed with less tolerance.
13
Chapter 2: Aircraft Equipment
Two main pieces of equipment were required for the experiment to be completed. This
equipment, an Angle of Attack Indicator and a flight data recorder, was provided to the Ohio
State University Center for Aviation Studies as a part of a PEGASAS research project. The
Partnership to Enhance General Aviation Safety, Accessibility, and Security (PEGASUS) is an
FAA sponsored consortium of colleges which focus on increasing safety in the general aviation
industry.
One such project studied the correlation between Angle of Attack Indicator usage and
steady descent rates on approach. This project analyzed the descent rates of aircraft on final
approach in an attempt to see if those who were received training on and were able to utilize
Angle of Attack Indicators flew more stable approaches, or those with more constant vertical
speeds. In order to best use resources and attain significant amounts of data with limited funds,
this project used the same equipment and data provided for the PEGASAS project. The Angle of
Attack Indicator used for this project, the Legacy model by Alpha Systems, was the company’s
bestselling model as of summer 2013.. At a cost of around $1,200, the system offers great
potential benefits for a relatively low cost. The Legacy model indicator, shown below in Figure
2, displays nine different lighting configurations which indicate the aircraft’s angle of attack
continuously throughout the flight. In cruise flight, where the angle of attack is significantly
smaller than the critical angle of attack, only the blue bar on the bottom of the indicator is
illuminated. As the angle of attack increases, the illuminated lights move upwards until reaching
the red chevrons, indicating a stall. The portion of the Operating Manual which addresses these
displays
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14
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16
Chapter 3: Study Methodology
As stated in the introduction, the main goal of the study was to determine the correlation
between Angle of Attack Indicator usage and airspeed control during approach to landing. Many
of the design specifics mirror those found in the FAA funded PEGASAS Angle of Attack
project, as that allowed a much greater and more meaningful amount of data to be analyzed.
Therefore, the methodology for the PEGASAS project is described below.
In order to study the effect Angle of Attack Indicator usage on airspeed control during
final descent to landing, four groups were developed which pilot participants would randomly be
assigned. These groups can be found in Table 1. Not only will the results show whether or not
Angle of Attack Indicators aided pilots in flying much more precise airspeeds, but also whether
or not receiving educational training on the systems proved beneficial.
Table 1: Participant Groups
For the purposes of this study, “training” was a 30 minute presentation on the
aerodynamics of stalls, the identification of the numerous lighting configurations on the indicator
and what they mean to the pilot, and how to properly utilize the indicator in the cockpit. Training
Participant Groupings Description Group 1 Received training and had access to the
Angle of Attack display during flight Group 2 Received training but did not have access
to the Angle of Attack display during flight Group 3 Did not receive training but had access to
the Angle of Attack display during flight Group 4 Did not receive training and did not have
access to the Angle of Attack display during flight
17
also involved the completion of basic maneuvers (such as climbing, descending, turning, and
both power on and power off stalls) and a total of six landings, all with an Angle of Attack
Indicator. Three airports were utilized in the study. The first two airports where landings were to
be performed were Delaware Municipal Airport (KDLZ) and Union County Airport (KMRT),
small airports located about 10 miles north and northwest of the Ohio State University Airport
(KOSU), respectively. The purpose of using these two airports was to have participants fly
landing approaches into airports they may not be incredibly familiar with. If a student has flown
in the local traffic pattern of an airport numerous times, they may resort to flying it in the same
way during the study. By utilizing other airports which they are less familiar with, their
performance will be the result of their training, or lack thereof, on the Angle of Attack Indicator
system.
