DOT/FAA/AM-02/19

Kathleen M. Roy

Aircraft Owners and Pilots Association

Air Safety Foundation

421 Aviation Way

Frederick, MD 21701

Dennis B. Beringer

Civil Aerospace Medical Institute

Federal Aviation Administration

Oklahoma City, OK 73125

October 2002

Final Report

This document is available to the public

through the National Technical Information

Service, Springfield, VA 22161.

General Aviation Pilot Performance

Following Unannounced In-Flight

Loss of Vacuum System and

Associated Instruments in

Simulated Instrument

Meteorological Conditions

Office of Aerospace Medicine

Washington, DC 20591
NOTICE

This document is disseminated under the sponsorship of

the U.S. Department of Transportation in the interest of

information exchange. The United States Government

assumes no liability for the contents thereof.
i

Technical Report Documentation Page

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No.

DOT/FAA/AM-02/19

4. Title and Subtitle 5. Report Date

General Aviation Pilot Performance Following Unannounced In-Flight Loss October 2002

of Vacuum System and Associated Instruments in Simulated Instrument

Meteorological Conditions 6. Performing Organization Code

7. Author(s) 8. Performing Organization Report No.

Roy, K.M.1, and Beringer, D.B.2

9. Performing Organization Name and Address 10. Work Unit No. (TRAIS)

1

Aircraft Owners and Pilots Assoc.

Air Safety Foundation

421 Aviation Way

Frederick, MD 21701

2

FAA Civil Aerospace Medical

Institute

P.O. Box 25082

Oklahoma City, OK 73125

11. Contract or Grant No.

12. Sponsoring Agency name and Address 13. Type of Report and Period Covered

Office of Aerospace Medicine

Federal Aviation Administration

800 Independence Ave., S.W.

Washington, DC 20591 14. Sponsoring Agency Code

15. Supplemental Notes

This report was performed under Task HRR-521.

16. Abstract

Forty-one instrument-rated pilots were exposed to an unannounced failure of attitude and heading

instrumentation during flight in single-engine general aviation aircraft: 25 in a Piper Archer PA-28 and 16 in

a Beechcraft Bonanza A36. The PA-28 flights consisted of three groups: (1) Group A — a failure of the

attitude indicator (AI) and directional gyro (DG), (2) Group B — same as Group A but received 30 minutes

of partial-panel instruction in a personal-computer-based aviation training device (PCATD) prior to the

flight, and (3) Group C – same as group A but had a failure-annunciator light (vacuum) on the panel. The

A36 flights consisted of two groups: (1) Group A – a failure of the AI only, (2) Group B – a failure of the AI

and the horizontal situation indicator (HSI). All of the PA-28 pilots maintained control of the aircraft, and 68

percent of them flew successful partial-panel approaches, and likely would have survived if it had been an

actual emergency. However, 25 percent of the Bonanza pilots could not maintain control, and the evaluator

had to assume control of the aircraft. Use of the PCATD prior to the data flight reduced the time required to

recognize a failure while airborne (mean A&C = 7.6 min., mean for B = 4.9 min.), but there were no other

observed differences in performance between the Archer groups. Recommendations are presented regarding

both training and instrumentation.

17. Key Words 18. Distribution Statement

Document is available to the public through the

National Technical Information Service;

Vacuum Failure, Partial Panel, Attitude Indicator, Pilot

Performance

Springfield, VA 22161

19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price

Unclassified Unclassified 13

Form DOT F 1700.7 (8-72) Reproduction of completed page authorized

iii

ACKNOWLEDGMENTS

Special thanks to the Aircraft Owners and Pilots Association (AOPA) for the use of Bonanza 7236W,

and to John Steuernagle, Capt. Jeff Jones, Craig Brown, and John Collins for assistance in the design and

implementation of the research protocol. The project was funded by the FAA through the Human

Resources Research Division of the FAA Civil Aerospace Medical Institute (AAM-500), which also

served as contract monitor (Task AAM-A-01-HRR-521), and was sponsored by Flight Standards General

Aviation and Commercial Division (AFS-800).

