COURSE NOTES FOR AS495L



COURSE TEXT

ALL ATTITUDE FLIGHT

AND

UPSET RECOVERY

© 2004 R. Rogers

Drs. Rogers is an Aeronautical Science Department faculty member at Embry-Riddle Aeronautical University, Daytona Beach, Florida. Educators including flight instructors are free to use the materials contained herein for non-profit or not-for-profit activities. Any other use of these materials may constitute a copyright violation. Feedback on our course is invited. Send email to

rogers@erau.edu

TABLE OF CONTENTS

|Subject |Page |

|I. Course Introduction |2 |

|II. Introduction to Microsoft Flight Simulator |3 |

|III. Aerobatics Maneuvers |5 |

|IV. Upset Aerodynamics |15 |

|V. Airplane Upset Recovery Techniques |35 |

|VI. Causes of Upsets |42 |

|VII. Loss of Control Accident Analyses |54 |

|VIII. Annotated Bibliography |80 |

COURSE WEB SITE



I. COURSE INTRODUCTION

Purpose. This course is a low-cost introduction to all attitude flight and to upset recovery (also called recovery from unusual attitudes). To this end, it uses desktop computers running inexpensive flight simulation software (currently the Microsoft Flight Simulator 2002 Professional Edition). While intended for pilots embarking on an airline career, the course contains information of interest to aviators in general. By all attitude flight, we mean upright or inverted flight throughout the subsonic flight regime, including bank angles up to and including 180o, and pitch angles that reach 90o nose-up or 90o nose-down. An upset, or unusual attitude, occurs when an aircraft reaches an attitude or airspeed outside its normal operating regime. In general, an unusual attitude is unanticipated by the flight crew, and has a distinct potential to compromise safety-of-flight. For airliners, such an attitude can involve a pitch angle as small as 15-20o, and a bank angle of only 35-45o. Indeed, a 10o nose-down attitude in an airliner at high altitude and airspeed above normal cruise mach can rapidly progress toward a dangerous situation.

Prerequisite Knowledge. Since recovery from unusual attitudes requires a through knowledge of instrument flight, a pilot should be instrument qualified before undertaking this course. ERAU students taking this course for credit should hold an instrument airplane rating and should have completed AS309, Basic Aerodynamics. In addition, they should be familiar with the Microsoft Flight Simulator or be able to learn how to use the software with minimal supervision.

Organization. The course content is divided into six parts. Currently you are reading the first part, an introduction to the course. The second part is a brief introduction to the Microsoft Flight Simulator 2002 Professional Edition. The third part of the course introduces basic aerobatic maneuvers: low level constant altitude flight at bank angles up to 180o; inverted flight; loops; half and full Cuban eights; aileron and barrel rolls, and Immelmanns. The goal here is to help a pilot become acquainted with the cues—both visual and those provided by aircraft instruments—available to control an aircraft throughout the flight regime. The fourth part is a review of Aerodynamics, while the fifth treats relevant loss of control accidents in airliners, and is a motivation for the sixth and last part of the course, which focuses on techniques for upset recovery in airliners. An appended seventh section lists potential topics for course projects.

Lab Sessions. The course requires eleven flights on the Microsoft Flight Simulator, each of duration about one hour. The flight syllabus is given below.

|Flight |Focus |Brief Description |

|Fam 1 |Sim and A/C Fam; Steep Turns |System Familiarization; Upright and Inverted Steep Turns |

|Aero 1,2 |VFR Acrobatics |Aileron Roll, Barrel Roll, Wingover, Loop, Half and Full Cuban Eight, |

| | |Immelmann, Split-S with Visual horizon |

|Aero 3,4 |IFR Aerobatics |Aero 1 and 2 Maneuvers by Instrument Reference |

|Aero 5 |Aerobatics Check Flight |Precision VFR and IFR Aerobatics Check Flight |

|Upset 1 |Upset Recovery |Fighter Aircraft Upset Recovery Techniques |

|Upset 2 |Upset Recovery |B767 Aircraft Fam; Airline Upset Recovery Techniques |

|Upset 3 |Upset Recovery |Airline Upset Recovery Techniques (IFR) |

|Upset 4 |Upset Recovery |Upset Recovery Check Ride Practice |

|Upset 5 |Upset Recovery Check Flight |Fighter/Transport Upset Recovery Check Flight |