The data recorder in the aircraft collected several pieces of data twice per second
throughout the entire flight. The data analyzed for the study was that which was produced when
the aircraft, on approach to land, was between 500 feet above the ground and the surface. This
parameter, for the average approach, represents the final portion of the approach where the
aircraft is aligned with the runway and flying at the proper approach speed. If performed, the
completion of training maneuvers was simply to allow the participant to see the Angle of Attack
Indicator and gain knowledge on the system operation. So, any participant with training would
have watched an educational video, performed basic maneuvers in flight, and performed six
landings all with the Angle of Attack Indicator. The testing for each group required only one
flight, except Group 2 which required two flights, because the pilot does not have access to an
Angle of Attack Indicator during the data collection flight but does during the training flight.
18
The data analysis of flights in Groups 1 and 4 will determine the value of training on and
access to an Angle of Attack Indicator. Group 4 is the control group, so it is important to
compare the performance of those in Group 1 to those in Group 4. If there is no significant
difference between the performances, then there is no advantage to having training in and access
to an Angle of Attack Indicator. Group 2 will serve to determine the value of the training module
and flight, as the participant will not be able to use the indicator during the flight where data was
analyzed. The data collected from this group will show if the training on stalls and the Critical
Angle of Attack has any advantage on flying without an Angle of Attack Indicator. Group 3 will
determine how intuitive the device is, as they will attempt to use it in flight having no prior
knowledge on its operation. Information from this study may be crucial to the industry, and may
lead to the development of training modules for Angle of Attack Indicator systems is they are
found to be a significant factor in the performance of pilots.
The pilot requirements to participate in the study were a Private Pilot License, the first
license most pilots receive, and between 50 and 250 total flight hours. Those requirements were
chosen as they represent the average pilot who is licensed yet does not have a large amount of
experience at the controls of an aircraft. Any pilot interested in participating in the study was
eligible as long as the experience requirements were met, although a majority of those in the
study are pilots in the aviation program at the Ohio State University. The original plan was to
place forty pilots randomly between the four groups, though time constraints on the project
resulted in only 22 participant flights completed.
The data analyzed after each flight was that produced during the final approach before
each landing, from 500 feet above the surface to the surface. Certain limits of the CAPACG data
recorder, such as it only being capable of recording the groundspeed of the aircraft, were taken
19
into account when deciding how to record the approaches. Though pilot’s fly an approach based
on a certain indicated airspeed, a speed relative to the air itself which does not take wind into
account, the actual speed of an aircraft on approach relative to the ground will change from day
to day as the wind changes. In other words, an approach with a strong headwind will result in a
slower ground speed than one with no headwind, even though both approaches were flown at the
same indicated airspeed. The flight data recorder has no way of recording the indicated airspeed
shown to the pilot, but uses GPS location and time to determine the aircraft speed over the
ground. Therefore, finding the average ground speed of all approaches in a given group and
comparing them would not yield useful data as different ground speeds for each data recording
flight may be the result of different winds and not pilot skill. Using this information, it was
decided that the best way to determine a pilot’s ability to hold a steady approach speed was to
determine the standard deviation for each approach. By doing so, the calculation will reveal how
much a pilot’s airspeed fluctuated during the final descent. A high standard deviation would
represent greater airspeed fluctuations than a smaller value, and therefore less airspeed control.
This assumes a constant wind from an altitude of 500 feet to the surface, which is not considered
an extreme assumption. Though the wind will change slightly during the final portion of the
descent on an average day, this assumption is not considered extreme and will not have
significant effect on the data results. In order to obtain the most accurate results, go-arounds,
approaches where the aircraft is unstable and results in a balked landing, would be disregarded as
the airspeed would remain much higher than one which results in a landing. After the standard
deviation of airspeed was calculated for each approach, the math for which can be seen in the
appendix, each standard deviation value for the entire group was added up and divided by the
number of approaches to determine an average value for the standard deviation.