1

GENERAL AVIATION PILOT PERFORMANCE FOLLOWING UNANNOUNCED IN-FLIGHT

LOSS OF VACUUM SYSTEM AND ASSOCIATED INSTRUMENTS IN SIMULATED

INSTRUMENT METEOROLOGICAL CONDITIONS

BACKGROUND

There has been a concern with instrument flight

and loss of attitude awareness for at least the last 50

years. There are two primary situations where loss of

attitude awareness may lead to a fatal accident. The

first is when a non-instrument-rated pilot inadvertently or intentionally enters instrument meteorological conditions (IMC), is unable to maintain the attitude

of the aircraft, and ultimately enters either a spiral

dive or increasingly severe oscillations that ultimately

lead to aircraft structural failure. The AOPA Foundation, Inc., funded a study at the University of Illinois

Institute of Aviation that was reported by Bryan,

Stonecipher, and Aron (1954) in which a procedure

was developed to help visual-flight-rules (VFR) pilots

who had inadvertently wandered into IMC to return

to visual meteorological conditions (VMC). Baseline

data were collected at the beginning of the study to

determine with what frequency pilots without instrument experience would enter potentially flight-terminating conditions. The 20 pilots ranged in age from

19 to 60 years, had no previous instrument experience, and had a minimum of experience with the

Beechcraft Bonanza. Total pilot time ranged from 31

to 1625 hours. In their first exposure to simulated

instrument conditions (created by wearing blue goggles

in a cockpit with orange plexiglas covering the front

and side windows), 19 of the 20 entered a “graveyard

spiral” within an average of 3 minutes after losing

their contact view of the outside world. The 20th

placed the aircraft into a whip-stall attitude. These

results were obtained with cockpit instrumentation

sufficient to conduct instrument-referenced flight.

The second contributing situation is the one in

which instrument-rated pilots in IMC lose their attitude reference through vacuum/pressure system or

instrument failure. The majority of the 207,000 airplanes in the general aviation (GA) fleet have vacuumpowered attitude indicators (AIs) and heading

indicators. Many of those same airplanes are not

equipped with back-up or secondary attitude indicators or a back-up vacuum pump. Therefore, instrument-rated pilots must demonstrate the ability to fly

airplanes in “partial-panel” (loss of vacuum instruments)

conditions as part of their initial and recurrent training. This usually entails maintaining controlled flight

using indications from the pitot/static system instruments (airspeed indicator, vertical speed indicator,

and altimeter), electric gyro instruments (turn coordinator), and magnetic instruments (compass). Inasmuch as partial-panel flying is usually simulated by

covering up the supposedly “failed” instruments, pilots do not have the opportunity to experience a

realistic vacuum failure, in which they would have to

detect and diagnose the failure - unless it is an actual

emergency. This type of mechanical failure (vacuum

system or related instruments) has been documented

as a causal factor in only about three accidents per

year, which is 11% of all documented spatial disorientation accidents. However, these accidents result in

fatalities approximately 90% of the time (Landsberg,

2002; data from the Air Safety Foundation, ASF,

database of National Transportation Safety Board

accident reports). If one was to look at the combination of a VFR pilot entering IMC and experiencing a

vacuum-system failure, thus losing any attitude reference, it is not difficult to imagine the fatality rate

being even higher (little data exist, however, on this

specific combination of factors). That is to say, if

pilots who flew primarily by visual reference had

difficulty flying by reference to a full set of instruments, it is likely that they would be completely

unable to continue under partial-panel conditions.

This study is a continuation of a study conducted

for the AOPA/ASF by Martinez (2000) and administered by Flight Safety International (FSI) in 2000.

Martinez reported on pilot performance following the

failure of an aircraft vacuum system in single-engine

Cessna 208 and Cessna 210 simulators, with motion

disabled. Beringer and Ball (2001) reported a similar

study in fixed-base single-engine Cessna 172 and

Piper Malibu simulators, with results comparable to

Martinez’.

In the Martinez study, 66.7 % of the 24 test flights

resulted in loss of control and 50 % of the flights

ended in a crash. Beringer and Ball’s results from a

sample of 60 pilots showed that 27 % of the 11 pilots

flying the Malibu with the electric horizontal situation

indicator (HSI) would have exceeded performance
2

limitations of the aircraft or struck the ground. A

simulated vacuum-driven directional gyro (DG) was

depicted in place of the HSI to represent the majority

of low-end GA aircraft for one group, and 83 % of

those 12 pilots lost control, exceeded performance

limitations of the aircraft, or would have struck the

ground. When a back-up AI was depicted in place of

the turn coordinator (TC), 33 % of the pilots in that

group were unsuccessful in continuing the flight. Best

performance was obtained with a back-up AI, HSI

and turn coordinator (only 8 % loss). The Cessna 172

pilots, with a warning flag on the AI, fared better, with

only one (8 %) loss of control. However, differences

in stability between the Malibu simulator and the

Cessna simulator (more stable in roll) placed limitations on interpretation. Beringer and Ball recommended replacing the DG and very-high-frequency

omni range (VOR) heads with an HSI, freeing up an

instrument location for a back-up electric AI.