Disclaimer. This course is NOT a substitute for experience in an airplane. While we believe the course will enable a pilot to learn aerobatic flight and in-flight upset recovery faster, no evidence currently exists to support this belief. No flight simulator known to the author faithfully reproduces all the cues available to pilots in all attitude flight. In particular, the Microsoft Flight Simulator provides rather accurate visual cues and realistic instrument responses; however, it is notably lacking in faithfulness with respect to G-forces and responses of the airplane to throttle changes, control inputs, and changes in AOA/induced drag, among other shortcomings. That said, full motion simulators used by airlines to teach upset recovery techniques suffer in lesser degree from some of the same shortcomings.

II. INTRODUCTION TO THE MICROSOFT FLIGHT SIMULATOR

Preliminary Comments

Microsoft Flight Simulator 2002 Professional Edition (MFS 2002) is a first rate piece of software at a low cost. The simulator views we have developed for training flights provide visually realistic cues for aerobatics and upset recovery training. Bear in mind, however, that even with three forward looking windows open—affording a forward field of vision exceeding 90o—what you see from a simulated fighter cockpit is severely restricted compared with what you can see from a real-world airplane. This problem is much less significant when you are flying a simulated airliner, since visibility from a real-world airliner cockpit is already very severely restricted.

Another drawback is that both fighter and airline type aircraft in MFS 2002 respond more erratically to control stick movement than do real world airplanes, a phenomenon more pronounced in airliners than in fighters, however. As an example, 8-point aileron rolls are fairly easy to perform in a real-world fighter, and quite difficult in a simulated fighter (the simulator control stick is so sensitive that stopping a roll precisely on increments of 45o requires a lot of practice). In fact, even 4-point rolls are challenging to perform in the simulator.

Other considerations include the fact that G forces do not decrease realistically when stick back pressure is released; simulated aircraft do not decelerate as rapidly as real-world aircraft when thrust is reduced at low altitude and high airspeed; and RPM required in simulated fighter-type aircraft to maintain a given flight regime is significantly less than in real-world aircraft.

For all this, the bottom line is that you can learn a lot about aerobatics and unusual attitude recovery by flying MFS 2002. You may also develop some habits about throttle and stick control that will need modification when you begin similar training in a real airplane, but we think that overall the gains from flying the simulator will far outweigh the drawbacks. In time, there will probably be an aerobatics flight course at ERAU Daytona Beach. At that point, we will be able to test the validity of our theory that MFS 2002 is an excellent way to introduce student aviators to all attitude flight.

Using the Simulator

Introductory Comments. Many if not most Air Science students are already familiar with MFS 2002. If you’re not one of these people, you will need to spend some time learning the features of the simulator. This isn’t difficult, but it may require you to schedule a few extra sessions in the lab or on your own or your friend’s computer. We anticipate that you will be able learn what you need to know about MSF 2002 pretty much on your own. However, feel free to ask questions of the lab instructors, and if necessary, of the course instructors. If you have a home computer with good enough graphics capability to run MFS 2002, you should very definitely consider buying and installing the software.

When you arrive for a flight in the lab (scheduled or unscheduled), the procedures to follow are straightforward. (The foregoing assumes MFS 2002 is already running; if this isn’t the case, first start the software.)

1. Receive a computer assignment from the lab instructor; if no instructor is present, sit down at an available computer.

2. From the start screen, click Select a Flight.

3. When the Select a Flight screen appears, click My Saved Flights in the Choose a Category list.

4. From the Choose a Flight list, pick the training flight you want.

5. You will find yourself in the cockpit in position at the takeoff end of the runway. Take off and proceed with the maneuvers relevant to your flight. (Use slew to depart the field if time is short.)

6. When you complete your maneuvers or when time runs out, pause the flight (You may fly or slew back to the field and land if time permits, assuming you can find the field using visual navigation).

7. Select File in the menu bar (ALT key toggles the menu bar on/off in full screen mode).

8. Click End Flight on the File pull-down menu. (Leave the Select a Flight screen on the display.)

Note: Please do NOT save any flights on the lab machines. If you want to port the course flights saved on a lab machine to another computer, ask the lab assistants for help. You will need first to install any airplanes not already on your personal installation of MFS 2002. Course saved flight and weather files and zipped aircraft software can be downloaded from the course web site. Each saved flight has two files (.flt and .wx extensions). These two files should be placed in the ..\FS2002\flights\myflights directory of your Flight Simulator installation. In a default installation, the full path is ordinarily C:\Program Files\Microsoft Games\ FS2002\flights\myflights. To install a new aircraft, unzip the aircraft file and follow the installation directions. The gauges should be installed in the ..\aircraft\gauges directory.

