(iv) The secondary effects of falling and blowing snow should also be considered in the evaluation of susceptibility to, and protection from, ice accumulations. The critical ambient temperatures should be defined;
this evaluation should take into consideration not only temperatures in the immediate vicinity of the freezing point, where "wet, sticky snow" is likely, but also colder temperatures where snow may adhere to partially heated interior inlet surfaces, melt, and refreeze in a colder location. Any hardware or ancillary systems (e.g., screens, particle separators, oil coolers) installed in turbine engine inlets may facilitate the accumulation of snow and potential for generation of ice. The effects of falling and blowing snow should be evaluated for both ground and flight conditions. The number of actual airplane tests within the critical snow and temperature environment should be maximized. The test article must be in production configuration with respect to surface finish, texture, and material type, to assure accurate representation of the in-service configuration. Recognizing that the desired atmospheric conditions may be difficult to find, some conditions may be substantiated by analysis, provided that analysis is supported by some form of test data (e.g., temperature survey data).
(v) It would be inconsistent to consider that § 25.1093(b) pertains only to the engine or engine inlet lip. The wording is broad and objective. It should be noted that the icing requirements pertaining specifically to the engine are included in § 33.68. Section 25.1093(b) was included to assure that the engine, as installed in the airplane, did not suffer adverse effects when the airplane was subjected to the icing envelope requirements covered by Appendix C to Part 25. It is not logical to exclude any part of the airplane that might shed ice so as to produce an adverse effect on the engine.
(vi) The foregoing discussions are intended to describe the minimum program to achieve certification for flight in icing conditions.
(2) Procedures . None.
[133. - 136.] [RESERVED]
Section 7. EXHAUST SYSTEM
137. GENERAL - § 25.1121 .
a. Explanation. Section 25.1121(a) - Carbon monoxide contamination. Carbon monoxide detection tests are conducted in accordance with this requirement to determine that the disposal of exhaust gases from each exhaust system does not cause carbon monoxide contamination of any personnel compartment.
b. References . Also see information related to the evacuation of other personnel compartment atmosphere contaminants contained in paragraphs 84 and 165 of this AC addressing the requirements of §§ 25.831 and 25.1197, respectively.
[138. - 140.] [RESERVED]
Section 8. POWERPLANT CONTROLS AND ACCESSORIES [141. - 154.] [RESERVED]
Section 9. POWERPLANT FIRE PROTECTION [155. - 158.] [RESERVED]
159. DRAINAGE AND VENTILATION OF FIRE ZONES - § 25.1187 .
a. Explanation. - There must be complete drainage of each part of each designated fire zone to minimize the hazards resulting from failure or malfunctioning of any component containing flammable fluids. The drainage means must be:
(1) Effective under conditions expected to prevail when drainage is needed;
(2) Arranged so that no discharged fluid will cause an additional fire hazard; and
(3) Arranged so that no discharged fluid will enter any other fire zone.
b. Procedures .
For detailed information related to flammable fluid drainage, ventilation, and fire protection, refer to Advisory Circular 25.1187, "Minimization of Flammable Fluid Fire Hazards," (to be released).
[160. - 164.] [RESERVED]
165. FIRE EXTINGUISHING AGENTS - § 25.1197 .
a. Explanation. Carbon Dioxide in Flightcrew Compartments. Carbon dioxide has been found to adversely affect flightcrew personnel in the performance of their duties. Therefore, in airplanes equipped with built-in carbon dioxide fuselage compartment fire extinguisher systems, the carbon dioxide concentration occurring at the flightcrew stations as a result of discharging the fire extinguishers should be determined in accordance with the procedures of this section (also see paragraph 84 of this AC), except that such determination is not considered necessary if:
(1) Five pounds or less of carbon dioxide will be discharged into any one such fu selage compartment in accordance with established fire control procedures; or
(2) Protective breathing equipment is provided for each flight crewmember on flight deck duty.
b. Procedures . Flight Test Investigation.
(1) The carbon dioxide concentra tions at breathing level at the flightcrew stations should be determined in flight tests during which fuselage compartment fire extinguishers are discharged in accordance with established fire control procedures. Since carbon dioxide is heavier than air, a nose-down attitude is likely to produce the critical concentrations in the crew compartment. Perform the tests described in paragraphs (2) and (3) below.