The Air Safety Foundation, in coordination with

the FAA Civil Aerospace Medical Institute (CAMI),

developed the present study to collect baseline aircraft

data evaluating pilots’ skills in dealing with an unannounced vacuum failure in flight for comparison with

results obtained in flight simulators. The following

sections describe the details of the effort and the

results obtained.

METHOD

Participants

Forty-one volunteer pilots (40 males, 1 female)

were selected from approximately 300 applicants who

responded to an announcement on the ASF Web site.

The primary goal in the selection process was to

choose a wide variety of pilots, regarding demographics and flight experience. Pilots participated without

monetary compensation. Table 1 presents demographic data for both the Archer and the Bonanza

pilot groups.

Equipment

Aircraft. The two aircraft used were a simple (Piper

Archer PA-28) (see Figure 1) and a complex (Beechcraft

Bonanza A36; see Figure 2) airplane. Each was

equipped with all Federal Aviation Regulation - FAR

- required items for a single-pilot IFR flight. Polarized

material was placed across the lower portion of the

windscreen and left side window of each aircraft (see

Figure 3) so that approximately the lower two-fifths of

the windscreen was covered. The Francis hood used to

simulate IMC (see inset, Figure 3) contained the same

polarized material in the eye openings, oriented 90° to

Table 1. Pilot demographic data by group.

ARCHER (n=25 males)

Variable name Mean Median Minimum Maximum

Age (years) 50 53 20 79

Total Pilot-in-Command time (hours) 4,358.0 1,477 161.0 24,750.0

Total instrument time (hours) 1,528.0 283 20.0 14,000.0

Total instrument time in the last 90 days 3.9 3 0.0 9.7

Years since rating received 19.2 19 0.3 58.0

BONANZA (n=15 males, 1 female)

Age (years) 46 44 32 62

Total Pilot-in-Command time (hours) 1,714.0 1,397.0 195.0 5,495

Total instrument time (hours) 294.0 212.0 45.0 816

Total instrument time in the last 90 days 4.7 2.5 0.0 25

Years since rating received 13.2 12.9 1.2 32
3

the windshield material. This arrangement allowed

the pilots to see inside the cockpit, but eliminated the

outside view immediately above the glare shield.

The PA-28 flights consisted of three groups (Table

2): (1) Group A - a failure of the AI and the DG, (2)

Group B - same as Group A but received 30 minutes

of partial-panel instruction in a personal-computerbased aviation training device (PCATD) prior to the

flight, and (3) Group C – same as group A but had a

failure-annunciator light (vacuum) on the panel. The

A36 flights consisted of two groups (see Table 2): (1)

Group A – a failure of the AI only, (2) Group B – a

failure of the AI and the HSI.

Data recording. Several forms of data were recorded

for each flight. Flight performance data were recorded

via a Cambridge Aero Instruments GPS Navigator

and Secure Flight Recorder. This generated a planview map and vertical-profile view of the flight path

for the purpose of assessing average and maximum

flight-path deviations. A color digital video recording

was also obtained during the flight with audio of all

intercom/radio communications. The field of view of

the camera, which was attached to the cabin headliner

Figure 3. Polaroid material across windscreen of

Piper Archer with (inset) Francis hood.

Figure 2. Bonanza instrument panel. Figure 1. Archer instrument panel.

Table 2. Operational instruments available for each pilot group. X indicates feature

was present.

Pilot Group AI DG HSI TC Compass Annunciator

Light Vacuum gauge

Archer A N/A X X X

Archer B N/A X X X

Archer C N/A X X X X

Bonanza A N/A X X X X X

Bonanza B N/A X X X X

behind and to the right of the front left seat, included

the subject pilot, key flight instrumentation, and the

forward view out of the windscreen. Additionally, the

pilot and evaluator each completed a post-scenario

questionnaire at the conclusion of the flight .
4

Archer implementation of vacuum failure. Prior to

each flight, an airframe and powerplant (A&P) mechanic disengaged the aircraft’s engine-driven vacuum

system. Therefore, the AI and DG were fully operational only via the standby vacuum system. During

the flight, the evaluator disengaged the standby system via a switch in the cockpit, thereby failing the

vacuum-driven instruments in a realistic manner.