20
Based on the experimental design established, the following three hypothesis were
developed:
1. Pilots with AOA training and access to and AOA will conduct approaches to landing with
greater airspeed control
2. Pilots with AOA training will conduct approaches to landing with greater airspeed
control even without the use of an AOA indicator
3. Pilots with AOA indicator access will conduct approaches to landing with greater
airspeed control even without training on the system
21
Chapter 4: Findings
Group 1 Group 2 Group 3 Group 4 Standard Deviation Approach Average
4.64 knots 8.06 knots 7.19 knots 6.33 knots
Number of Approaches
6 43 26 37
Number of Participants
1 8 5 7
Table 2: Cumulative Data Findings
The experimental findings can be found in the table below. The two data values found
associated with each group are the standard deviation approach average by the pilots in that
particular group, as well as the number of approaches analyzed in each group. The approach
numbers between each group differ due to time constraints on the project, a few equipment
issues, and the decision not to analyze go-arounds. Looking strictly at the average standard
deviation data, it appears that the pilot from Group 1 maintained airspeed within the closest
parameters, followed by Group 4, then Group 3, then Group 2. These results support the first
hypothesis, but not the second or third. Group 1, whose pilots received training and were able to
use the AOA indicator, performed the best of all other groups. However, it must be noted that
only 6 approaches were analyzed in this group, significantly less than the number analyzed in the
other three groups. The next best performing group was Group 4, the control group. This is
surprising as the pilots in this group flew the aircraft without the aid of training or the indicator
during approaches. However, it is important to note that the data from groups 2,3, and 4 are all
within 1.73 knots of each other, a relatively low number given a normal approach speed of 73
knots. This does not show any major difference in performance between the four groups.
22
Chapter 5: Conclusion
Though the experiment did not find conclusive evidence regarding the aid of Angle of
Attack Indicators in the cockpit of general aviation aircraft, there is much to take away from the
results. Numerous variables may explain why no conclusive evidence was found, such as the
type of aircraft used in the experiment. To begin with, completing more flights to bring the total
approach number of Group 1 to the level of the other groups will allow the results to hold more
value. The pilot participant responsible for the Group 1 data may have been either well above or
well below average skill, therefore resulting in data that may not replicate the data seen if several
pilots had been in that group. Another explanation for the results may be the type of aircraft
flown in the study. The Piper Arrow is a complex aircraft, meaning it has retractable landing
gear, flaps, and a constant speed propeller. Only two of the participants in the study had prior
complex aircraft experience, so many may have felt uncomfortable in the new aircraft to the
extent that they were more focused on flying the aircraft than utilizing the indicator. Performing
this experiment in an aircraft type which most of the participants have prior experience in may
result in more conclusive evidence and show the indicator’s ability to help participants hold a
more constant airspeed on approach. One last thought on the reason behind the lack of
conclusive evidence in the study is the fact that gathering data after only one flight with the
Angle of Attack Indicator may simply be asking too much of the pilots. The participants, none of
whom indicated they regularly flew an aircraft with a similar angle of attack device, learned how
to manipulate the aircraft in response to other stimuli and instrument readings. Introducing them
to this new equipment to see how they react to it in one flight may have resulted in them ignoring
23
the device and focusing on only the instruments and procedures they are used to. Introducing a
change like this may take several flights to get the participant comfortable with the device, at
which point collecting data on the flight may yield much different results. Additionally, changes
to the research methodology, such as a new educational module or different experience
requirements for participants, may lead to more conclusive results as well. This paves the way
for future research on the topic, where the effect of individual variables can be tested to see their
overall impact on Angle of Attack Indicator usage.
24
References
Installation and Operations Manual for the “Legacy” Angle of Attack (AOA) Indicator. (2010, October 28). Retrieved September 14, 2014. Airplane Flying Handbook. Washington, D.C.: U.S. Dept. of Transportation, Federal Aviation Administration, Flight Standards Service, 2004. Print.
Angle of Attack Equipment in General Aviation Operations. FAA Report, December 2014
AAppendix AA: Legacy Mo
25
odel Lightinng Display OOverview
26
Appendix B: Sample Group 1Participant Data Calculation