Chapter 3: Instrument Flight Rules (IFR) PDF

CHAPTER 3 Instrument Flight Rules (IFR) 3.1 Aircraft Instruments and Equipment 3.1.1 Aircraft shall be equipped with suitable instruments and with navigation equipment appropriate to the route to be flown. 3.2 Clearances 3.2.1 Scope and Purpose 3.2.1.1 Clearances are issued solely for expediting an...

CHAPTER 3 Instrument Flight Rules (IFR) 3.1 Aircraft Instruments and Equipment 3.1.1 Aircraft shall be equipped with suitable instruments and with navigation equipment appropriate to the route to be flown. 3.2 Clearances 3.2.1 Scope and Purpose 3.2.1.1 Clearances are issued solely for expediting and separating air traffic and are based on known traffic conditions which affect safety in aircraft operation. 3.2.1.2 If an air traffic control clearance is not suitable to the pilot-in-command of an aircraft, the flight crew may request and, if practicable, obtain an amended clearance. 3.2.1.3 If a pilot receives a clearance or an instruction which cannot be complied with, the controller shall be advised so using the phrase “UNABLE” supplemented by the reason for this inability, in which case the controller shall issue an alternative clearance or instruction. 3.2.1.4 The issuance of air traffic control clearances by Tehran ACC constitutes authority for an aircraft to proceed only in so far as known air traffic is concerned. ATC clearances do not constitute authority to violate any applicable regulations for promoting the safety of flight operations or for any other purpose; neither do clearances relieve a pilot-in-command of any responsibility whatsoever in connection with a possible violation of applicable rules and regulations. 3.2.1.5 Tehran ACC shall issue such ATC clearances as are necessary to prevent collisions and to expedite and maintain an orderly flow of air traffic. 3.2.1.6 ATC clearances must be issued early enough to ensure that they are transmitted to the aircraft in sufficient time for it to comply with them. 3.2.1.7 Tehran ACC may request an adjacent ATC unit to clear aircraft to a specified point during a specified period of time. 3.2.1.8 In case at the point of departure, the clearance to enter Tehran FIR for an aircraft has not been issued due to any possible reason, before the aircraft enters Tehran FIR, the transferring unit shall issue holding instructions to keep the aircraft out of Tehran FIR until such time that the relevant entry clearance has been obtained from Tehran ACC. It is the responsibility of Tehran ACC to take all possible measures for such a clearance to be issued and in case of its impossibility, the flight shall not enter Tehran FIR under no circumstances. 3.2.2 Aircraft Subject to ATC for Part of Flight 3.2.2.1 When a flight plan specifies that the initial portion of a flight will be uncontrolled, and 1 that the subsequent portion of the flight will be subject to ATC, the ATS unit at departure aerodrome shall obtain clearance from the Tehran ACC sector, in whose area, controlled flight will be commenced, before departure. TEHRAN ACC OPERATIONS MANUAL 111 3.2.2.2 When a flight plan specifies that the first portion of a flight will be subject to ATC, and that the subsequent portion will be uncontrolled, the aircraft shall normally be cleared to the destination aerodrome, while the limit of that clearance shall be considered to be the controlled airspace boundary. 3.2.2.3 When an aircraft intends to leave a control area for flight outside controlled airspace, and will subsequently re-enter the same or another control area, a clearance from point of departure to the aerodrome of first intended landing may be issued. Such clearance or revisions thereto shall apply only to those portions of the flight conducted within controlled airspace. 3.2.2.3.1 The phraseologies used shall be: a) LEAVE CONTROLLED AIRSPACE (or CONTROL ZONE) [VIA (significant point or route)] AT (level) (or CLIMBING, or DESCENDING); b) JOIN (specify) AT (significant point) AT (level) [AT (time)]. 3.2.3 Flights Through Intermediate Stops When an aircraft files, at the departure aerodrome, flight plans for the various stages of flight through intermediate stops, the initial clearance limit will be the first destination aerodrome and new clearances shall be issued for each subsequent portion of flight. 3.2.4 Approval for Controlled Flights Tehran ACC shall forward a clearance to approach control units, aerodrome control towers or AFISUs with the least possible delay after the receipt of a request made by these units. In case issuance of such a clearance is not feasible, the requesting unit shall be so informed and the reason specified. 3.2.5 Contents of Clearances 3.2.5.1 Clearances shall contain positive (i.e. certain and doubtless) and concise data and shall, as far as practicable, be phrased in a standard manner. 