(2) A rapid descent at the "maximum operating limit speed" of the airplane with the flaps and landing gear up.
(3) A rapid descent with the flaps and landing gear down, at the maximum permissible speed for this configuration. If it appears that any other condition is likely to be critical on a particular airplane, it should also be investigated.
(4) In the flight tests specified above, it will be permissible to institute emergency ventilating procedures immediately prior to or following the discharge of carbon dioxide, provided such procedures can be accomplished easily and quickly by the flightcrew, and do not appreciably reduce the effectiveness of the fire protection system.
(5) If the measured carbon dioxide concentrations exceed three percent by volume (corrected to Sea Level, Standard Day conditions), protective breathing equipment should be provided for each flight crewmember on flight deck duty.
(6) Appropriate emergency operating procedures should be entered in the Airplane Flight Manual.
[166. - 169.] [RESERVED]
CHAPTER 6 - EQUIPMENT Section 1. GENERAL
170. EQUIPMENT - FUNCTION AND INSTALLATION - § 25.1301 .
a. Explanation.
(1) Certification of the installation of modern avionics/electrical systems on airplanes can be summarized, generally, by stating that the systems/equipment must:
(i) Perform its intended function (§ 25.1301);
(ii) Be adequately protected for failure conditions (§ 25.1309);
(iii) Be arranged to provide proper pilot visibility and utilization (as appropriate) (§ 25.1321);
(iv) Be protected by circuit breakers to preclude failure propagati on and/or minimize distress to the airplane's electrical system (§ 25.1357); and
(v) Be installed in a manner such that operation of the system will not adversely affect the simultaneous operation of any other system (§ 25.1431).
(2) Accordingly, th e recommended flight test procedures for equipment covered in Subpart F of 14 CFR part 25 (excluding powerplant instruments, airspeed calibration, safety equipment, and lights) are grouped together under this section and are organized as follows:
Para. b. Communication Systems.
Para. c. Navigation Systems.
Para. d. Instruments and Displays.
Para. e. Sensors and Warning Systems.
Para. f. Recording Systems.
Para. g. Engine Interfacing Systems ( autothrottle, thrust rating, ATTCS, etc.).
Para. h. Stability Augmentation Systems.
Para. i. All Weather Operation (Reduced Visibility) Systems.
(3) Section 25.1301 refers to each item of installed equipment. Type certification involving equipment approvals is governed by Title 49 United States Code, section 44704. That section requires the Administrator to make, or require the applicant for a type certificate to make, such tests "necessary in the interest of safety." Tests may be necessary in order to allow the Administrator to find that the aircraft is properly designed and manufactured, performs properly, and meets the regulations and minimum standards prescribed under section 44701(a) of Title 49.
“Optional equipment," however, is not a term that is meaningful under the Act in connection with the type certification of any given airplane. Where equipment or a system is a part or appurtenance of the airplane and is designed to aid and will obviously be used by the crew, the statutorily required tests and findings that must be used necessarily account for that equipment, whether or not it is characterized as "optional." Moreover, the regulatory requirement of § 21.21(b)(2) specifies that the Administrator must find that no feature or characteristic of the airplane makes it unsafe for the category in which certification is requested. Therefore, the extent to which that equipment must be tested or evaluated, in order that the Administrator may make the necessary finding with respect to the whole airplane, is a technical determination within the engineering and operational expertise of the Administrator. (See Advisory Circular 25-10, "Guidance for Installation of Miscellaneous, Non-Required Electrical Equipment," dated March 6, 1987, for further information.)
(4) Criteria and requirements f or flight must consider the applicant's engineering analysis and laboratory (simulator) test program. The combination of analysis, laboratory, and flight evaluation will form the whole of the certification requirement and, as such, must be in harmony and provide full evaluation. The flight evaluations supplement the analysis and simulation as required in both the representative operating conditions of the airplane and for analysis of the airplane modes of operation and configuration conditions.
(5) The requirement for demonstrating safe operation should normally include induced failures during flight. The requirement for failure demonstrations is also an outgrowth of the analysis and laboratory test results submitted by the applicant and is a result of the particular design being evaluated (e.g., consideration should be made for multiple channel systems specifically designed to be self-adaptive to failure conditions). Performance and malfunctions testing should include those flight conditions and airplane configurations that have been identified to be the most critical by analysis and/or test. Such items as weight, c.g., speed, altitude, flaps, slats, gear, speed brakes, and airplane system degradation should be considered.