Bonanza implementation of vacuum failure. Prior

to each flight, an A&P mechanic disabled the aircraft’s

engine-driven instrument air pressure pump. The AI

was powered by the standby system. The HSI is

electrically powered, so no maintenance was required

before the flights for that instrument. During the

flight, the evaluator disengaged the standby air pressure system and HSI via their individual circuit breakers in the cockpit. It is important to note that, because

the AI was vacuum-driven and the HSI was electric,

this was not a “real world” failure. It would be rare for

both vacuum systems and one electric instrument to

fail during flight.

Procedures and Tasks

Each scenario began with a pre-flight interview and

briefing involving the volunteer pilot and the evaluator. Safety information for the flight was discussed,

and each volunteer completed a consent form and a

flight-experience questionnaire. The pilot was briefed

about proper aircraft operation, including airspeed

and power settings, and the flight plan was discussed.

The volunteers were told they would be evaluated on

their execution of IFR procedures. The autopilot was

turned off for the duration of each flight.

After the briefing and pre-flight inspection of the

airplane, the pilots departed the Frederick Municipal

Airport (FDK) in Frederick, Maryland. The evaluator

acted as an air traffic controller (ATC), giving the

pilot heading vectors for the instrument landing system (ILS) Runway 23 approach at FDK. The purpose

of this first approach was to allow the volunteer to

practice flying in simulated IMC and to allow the

flight data recorder to establish a baseline for the

pilot’s performance under normal conditions. The

approach was discontinued at approximately 800 feet

above ground level (AGL). At that time, “ATC” issued

a clearance for the pilot to climb to 3,000’ and fly a

heading of 270o to the Eastern WV Regional/Shepherd

Airport (MRB) in Martinsburg, West Virginia.

During the climb after the ILS approach, at a

specific standardized point (point D in Figure 4), the

aircraft vacuum/pressure pump (and HSI in the Bonanza) was disengaged without the pilot’s knowledge,

leading to the eventual loss of the aircraft’s attitude

and heading indicators. The pilot’s task was to maintain control of the aircraft, select the best option(s) to

pursue, navigate accurately, communicate effectively

with ATC, and complete the flight with a safe landing

at either the destination or an alternate airport.

The simulated weather conditions were such that

FDK was the best alternate airport. “ATC” provided

vectors to a point that provided the pilot an intercept

heading and altitude to the ILS Runway 23 approach

at FDK. If requested by the pilot, “ATC” provided

no-gyro vectors above 2,500’. No-gyro vectors consisted of the direction of turn, and when to start and

stop that turn. The evaluator took control of the

airplane if the pilot at any time maneuvered to a bank

angle approaching 60 degrees and increasing, if the

aircraft’s airspeed was approaching Vne (never-exceed

speed) and increasing, if the aircraft was approaching

a stall condition, or for any other reason deemed

necessary for the safety of the flight. All flights were

conducted in weather conditions that would allow the

scenario to be completed in VFR conditions. The

evaluator acted as pilot in command for each flight

relative to flight safety issues.

RESULTS

Response to the vacuum-failure event required two

tasks to be performed. First, the pilot had to recognize

that a failure of some kind had occurred and correctly

diagnose it and, second, the pilot then had to successfully control the aircraft using the flight data remaining. The following sections consider each component

in turn.

Recognition Time

Time to detect/recognize the failure was measured

from the time the failure was initiated to the first

verbal report by the participant of something being

“wrong.” Although some pilots attempted adjustments to the AI prior to verbal reporting, the only

consistent scoring point that could be used was the

verbal report.

The PA-28 pilots averaged a higher recognition

time, with an average of 6.9 minutes for the entire

group. (See Table 3.) The pilots who flew partialpanel on the PCATD prior to their flight recognized

the instrument failure more quickly; however, the

differences among the Archer groups did not attain

statistical significance [F (2,21)=2.54, p>0.1] (4.9

minutes, vs. 7.6 for the other groups). Neither can

comparisons be made between the two aircraft because

of potential differences in the rate at which the vacuum/

pressure-driven instruments in each failed. The
5

Bonanza pilots who experienced a failure of the HSI

as well as the AI recognized the failure in an average of

2.6 minutes, which was significantly faster than the

average 4.6 minutes for those pilots who had only an

AI failure [F (1,14)=6.372, p<. 05]. The HSI was

equipped with a warning flag to announce instrument

failure, and this undoubtedly aided pilots in the

Bonanza B group.