3.2.5.2 Clearances shall contain the following in the order listed: a) aircraft identification; b) clearance limit; c) route of flight; d) level(s) of flight for the entire route or part thereof and changes of levels if required; Note. If the clearance for the levels covers only part of the route, it is important for the air traffic control unit to specify a point to which the part of the clearance regarding levels applies 2 whenever necessary to ensure compliance with RCF procedures of Annex 2. e) any necessary instructions or information on other matters such as SSR transponder operation, approach or departure maneuvers, communications and the time of expiry of the clearance. Note. The time of expiry of the clearance indicates the time after which the clearance will be automatically cancelled if the flight has not been airborne. TEHRAN ACC OPERATIONS MANUAL 112 3.2.5.3 Instructions included in clearances relating to levels shall consist of: a) cruising level(s) or, for cruise climb, a range of levels, and, if necessary, the point to which the clearance is valid with regard to the level(s); b) levels at which specified significant points are to be crossed, when necessary; c) the place or time for starting climb or descent, when necessary; d) the rate of climb or descent, when necessary; e) detailed instructions concerning departure or approach levels, when necessary. Note. Although in some states like England (CAP 413) the phrase “RECLEARED (level)” is not authorized to be used, ICAO in Doc.9432 approves of its application. 3.2.6 Clearance Limit 3.2.6.1 A clearance limit shall be described by specifying the name of the appropriate significant point, or aerodrome, or controlled airspace boundary. Note. This specification is not for taxi clearances, as Doc. 9432, 4.4.1 ascertains that Taxi instructions issued by a controller will always contain a clearance limit, which is the point at which the aircraft must stop until further permission to proceed is given. For departing aircraft, the clearance limit will normally be the taxi-holding point of the runway in use. The clearance limits for take-off and landing clearances are derived from the ATC clearance issued before departure, which is normally the destination aerodrome. 3.2.6.2 When prior coordination has been affected with units under whose control the aircraft will subsequently come, or if there is reasonable assurance that it can be affected a reasonable time prior to their assumption of control, the clearance limit shall be the destination aerodrome or, if not practicable, an appropriate intermediate point, and coordination shall be expedited so that a clearance to the destination aerodrome may be issued as soon as possible. 3.2.7 Clearance for a Requested Change in Flight Plan Note. Request for route change before departure, requires a new flight plan to be filed with a minimum lead tome of one hour to the EOBT. 3.2.7.1 When issuing a clearance covering a requested change in route or level, the exact 3 nature of the change shall be included in the clearance. 3.2.7.2 When traffic conditions will not permit clearance of a requested change, the word “UNABLE” shall be used. When warranted by circumstances, an alternative route or level should be offered. 3.2.7.3 When a change is made to any part of a level clearance, the entire level clearance shall be re-stated. 3.2.7.4 When an alternative route is offered and accepted by the flight crew the amended clearance issued shall describe the route to the point where it joins the previously cleared route, or, if the aircraft will not re-join the previous route, to the destination. 3.2.8 Change from IFR Flight to VFR Flight TEHRAN ACC OPERATIONS MANUAL 113 3.2.8.1 Change from instrument flight rules (IFR) flight to visual flight rules (VFR) flight is only acceptable when a message initiated by the pilot-in-command containing the specific expression “CANCELLING MY IFR FLIGHT”, together with the changes, if any, to be made to the current flight plan, is received by an air traffic services unit. No invitation to change from IFR flight to VFR flight is to be made either directly or by inference. 3.2.8.2 This request shall not be refused by the controller and no reply, other than the acknowledgment “IFR FLIGHT CANCELLED AT (time)”, should normally be made by an air traffic services unit. 3.2.8.3 When an air traffic services unit is in possession of information that instrument meteorological condition are likely to be encountered along the route of flight, a pilot changing from IFR flight to VFR flight should if practicable, be so advised. 3.2.8.4 Information to a pilot changing from IFR flight to VFR flight, where it is likely that flight in VMC cannot be maintained, shall be given in the following manner: “INSTRUMENT METEOROLOGICAL CONDITIONS REPORTED (or forecast) IN THE VICINITY OF (location)”. 3.2.8.5 ACC controller who receives notification of an aircraft’s intention to change from IFR to VFR flight shall, as soon as practicable thereafter, take proper measures to inform all other ATS units to whom the IFR flight plan was addressed, except those units through whose regions or areas the flight has already passed. 3.2.9 Change from VFR Flight to IFR Flight 4 Refer to part 4, 4.1.9 and 4.2.7. 3.2.10 Departure Procedures 3.2.10.1 Departure Restrictions 3.2.10.1.1 Take-off and Clearance Expiry Times 3.2.10.1.1.1 Time of take-off shall be specified by the Tehran ACC when it is necessary to: a) coordinate the departure with traffic not released to the unit providing approach control service; and b) provide en-route separation between departing aircraft following the same track. 