Some Important MFS 2002 Key Commands. The simulator has many key commands to assist you in operating the software and flying the airplane. To see a full list of these commands, select Aircraft on the menu bar at the in-flight screen, and then Kneeboard in the resulting pull-down menu. (See the table below: F10 is a shortcut for these two actions.) You will probably want to make your own list of the key commands you use frequently in flight. We list below some of the commands we have found most useful when the simulator is in the in-flight mode (i.e., when you are sitting in the simulated airplane’s cockpit). It wouldn’t be a bad idea to have this page of the notes open on the table next to you when you are using MFS 2002.

|Command |Result |

|Simulator Controls |

|ALT |Toggles menu bar on/off at top of screen (flight is paused when menu is on) |

|CTRL - ; |Resets simulator to beginning of saved flight |

|SHIFT - Z |Cycles between various presentations of statistics at top of screen |

|P |Toggles between pause/fly simulator |

|F10 |Open Kneeboard and cycle through its four tabs. (Fifth F10 closes kneeboard.) |

|Aircraft Controls |

|. (Period) |Release parking brake when set; apply brakes when parking brake is released |

|F11 |Apply left brake |

|F12 |Apply right brake |

|CTRL - . |Set parking brake |

|G |Toggle landing gear up/down |

|F5 |Retract flaps fully |

|F6 |Retract flaps in increments |

|F7 |Extend flaps in increments |

|F8 |Extend flaps fully |

|/ |Toggles spoilers (airbrakes) between extend/retract |

|I |Toggles smoke on/off |

|View Commands |

|W |Cycle instrument through instrument panel presentations (including off) |

|S |Cycle views in the currently selected view window (must be in cockpit mode) |

|Slew Commands |

|Y |Toggle slew mode on/off |

|SPACEBAR |Heading north; straight and level; cancel current commanded slew movement |

|Num Pad 5 |Freeze all slew movement |

|Num Pad 8 |Move forward |

|Num Pad 2 |Move backward |

|Num Pad 4 |Move left |

|Num Pad 8 |Move Right |

|F3 (or Q) |Move up slowly |

|F4 |Move up quickly |

|A |Move down slowly |

|F1 |Move down quickly |

|F2 |Freeze vertical movement |

III. AEROBATIC MANEUVERS

| |

|Safety Note: DON’T QUALIFY FOR THE DARWIN AWARD |

| |

|Practice the following maneuvers only in a simulator. In particular, actual low level flight can be very |

|hazardous and not infrequently involves violations of FARs. (Stop a minute to recall The many times you flew|

|the simulator aircraft into the terrain while flying close to the ground.) Many pilots including Blue Angels|

|and Thunderbirds have been killed due to inadvertent terrain contact during low level flight. Regrettably, |

|it is likely that more pilots in all categories will be killed in the future. Many flying maneuvers are best|

|learned through airborne experience. Flying into the ground is not one of these maneuvers. |

| |

|Never attempt any kind of aerobatic or non-standard maneuvering in an airplane unless a flight instructor |

|qualified to instruct you in aerobatic flight accompanies you. An Embry-Riddle student pilot was killed |

|around 1990 at Daytona Beach while involved in unauthorized low level flight. He accidently flew a C-172 |

|into his girlfriend’s house in Flagler County while making a low pass to impress her. Many other pilots, |

|including a number of military pilots known personally to the author, have lost their lives by ignoring |

|simple safety considerations about unusual attitude flight. One of the most important of these safety |

|considerations is that if you are a pilot, you are not nearly as hot a pilot as you think you are, and should|

|act accordingly. A correlary to this idea is that you don’t really believe what you just read in the last |

|sentence. Remember the famous aviation maxim: “There are old pilots, and there are bold pilots, but there |

|are no old, bold pilots.” |

Terminology, Abbreviations, &c.