Flight Performance Data

Outcomes of all flights were categorized as follows:

Category 1: The pilot had no problem controlling

the aircraft. The deviation was less than 20 degrees

and 200 feet.

Category 2: The aircraft remained under the pilot’s

control, but with more effort than the category 1

pilots. The deviation was between 20 and 40 degrees

inclusive and 200 to 400 feet.

Category 3: The aircraft was barely under control

– the pilot was struggling significantly.

Category 4: The evaluator had to take control of

the aircraft. Had this been a real instrument failure in

IMC, the flight likely would have resulted in a crash.

Archer. All of the PA-28 pilots were able to maintain control of the aircraft under partial-panel conditions. However, some became disoriented and were

not able to successfully execute an approach and

landing to the airport. Thirty-two percent of the

pilots did not successfully complete the approach

(i.e., 68 % were successful). An example of the horizontal and vertical-profile plots obtained from each

flight is depicted in Figure 5.

Archer Procedures. Upon noticing the vacuum system failure, 28 percent of the PA-28 pilots declared an

emergency to “ATC” (the experimenter). At the next

level of urgency, 68 % notified ATC of the problem

without declaring an emergency, but one pilot (4 %)

gave “ATC” no notification of the problem. Distraction played a significant role in that 28 % of the pilots

covered the failed instruments to prevent distraction

or being mislead by the now-failed indicators. The

remaining 72 % did not do so and tended to include

the failed instruments in their scan, indicating to the

experimenter that this was a distracting situation.

Bonanza. Twenty-five percent of the A36 pilots lost

control of the aircraft. All four of those pilots (3 males,

1 female) were in Bonanza Group B (loss of AI and

Figure 4. Scenario chart showing point (D) where vacuum/pressure system was disengaged

and intended flight paths.
6

HSI). Two of these four were experienced

Bonanza pilots, thus the effect is not solely

attributable to lack of familiarity with the

aircraft type. Sixty-nine percent of the pilots

were able to successfully complete a partialpanel approach, including two of the pilots

who had lost aircraft control but were given a

second chance to fly the aircraft partial panel.

Bonanza procedures. None of the A36 pilots declared an emergency to “ATC,” choosing instead to simply notify “ATC” of the

problem and request assistance. One pilot (6

%) covered the failed instruments – that pilot

had no problem controlling the aircraft, and

is classified as a Category 1 flight. Flight

performance outcomes for each group are

summarized in Table 4.

The distribution of loss-of-control flights

was such that a chi-square analysis was not

appropriate given the number of cells having

expected and/or observed frequencies less than

five. The distribution of failed approaches by

aircraft type was amenable to such an analysis, but there was no significant effect attributable to

aircraft type.

Response to Warning Indicators/instruments

Only one (12.5 %) of the Archer pilots in Group C

(vacuum annunciator light available) actually noticed

the light. Only one (4 %) out of all of the Archer pilots

noticed the vacuum gauge at the onset of the emergency. The others simply used it to verify the system

failure once the instruments were tumbled. Previous

studies (Beringer and Ball, 2001) listed the vacuumlow annunciator light as one of the first failure indications detected by the participants in the conditions

where it was available. The present finding is not too

Table 3. Mean recognition time per pilot group.

Pilot Group Average time to recognize failure

(minutes)

Archer A 7.3

Archer B 4.9

Archer C 7.9

Bonanza A 4.6*

Bonanza B 2.6*

Note:

* The only significant difference was between these means. Archer A pilots

used standard Archer instrumentation.

Archer B pilots received 30 minutes of partial panel PCATD training prior

to their flight.

Archer C pilots had a vacuum annunciator light as a warning when the

vacuum system failed.

Bonanza A pilots experienced an AI failure.

Bonanza B pilots experienced both AI and HSI failures.

Table 4. Frequency of outcome by aircraft group. Subscripts indicate

number of pilots in that category who failed to complete the instrument

approach in partial-panel conditions.