(6) The amount of flight testing should be determined through the cooperative efforts of the assigned project personnel. It is recommended that the procedures that follow be used as a guide in preparing for the flight testing of an initial certification program. Follow-on items relevant to system derivative certification may result in considerably reduced flight testing. However, sufficient testing should be accomplished to assure satisfactory performance. When ground or flight test data, available from similar previously approved installations, are sufficient to properly evaluate a system's performance, additional testing may not be required. In the absence of such data, additional testing or analysis should be presented to substantiate the areas potentially affected.
(7) Particular attention should be given to those installations where an external piece of gear, such as an antenna, could affect the flight characteristics. All installations of this nature should be evaluated by the flight test pilot.
(8) Installations th at can or may change the established limitations, flight characteristics, performance, operating procedures, or any required systems require approval by an FAA Aircraft Certification Office (ACO) flight test branch. New installations of equipment in the cockpit, or modifications that affect existing equipment in the cockpit, should be evaluated through the cooperative efforts of the FAA, the project engineer, and the assigned flight test pilot, and assessed for the need of an FAA flight test demonstration.
(9) Throughout the systems/equipment evaluation, the operation of annunciators should be assessed to determine that proper conspicuity and display are provided to the appropriate flight crewmember. Any mode of operation selected by a manual action, or automatically, should be positively identified. Any submode should be evaluated to determine the need for annunciation.
b. Procedures - Communications .
(1) VHF Systems.
(i) Airplanes to be operated above 18,000 ft. Intelligible communications s hould be provided between the airplane and facilities throughout circle-turns within 160 NM of an FAA approved ground facility and above minimum radio line-of-sight with no intervening terrain. Bank angles up to 10 degrees on all headings should be used. Drop outs that are relieved by a reduction in bank angle at the same relative heading to the station are satisfactory. It is suggested that the Long Range Reception Test of paragraph (iv), below, be conducted first. If this test is successful, the circle-turns (within 160 NM) of this paragraph need not be conducted. Skidding turns may be used to minimize turn radius.)
(ii) Airplanes to be operated below 18,000 ft. For airplanes limited to operation below 18,000 ft., intelligible communications should be provided as given in ( i) above, except that the distance from the ground facility need not exceed 80 NM.
(iii) Antenna Coverage Measurement. If antenna locations are symmetrical, tests may be conducted using only one direction of turn. When antenna radiation pattern data are available, flight testing in a 360 degree turn may not be necessary, if satisfactory communication is achieved during checks in the vicinity of the predicted bearings and bank angles of worst performance.
(iv) Long Range Reception. At a distance of at least 160 NM (or 80 NM for airplanes to be operated below 18,000 ft.) from the ground facility antenna on a heading and above radio line-of-sight, perform a right and/or left 360 degree turn at a bank angle of at least 10 degrees. Communicate with the ground facility every 10 degrees of turn to test the intelligibility of the signals received at the ground station and in the airplane. The minimum line-of-sight altitude for 160 NM is approximately 17,000 ft. and for 80 NM is approximately 4,000 ft.
(v) High Angle Reception. Repeat the above test at a distance of 50 to 70 NM from the ground station and at an altitude of 35,000 ft., or the maximum operating altitude of the airplane (20 to 30 NM for airplanes to be operated below 18,000 ft.).
(vi) Approach Configuration. With the landing gear extended and the flaps in the approach configuration, demonstrate intelligible communications between the airplane and the ground facility.
(vii) Electro-Magnetic Compatibilit y (EMC). With all systems operating in flight, if practicable, verify, by observation, that no adverse effects are present in the required flight systems.
(2) HF Systems .
(i) Acceptable communication should be demonstrated by contacting one or more ground stations on several of the frequencies allowed by HF propagation conditions. Distances may vary from 100 to several thousand NM, and at least one over-the-horizon communication link should be established via ionospheric propagation. When a new HF antenna installation is being evaluated, the test should be conducted at an altitude of not less than 90 percent of the maximum altitude to check for possible corona arcing at the antenna.
(ii) The effect of precipitation static should be considered. This type of static is normally found in areas of high cirrus clouds, dry snow, dust storms, etc.
(iii) Electro-Magnetic Compatibility (EMC) should be evaluated with all systems operating in flight, if practicable, to verify, by observation, that no adverse effects are present in the required flight systems.