1. AI = attitude indicator; AS = airspeed indicator; DG = directional gyro (gyro compass); VSI = vertical speed indicator; TI = turn indicator (also a roll indicator); G = g meter.

2. Upright (inverted) flight = A/C bank angle less than (more than) 900, or pitch angle less than (more than) 90o with bank angle less than 90o.

3. Nose up (nose down) = nose up (nose down) refer to the nose attitude relative to the horizon. Note that nose position is NOT necessarily relative to pilot’s orientation in cockpit, depending on whether the aircraft is in upright or inverted flight. In both upright and inverted flight, nose up (down) means to aircraft’s nose is above (below) the horizon and the AI is in the light (dark) area. In upright straight and level flight, the pilot of course must pull back (push forward) on the stick to achieve a nose up (nose down) attitude. In inverted straight and level flight, however, the pilot must push forward (pull back) on the stick to achieve a nose up (nose down) attitude.

4. Nose rising (nose falling) = movement of nose position relative to the horizon, not the pilot’s orientation in the cockpit. In upright straight and level flight, a pilot pushes forward (pulls back) on the control stick to stop nose rising (nose falling) motion. In inverted straight and level flight, to stop nose rising (nose falling) motion, the pilot must pull back (push forward) on the stick.

5. Wing up (down) = wing position relative to the horizon, not to the pilot’s position in the cockpit. (See notes immediately following on Upright vs. Inverted Flight.)

Upright vs. Inverted Flight

From straight and level upright flight, inverted flight can be achieved by exceeding 90o of roll (without changing altitude) or 90o of pitch (without rolling). If you are in upright straight and level flight, a roll of 180o, together with proper pitch control, will put you in inverted straight and level flight. You can achieve the same attitude by pulling back on the stick. As the airplane goes past the vertical position (90o of pitch), the AI “flips” so that the light part is toward the pilot’s feet and the dark part toward the pilot’s head. The DI also reverses 180o. You are now in inverted flight with a very nose high attitude. If you continue stick backpressure, the nose will fall through to the horizon, at which time—if you stop the nose from falling further—you will be in inverted straight and level flight. Note that in the vertical maneuver, you converted airspeed (kinetic energy) to altitude (potential energy). That is, airspeed decreased and altitude increased. You didn’t roll at all. (The vertical maneuver described is the first half of a loop, of course, or all of an Immelmann except the final 180o roll from an inverted to upright position.)

In upright straight and level flight, you pull back on the stick to move the nose up, and push forward to move it down (inducing negative G forces, of course). The light portion of the AI is oriented toward the top of the instrument panel. If the reference airplane on the AI is in the light (dark) portion, the nose is above (below) the horizon. Visually, the sky is oriented toward the pilot’s head and the ground toward the pilot’s feet. To drop the right (left) wing, you push right (left) on the control stick. When the right (left) wing drops, the aircraft turns toward the pilot’s right (left); the AI shows the right (left) wing in the dark area; the DG tracks clockwise (counterclockwise), and the TI shows a right (left) turn.

In inverted straight and level flight, lift is being developed by excess static pressure acting on the upper surface of the wing (as opposed to the lower surface in upright flight). Thus some of the control and instrument parameters of upright flight are “reversed.” you pull back on the stick to move the nose down, and push forward to move it up (inducing negative G forces). When the nose is moving up (down), it is moving toward the pilot’s feet (head). The light portion of the AI is oriented toward the bottom of the instrument panel. Of course, if the reference airplane on the AI is in the light (dark) portion, the nose is still above (below) the horizon. Visually, the sky is oriented toward the pilot’s feet and the ground toward the pilot’s head. To drop the right (left) wing, you push left (right) on the control stick. When the right (left) wing drops, the aircraft still turns toward the pilot’s right (left); the AI still shows the right (left) wing in the dark area. However, the DG tracks counterclockwise (clockwise.) while the TI still shows a right (left) turn even though the aircraft is tracking counterclockwise (clockwise). If it surprises you that the DG can track counterclockwise (clockwise) when the aircraft is turning to the pilot’s right (left) hand side and the TI shows a left (right) turn, remember that the pilot—like the airplane—is inverted. As discussed in above, if the aircraft and pilot were upright, counterclockwise (clockwise) DG motion would correspond to a turn to the pilot’s left (right) and the TI would show a turn to the left (right).