Pilot Group

Category 1:

Successful

partial panel

Category 2:

Required

more effort

Category 3:

Barely

controlled

Category 4:

Safety pilot

took over

Total

sample size

Archer A 61 52 1 0 12

Archer B 32 21 005

Archer C 4 42 008

Bonanza A 1 211 0 4

Bonanza B 4 21 21 42 12

surprising, given that the pilots were wearing the

Francis hood, which greatly limits peripheral and

parafoveal vision, and that the vacuum gauge was

located on the right side of the cockpit, well out of the

area of vision when viewing the primary instrument

cluster. It should also be noted that the vacuum

annunciator light was small and not very bright. This

suggests that some attention may need to be given to

indicator placement and conspicuity.

Pilot Experience Variables

Several questionnaire forms were administered to

gather experience data from each pilot. Questions

assessed experience with the specific model of aircraft

to be flown, certificates/

ratings, date of instrument rating, pilot-incommand hours (total

and last 90 days), instrument hours (total,

last 12 months, last 90

days, and “actual”), instrument training in the

last 90 days (last date of

partial-panel training,

amount of partial-panel

training, description of

that training), hours

flown annually, number

of approaches in the last
7

Figure 5. Example plan-view (top) and vertical profile (bottom) plots of a flight.
8

90 days, and experience using GPS equipment. Only

two observations are worth noting. The PA-28 pilots

who had more than three instrument flight hours

during the 90 days preceding their flight (40 %) were

noticeably more proficient. Nine (75 %) flew Category 1 flights and three (25 %) flew category 2

flights. However, it should be noted that no significant correlations were found between pilot experience

variables and performance variables, including categorization, for the PA-28 sample. This is undoubtedly due to the small sample size. The only

pilot-experience variable that showed a significant

correlation with performance (Spearman’s rho) was

total pilot-in-command (PIC) hours (rho = -.622,

p=.01). Practically speaking, a higher PIC total was

associated with a greater likelihood of obtaining a

better (lower-numbered) category of performance.

Simulator and Training Benefits

Pilots reported that flying the partial-panel trial

was a beneficial experience and that they felt better

prepared to handle this type of emergency situation in

actual IMC after having participated in the study.

Those who flew the Elite PCATD flight simulator

prior to their aircraft flights indicated that the training helped them. Specifically, they said that they were

better prepared to handle an emergency and were

already “warmed up” to fly after the training-device

practice. Recommendations related to training included encouraging flight schools to have at least one

aircraft configured to present vacuum-system failures

in flight, providing students with the opportunity to

detect and diagnose this failure in a realistic-onset

environment. This was proposed in contrast with the

present practice of having the instructor place covers

over instruments to be deemed “failed” during practice. In addition, it was the majority opinion that

pilots need more practice flying with partial-panel

instrumentation.

Limitations on Interpretation of the Data

A number of factors could have affected the outcome of the airborne trials and should be considered

when interpreting the data. Each is listed and discussed briefly below:

Artificial termination of flight: The evaluator terminated any flight at the onset of unsafe conditions.

Therefore, it was not possible to determine if a crash

would have actually occurred. For the four A36 losses of

control, it is likely that the flight would have been fatal.

Source of participants: As volunteers, the pilots

may not have been an accurate representation of the

309,000 instrument-rated pilots in the US. It is likely

that volunteers were more proficient than average

general-aviation pilots.

Experience with test aircraft: The pilots had no

flight time in the actual study aircraft (8121K and

7236W) prior to the experiment. However, most

pilots are familiar with PA-28 aircraft and have flown

them as part of their initial flight training. That may

be why a higher percentage of the PA-28 pilots were

able to control the aircraft during the loss of horizon

reference as compared with the Bonanza groups.

Simulated IMC and safety pilot: The scenario was

simulated single pilot IMC, actually flown under

VFR conditions with a safety pilot. The pilots likely

would perform differently if they had actually been

alone during a real emergency (more may have declared an emergency; fewer may have pressed the

limits of the performance envelope).

Compound systems failure, Bonanza: The A36

had a pressure-air-powered attitude indicator (AI),

but an electric horizontal situation indicator (HSI).

Therefore, there was not an actual pressure-air-system

failure, but instead, two individual instrument failures. That may have resulted in more confusion for

the pilots flying the scenario who were familiar with

redundant systems, although none of the A36 pilots

mentioned this issue during the post-flight briefing.