(3) Audio Systems .
(i) Acceptable communications should be demonstrated for all audio equipment, including microphones, speakers, headsets, interphone amplifiers, and public address systems. If provisions for passenger entertainment are included, adequate override of the music/audio by the cockpit crew attendants, or by prerecorded announcements, should be demonstrated. All modes of operation should be tested, including operation during emergency conditions (e.g., emergency descent with oxygen masks) with all airplane engines running, all airplane pulse equipment transmitting, and all electrical equipment operating. Flight tests, except as described in (ii) below, are generally not necessary unless airstream noise is considered a factor or unless excessive feedback with cockpit speakers is encountered during ground tests, which makes cockpit sound levels questionable.
(ii) If a flight evaluation of the passenger address (PA) system is d eemed to be necessary (or the prudent thing to do), PA announcements should be made from each handset station, including the cockpit, with the airplane operated at mid to high altitude at speeds approaching V MO/MMO.
(iii) Electro-Magnetic Compatibility (EMC). With all systems operating during flight, verify, by observation, that no adverse effects are present in the required flight systems.
(4) ARINC Communication Addressing and Reporting System (ACARS) . ACARS is an addressable VHF digital data link system that permits communication between the airplane and a ground-based facility. The display medium is a printer and/or electronic multifunction display.
(i) Acceptable performance should be demonstrated by verifying that pre-flight loaded data is retained in memory during power interrupts generated during transfers from ground to APU power, from APU to engine generators, and during cross-tie switching in all possible combinations.
(ii) Verify that simultaneous transmission of ACARS data and VHF voice communication is satisfactory and is free of interference between the two independent VHF systems, considering frequency and VHF antenna isolation. Exercise all possible switching combinations.
(iii) If the Selective Calling System (SELCAL) can be initiated by the ground station via the ACARS data link, demonstrate performance of that function by observing that the cockpit aural and visual indications are proper.
(iv) If the Flight Management System (FMS) data base or operational program can be accessed in flight by ACARS, exercise all interface functions to demonstrate performance of the intended function.
(v) ACARS should not cause interference with the operation of any of the airplane's radio/navigation systems. Particular attention should also be devoted to non-interference with flight guidance takeoff and approach functions, particularly autoland, since these are likely flight regimes for ACARS transmissions.
(5) Selective Calling System (SELCAL) . Verify performance of intended function of the SELCAL by receipt of VHF and HF (if appropriate) calls from ground stations.
(6) Satellite Communications (SATCOM) . [RESERVED]
(7) Portable Battery-Powered Megaphones .
(i) Conduct hearing range and intelligibility tests on the airplane to demonstrate that the amplified speech is heard and clearly understood throughout the interior cabin region served by the megaphones, with the engines off and with all significant conditions associated with an accident (including the presence of passengers and the normally-attendant confusion din) appropriately simulated.
(ii) Verify that the megaphone reliably performs its intended function and is designed:
(A) For ease of handling, and use, with one hand;
(B) With sufficient acous tical feedback suppression; and (C) With a volume control.
c. Procedures - Navigation .
(1) Very High Frequency Omnidirectional Range (VOR) Systems .
(i) These flight tests may be reduced if adequate antenna radiation pattern studies have been made and these studies show the patterns to be without significant holes (with the airplane configurations used in flight,
i.e., flaps, landing gear, etc.). Particular note should be made in recognition that certain propeller r.p.m. settings may cause modulation of the course deviation indication (prop-modulation). This information should be presented in the Airplane Flight Manual (AFM).
(A) The airborne VOR system should operate normally with warning flags out of view at all headings of the airplane (wings level) throughout the airspace within 160 NM of the VOR facility (for airplanes to be operated above 18,000 ft.), from the radio line-of-sight altitude to within 90 percent of the maximum altitude for which the airplane is certified or the maximum operating altitude.
(B) The accuracy determination should be made such that the indicated reciprocal agrees within 2 degrees. The test should be conducted over at least two known points on the ground such that data are obtained in each quadrant. Data should correlate with the ground calibration and in no case should the absolute error exceed ±6 degrees. There should be no excessive fluctuation in the course deviation indications.