Finally, recall that both the AI and the DG “flip” whenever the pitch angle exceeds 90o nose up or nose down with bank angle not equal to 90o. For example, in a loop, the AI flips from upright to inverted when the airplane passes through the vertical nose up; at the same time, the DG flips to show the reciprocal of the current heading. When passing through the vertical nose down in the same loop, the AI flips from inverted to upright, and the DG flips to its reciprocal, i.e., flips 180o to the original heading.

The following table summarizes instrument responses in level coordinated turns in upright (+1.0 G force) and inverted flight (-1.0 G force). The fighter aircraft we currently use for the course in MF 2002 doesn’t have a turn indicator. You can verify the following however, by choosing a simulated aircraft that has the required instrumentation.

|Turn |Instrument Responses |

|Upright right wing down |DG tracks clockwise; TI shows starboard (right) turn; Positive G. |

|Upright left wing down |DG tracks counterclockwise; TI shows port (left) turn; Positive G. |

|Inverted left wing down |DG tracks clockwise; TI shows port (left) turn; Negative G. |

|Inverted right wing down |DG tracks counterclockwise; TI shows starboard (right) turn; Negative G |

Table Shows Turn Directions in Inverted Flight at Negative G of -1.0 or less

The comments above assume negative G in inverted flight; i.e., assume straight and level inverted flight. However, if you think a bit—perhaps draw some pictures and watch the simulator response with spot aircraft windows open—you will see that the situation with positive G inverted flight turn is a little different. If your right wing is down in inverted flight when lift is being developed on the underside of the wing (i.e., at positive G), you will turn to your right (clockwise on the DG). If your left wing is down, you will turn to your left (counterclockwise on the DG). Note also that positive G inverted flight means the nose is falling toward or below the horizon, depending of course on the nose position of the aircraft at the moment the inverted flight attitude is achieved. The following table summarizes these ideas.

|Turn at Positive G |Instrument Responses |

|Upright right wing down |DG tracks clockwise; TI shows starboard (right) turn |

|Upright left wing down |DG tracks counterclockwise; TI shows port (left) turn |

|Inverted right wing down |DG tracks clockwise; TI shows starboard (right) turn |

|Inverted left wing down |DG tracks counterclockwise; TI shows port (left) turn |

Table Shows Turn Directions in Inverted Flight at Positive G of 1.0 or Greater

The following ideas can be summed up in a simple statement: An aircraft—upright or inverted—turns whenever its lift vector is oriented away from the vertical, and it turns in the direction that the lift vector is oriented.

Low Level Flight

You will practice maintaining (as closely as possible) a constant low altitude (100-200 feet AGL) in flight over mountainous terrain. This involves maneuvering up and down canyons at bank angles up to 180o; pitch angles may vary as much as 45o nose up or nose down. This type of flight is accomplished primarily using visual cues, with aircraft instruments as a backup. The purpose of this activity is to help you become accustomed to steep and constantly varying bank and pitch angles. Note that the G force on the aircraft will change with bank angle, and will probably vary between +4-5 G and –1 G. When cresting a ridge with a steep drop off on the far side, you will need to roll inverted to avoid gaining altitude, since pushing over in wings level flight at the top of the ridge to start “downhill” produces uncomfortable negative G’s in a real airplane. Once you are inverted, you will need to “pull through” on the stick to move the nose somewhat below the horizon; then roll out as the aircraft’s longitudinal axis becomes parallel with the ground. Be careful not to fly into the ground by pulling through too hard or rolling out late.

While maneuvering, it is easy to “manhandle” the controls of the simulator without much evidence that you have done anything improper, i.e., something that might be dangerous in a real airplane. Watch the G meter; if you apply excessive G force in a real airplane, there is a danger of causing structural damage or even breaking the wing spars and removing the wings from the aircraft. Every airplane has a maximum allowable G and an ultimate G, the latter of which is the point where the danger of catastrophic structural failure becomes a major concern.