Supplemental visual cues: Pilots may have received some supplemental cues to aircraft attitude

from shadows playing across the cockpit or from

occasional fragmentary outside-world views when the

head was tilted (polarizing fields not fully orthogonal). This could have improved their performance

slightly as a group.

CONCLUSIONS AND

RECOMMENDATIONS

The results of this study reflect to some degree the

simulator data obtained by Beringer and Ball (2001).

On one hand, the results with the simple airplanes

(Cessna 172 and Piper PA-28) were very similar in

that pilots flying that simulator and aircraft were

largely in control of the flight to completion. It was

also true that virtually all of the recorded losses of

control occurred with the complex aircraft (Malibu

simulator and Bonanza A-36 aircraft; only one loss

was recorded in the Skyhawk simulation). These re-
9

sults, taken together, suggest that we are likely to see

more problems associated with high-performance aircraft than with the simpler fixed-gear, fixed-pitchpropeller aircraft when vacuum failures are

encountered. However, it should also be noted that

the loss rate using the airborne platform (Bonanza)

was far lower, for equivalent instrument equipage,

than that found in the Malibu simulator. Given the

factors listed that may have compromised, to some

degree, the outcomes of the airborne observations and

the simulator trials, we believe that the likely rate at

which serious controlling difficulties may be encountered in actual aircraft lies somewhere between the

two figures.

Unfortunately, no airborne assessment of the efficacy of a back-up attitude indicator was obtained.

Nonetheless, it is reasonable to believe, based upon

the simulator data, that the percentage of complexairplane pilots experiencing serious difficulties after

loss of the horizon reference could be further reduced

with the proper instrumentation. The presence of the

functioning electric HSI after the vacuum failure

appeared to greatly reduce the difficulty of maintaining control in the Bonanza, and this was consistent

with data from the Beringer and Ball simulator study.

Given the results to date, as represented in the cited

simulator studies and the airplane study reported

herein, the following recommendations are submitted

for consideration:

Training: Attention should be given to any training wherein detection and diagnosis can be practiced

in as realistic a setting as possible. This should include

simulators, aviation training devices, and/or aircraft

with vacuum systems specially adapted for the removal of power from at least one attitude instrument

on the trainee’s side.

Warnings indicators: Attention should be given to

the location and design of indicators for system or

instrument failure. Vacuum-failure indicators should

be located as close to the primary panel as possible and

should be very conspicuous. Attitude indicators should

be flagged to indicate loss of power, whether electrical

or air. This can also be done by referencing gyro RPM

and indicating whenever it drops below the minimum

deemed necessary for reliable operation.

Instrumentation: Three levels of instrumentation

modification are suggested in order of increasing cost/

complexity:

1) Given that a reliable heading reference has

been demonstrated to be useful in maintaining control, one option is to replace the directional gyro with

a heading indicator that is independent of the attitude

indicator’s power source and that will continue to

function, obviating the need for referencing a fluidfilled compass. This also brings the heading indicator

into the primary panel/scan and reduces both scanning requirements and the need to compensate for the

known behaviors of a fluid-filled compass.

2) Integrating instrumentation can also produce

decluttering benefits and free up space for additional

instrumentation. Thus, another option is to install an

HSI in place of a directional gyro and a VOR head,

providing multiple indications within one instrument, reducing required scan, and opening up an

instrument location on the panel.

3) It is also likely that a back-up attitude indicator

would be useful in reducing pilot workload based upon

the simulator-study results. This must be independently

powered relative to the primary attitude indicator, and

each should be flagged to indicate instrument or powersystem failure. This option, however, is not directly

addressed by data in this study.

REFERENCES

Beringer, D.B. and Ball, J.D. (2001). When gauges fail

and clouds are tall, we miss the horizon most of all:

General Aviation pilot responses to the loss of

attitude information in IMC. In Proceedings of the

Human Factors and Ergonomics Society 45TH Annual Meeting. Santa Monica, CA:HFES, 21-5.

Bryan, L.A., Stonecipher, J.W., and Aron, K. (1954).

180-degree turn experiment. University of Illinois

Bulletin, Aeronautics Bulletin Number 11.

Landsberg, B. (2002). Safety Pilot: The 90-percent

solution. AOPA Pilot, February, 87-94.Martinez,

C.R., Jr. (2000). AOPA Air Safety Foundation

study on pilot performance following instrument

failures in a simulated scenario.Preliminary report. (Draft not yet released publicly.)