(ii) Enroute Reception. Fly from a VOR facility rated for high altitude along a radial at an altitude of 35,000 ft. (or to within 90 percent of the airplane maximum certificated altitude or the maximum operating altitude) to a range of 160 NM (80 NM for airplanes not to be operated above 18,000 ft.). The VOR warning flag should not come into view, nor should there be deterioration of the station identification signal. The course width should be 20 degrees ±5 degrees (10 degrees either side at the selected radial). If practical, perform an enroute segment on a doppler VOR station to verify the compatibility of the airborne unit. Large errors have been found when incompatibility exists.
(iii) Long Range Reception. Perform a right and/or left 360 degree turn at a bank-angle of at least 10 degrees, at an altitude just above the radio line-of-sight, and at a distance of at least 160 NM (80 NM for
airplanes to be operated below 18,000 ft.) from the VOR facility. Signal dropout should not occur as evidenced by the malfunction indicator appearance. Dropouts that are relieved by a reduction of bank angle at the same relative heading to the station are satisfactory. The VOR identification should be satisfactory during the left and right turns.
Skidding turns may be used to minimize turn radius.
(iv) High Angle Recep tion. Repeat the turns described in (iii) above, but at a distance of 50 to 70 NM (20 to 30 NM for airplanes not to be operated above 18,000 ft.) from the VOR facility and at an altitude of at least 35,000 ft. (or to within 90 percent of the maximum certificated altitude of the airplane or the maximum operating altitude).
(v) Enroute Station Passage. Verify that the TO-FROM indicator correctly changes as the airplane passes through the cone of confusion above a VOR facility.
(vi) VOR Approach. Co nduct VOR approach( es) with gear and flaps down. The facility should be 12-15 NM behind the airplane with the approach conducted after a 30 degree radial change over the station. Use sufficient maneuvering in the approach to assure the signal reception is maintained during beam tracking.
(vii) Electro-Magnetic Compatibility (EMC). With all systems operating in flight, verify by observation that no adverse effects are present in the required flight systems.
(viii) Identifier. The audio identifi er should be checked, as should the decoded ident, if equipped with a digital bus.
(ix) Station tuning. Evaluate various methods of station tuning, including automatic and manually through a Flight Management System (FMS.) Also, a check for proper indications for loss of signal or receiver failure should be done.
(2) Localizer Systems .
(i) These flight tests may be reduced if adequate antenna radiation pattern studies have been made, and those studies show the patterns to be without significant holes. A significant hole is one that is greater than 10 decibels (dB) from the average within 30 degrees horizontally and 15 degrees vertically of the nose of the airplane (A) The signal input to the receiver, presented by the antenna system, sho uld be of sufficient strength to keep the malfunction indicator out of view when the airplane is in the approach configuration (landing gear extended-approach flaps) and at least 25 NM from the station. This signal should be received for 360 degrees of airplane heading at all bank angles up to 10 degrees left or right, at all normal pitch attitudes, and an altitude of approximately 2,000 ft.
(B) Satisfactory results should also be obtained at bank angles up to 30 degrees, when the airplane heading is within 60 degrees of the inbound localizer course. Results should be satisfactory with bank angles up to 15 degrees on headings from 60 degrees to 90 degrees of the localizer inbound course, and up to 10 degrees bank angle on headings from 90 degrees to 180 degrees from the localizer inbound course.
(C) The deviation indicator should properly direct the airplane back to course when the airplane is right or left of course.
(D) The station identification signal should be of adequate strength and su fficiently free from interference to provide positive station identification, and voice signals should be intelligible with all electrical equipment operating and pulse equipment transmitting.
(ii) Localizer Intercept. In the approach configuration an d at a distance of at least 25 NM from the localizer facility, fly toward the localizer front course, inbound, at an angle of at least 50 degrees. Perform this maneuver from both left and right of the localizer beam. No flags should appear during the time the deviation indicator moves from full deflection to on-course.
(iii) Localizer Tracking. While flying the localizer inbound and not more than 5 miles before reaching the outer marker, change the heading of the airplane to obtain full needle deflection; then fly the airplane to
establish localizer on-course operation. The localizer deviation indicators should direct the airplane to the localizer on-course. Perform this maneuver with both a left and a right needle deflection. Continue tracking the localizer until over the transmitter. Acceptable front course and back course approaches should be conducted to 200 ft. or less above the threshold.
(iv) Electro-Magnetic Compatibility (EMC). With all systems operating in flight, verify by observation that no adverse effects are present in the required flight systems.