At higher altitudes and lower airspeeds, high G may not be available, since an accelerated stall occurs first. A number of real world swept-wing fighters will experience a post-stall departure if the pilot fails to release stick backpressure when an accelerated stall occurs. Such a departure involves a loss of control with rapid and prolonged changes in roll and pitch angles. In short, the airplane gyrates wildly. Recovery may be initiated by releasing backpressure and lowering the angle of attack—in essence, by letting go of the stick. We might refer to this recovery technique as the “hands on head” procedure. Failure to release back pressure after an accelerated stall—particularly if the ailerons/spoilers are positioned for a roll—in some aircraft can lead to rapid loss of airspeed and a spin, recovery from which typically is problematic in swept-wing aircraft. We have not seen MFS 2002 simulate a post-stall departure, but in the real world inadvertent post-stall departure maneuvers are not at all uncommon. Some swept-wing fighters depart quickly if the pilot keeps on stick back pressure through an accelerated stall; others simply remain stalled without experiencing uncalled roll and pitch.

Inverted Flight Maneuver

To achieve inverted straight and level flight from upright straight and level flight, raise the nose slightly and roll quickly to the inverted position. It will be necessary to use forward stick to keep the nose slightly above the horizon while inverted. (Lift is being developed on the top of the wings, which are oriented toward the ground in inverted flight.)

Pilot Procedures. The roll can be in either direction. The cues assume a starboard (right) roll.

1. Achieve straight and level flight on a cardinal heading or section line at 350 KIAS and an altitude where a projection of the airplane’s flight path ahead 10 miles will not intersect the terrain. Choose an even thousand-foot altitude (1000, 2000, &c.)

2. Raise the nose of the aircraft 5-10o above the horizon (a gentle climb results) and—without hesitating—initiate a smart roll to the inverted position at a roll rate not less than 60o per second. That is, you should reach the inverted position in not more than 3 seconds.

3. As you roll, release backpressure on the sick to avoid turning; your heading must remain unchanged. The nose will begin dropping toward the horizon. At 90o of bank, the G force will be zero, and the nose should be dropping close to the horizon; the altimeter will be decreasing toward your original altitude. The direction of the aircraft should not change. As the aircraft exceeds 90o of bank (past the vertical position), begin to apply forward stick to keep the nose from dropping below the horizon.

4. As you reach 180o of bank, apply enough forward stick to maintain a negative angle of attack sufficient to keep the inverted aircraft in level flight. (That is, the altimeter will return to and then not vary from your original altitude.) The G force on the aircraft will be –1.0 G.

5. Maintain altitude in straight and level inverted flight for 10-20 seconds.

6. Now continue the roll to an upright position. To maintain level flight, decrease forward stick pressure from 180o to 90o of bank, and increase aft stick pressure from 90o to 0o of bank. Note that this is the reverse of what you did while rolling inverted.

7. The nose of the aircraft may be slightly below the horizon when you return to upright straight and level flight. Adjust it accordingly to maintain your original altitude and heading.

Visual Cues for Starboard (Right Roll) to Inverted Flight

|Phase |Visual Cues |

|Level Flight |Azimuth and elevation steady; nose on horizon. |

|5 – 10o Nose Up |Horizon drops; azimuth steady. |

|0o - 90o Bank |Horizon shows rapid roll; nose falling toward horizon; azimuth steady. |

|90o - 180o Bank |Horizon shows continuing roll; nose falling through horizon; azimuth steady. |

|180o Bank |Azimuth and elevation steady with nose on horizon in inverted flight. |

|180o – 90o Bank |Horizon shows rapid right roll; nose dropping (horizon rising); azimuth steady. |

|Level Flight |Azimuth and elevation steady; nose on horizon. |

Instrument Cues for Right Roll to Inverted Flight

|Phase |Instrument Cues |

|Level Flight |AI = wings level flight; AS; DG, ALT, TI = steady; VSI = 0; G = 1.0. |

|5o Nose Up |AI = nose up; AS; DG steady; ALT, VSI climbing; G = 1.0. |

|0o - 90o Bank |AI = right roll, nose falling; AS, DG steady, ALT, VSI decreasing climb; TI = right roll; G = 0.0 at |

| |90o of bank. |

|90o - 180o Bank |AI = right roll, nose rising; AS, DG = steady; ALT, VSI decreasing descent; TI = left roll; G < 0.0. |

|180o Bank |AI = wings level flight; AS; DG, ALT = steady; VSI = 0; TI = no turn; G = -1.0. |

|180o – 90o Bank |AI = right roll, nose rising inverted, nose falling upright; DG = cardinal heading, ALT decreasing; |

| |VSI < 0; TI = left turn inverted, right turn upright; -1.0 ................
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