(3) Glideslope Systems .
(i) These flight tests may be reduced if adequate antenna radiation pattern studies have been made, and those studies show the patterns to be without significant holes. A significant hole is one that is greater than 10 db from the average within 30 degrees horizontally and 15 degrees vertically of the nose of the airplane.
(A) The signal input to the receiver should be of sufficient strength to keep t he warning flags out of view at all distances up to 10 NM from the final approach fix. This performance should be demonstrated at all airplane headings between 30 degrees right and left of the localizer course. The deviation indicator should properly direct the airplane back to the glideslope path when the airplane is above or below the path.
(B) Interference with the navigation operation should not occur with all airplane equipment operating and all pulse equipment transmitting. There should be no interference with other equipment as a result of glideslope operation.
(ii) Glideslope Interception. Fly the localizer course inbound, at the altitude at which the glideslope beam intercepts the final approach fix, and at least 10 NM from the fix. The glideslope deviation indicator should be centered (± 25 percent of full scale) at the final approach fix. There should be no flags from the time the needle leaves the full scale fly-up position until it reaches the full scale fly-down position.
(iii) Glideslope Tracking. While tracking the glideslope, maneuver the airplane through normal pitch and roll attitudes. The glideslope deviation indicator should show proper operation with no flags. Acceptable approaches to 100 ft. or less above the threshold should be conducted.
(iv) Electro-Magnetic Compatibility (EMC). With all systems operating in flight, verify by observation that no adverse effects are present in the required flight systems.
(4) Marker Beacon System .
(i) In low sensitivity , the marker beacon annunciator light should be illuminated for a distance of 2,000 to 3,000 ft. when flying at an altitude of 1,000 ft. on the localizer centerline in all flap gear configurations.
NOTE: An acceptable test to determine distances of 2,000 to 3,000 ft. is to fly at a ground speed listed in Table I and time the marker beacon light duration.
Table I Altitude = 1,000 ft. (AGL) Ground Speed (Knots) Light Time (Seconds) 2,000 ft. 3,000 ft.
90 13 20 110 11 16 130 9 14 150 8 12 For ground speeds other than tabled values, the following formulas may be used:
Upper limit 1775 Ground Speed in Knots (secs.)= Lower limit 1183Ground Speed in Knots (secs.)=
(ii) If a high/low sensitivity feature is installed and selected, the marker beacon annunciator light and audio will remain on longer than when in low sensitivity.
(iii) The audio signal should be of adequate strength and sufficiently free from interference to provide positive identification.
(iv) As an alternative procedure, cross the outer marker at normal ILS approach altitudes and determine adequate marker aural and visual indication.
(v) Illumination shall be adequate in bright sunlight and at night.
(vi) Electro-Magnetic Compatibility (EMC). With all systems operating in flight, verify by observation that no adverse effects are present in the required flight systems.
(5) Automatic Direction Finding System (ADF) .
(i) Receiving Function. Determine that, when flying toward or away from a station with all required electrical and radio equipment in operation, the average bearing pointer indications are such that they present a usable bearing at the below-listed distances, during daylight hours (between one hour after local sunrise and one hour before local sunset) and when atmospheric disturbances are at a minimum. The amplitude of pointer oscillation is relatively unimportant, provided a valid direction to the station can be determined. At the distances shown below, the tone audio identification of the station to which the receiver is tuned shall be intelligible.
(ii) General Information.
Classification of LF/MF Radio Beacons in U.S. National Service Class Power (watts) Comlo under 25 MH under 50 H 50-1999 HH 2000 or more Facility Range Compass Locators 15 Nautical miles Transmitters <50 watts 25 Nautical miles Transmitters 100-200 watts 50 Nautical miles Transmitters 200-400 watts 60 Nautical miles Transmitters over 400 watts 75 Nautical miles NOTE: In areas where the known service range is less than the above distances, the actual shorter range may be used. It is advisable to ascertain the operating status of the facility to be used, prior to flight testing.
(iii) Pointer Reversal. While flying the airplane directly over a properly operating ground-based station, note the airplane's altitude and measure the complete pointer reversal time by using a stopwatch. Using the airplane's true ground speed and the airplane's altitude, determine that the pointer reversal occurred within a circular area centered over the station, having a radius equal to the altitude being flown. Partial reversals that lead or lag the main reversal are permissible.