3.2.10.1.1.2 A clearance expiry time shall be specified by the ACC if a delayed departure would conflict with traffic not released to the unit providing approach control service. If, for traffic reasons of its own, a unit providing approach control service has to specify in addition its own clearance expiry time, this shall not be later than that specified by the ACC. 3.2.10.1.2 Forwarding Departure Delay Information The unit providing ATS for departure aircraft should be informed of any anticipated departure delays by Tehran ACC. 3.2.10.1.3 Forwarding Departure Times TEHRAN ACC OPERATIONS MANUAL 114 All departure times shall be forwarded to Tehran ACC in accordance with procedures prescribed in the relevant LoC. 3.2.11 Traffic Orientation Scheme (TOS) 3.2.11.1 In order to reduce controllers’ workload, frequency congestion and airway saturation, a standard and mandatory traffic orientation scheme (TOS) is published in I.R. Iran AIP, as an AIP Supplement, to enable all aircraft operators, planning to fly within Tehran FIR (Including transit, domestic and international departure and arrival) to file flight plans as required by that supplement. 3.2.11.2 On special request of operators, permission may be granted to a flight by appropriate ATS authority to fly contrary to the published TOS. In case a filed route is found to be inappropriate in this regard, the controller should take appropriate action for flight to be continued in accordance with the filed flight plan and report the situation for further investigation by the authorities. 3.2.11.3 Should it be found out that the filed route is not possible to be followed by the flight due to any reason, the controller shall take action to amend the route of flight and coordinate the changes thereto with the ADOC and affected ATS units or sectors. 3.2.12 Alternative Routes 5 Should any part of an airway or route be unavailable, aircraft shall be cleared to operate on alternative routes. Note. There is no requirement for those flights which have not departed yet and who have filed a flight plan (or RPL) to submit a new flight plan to use the newly specified alternate routes. 3.2.13 Direct Routing Clearance, Approval and Instruction 3.2.13.1 Unless a flight plan is filed via direct routing and an ATC clearance for it is obtained before departure, the subsequent requests of flight crew for bypassing the route angles, shall require the controllers’ approval. 3.2.13.2 The controller may or may not approve such a request based on traffic, SUA, military and/or any other restrictions or reasons. 3.2.13.3 If such an approval is issued, the respective flight plan in the automation system shall be so modified and this modification shall be considered as the required notification to the ADOC. Note. Sending a flight plan to ADOC with the format number of 5, will alleviate this notification. 3.2.13.4 The approval of direct routing requests that affect other sectors or ATS units shall be withheld until the approval from those sectors or units is obtained. 3.2.13.5 Although it shall be inherent in such a request that the flight crew are aware of the change to the airspace classification and service provided, but still the specification by the controller of any possible entrance into uncontrolled airspace should be taken into consideration. 3.2.13.6 The phraseology used for approving the direct routing requests of the flight crew shall be: DIRECT [FROM (position)] TO (position) APPROVED. 3.2.13.7 For traffic or other reasons, the controller may find it necessary for a flight to proceed direct form a specific point to another, as for vectoring an aircraft in radar control service. This will be achieved by issuing an instruction. 3.2.13.8 In issuing such an instruction, if the direct routing (or vectoring) takes the aircraft off an ATS route, the responsibility for terrain clearance shall be transferred to the controller and shall remain with him until the aircraft rejoins the flight planned routing. Note 1. See Doc. 4444, 2.1, Note. 2. 6 Note 2. There are no obstructions at FL220 or above within the boundaries of Tehran FIR. 3.2.13.9 The phraseology used for instruction issuance of direct routing shall be: PROCEED DIRECT [FROM (position)] TO (position). 3.2.14 level assignment and verification 3.2.14.1 Standard Cruising Levels See 2.37. 3.2.14.2 Minimum Levels Except when necessary for take-off or landing, an IFR flight shall be flown at a level that is not below the minimum flight level established or, where no such minimum flight level has been established at a level which is at least 2500 feet above the highest obstacle located within 5 NM of the estimated position of the aircraft. Note 1. The estimated position of the aircraft will take account of the navigational accuracy which can be achieved on the relevant route segment, having regard to the navigational facilities available on the ground and in the aircraft. Note 2. The above calculated figure should be rounded up to the nearest thousand feet and should be selected from the table of cruising levels of 2.37. 3.2.14.3 Non-radar Level Confirmation Where radar service is not provided in a specific airspace, on initial contact, the flight crew shall report to the procedural controller the level of the aircraft in form of the specifications of voice position reporting as defined in Doc.4444, 4.11.2. If the pilot fails to do so, the controller shall issue appropriate instruction for this confirmation to be made in order to ensure that the aircraft is following the current flight plan. Note. For the purpose of this paragraph, “initial contact” means the pilot's first radio contact with the sector. 3.2.15 En route Holding and Delay TEHRAN ACC OPERATIONS MANUAL 116 3.2.15.1 When delay is expected, Tehran ACC shall normally be responsible for clearing aircraft to a specified holding fix, and for including holding instructions and onward clearance time, as applicable. 3.2.15.2 Aircraft should normally be held at a designated holding point and other aircraft shall be separated vertically or laterally from the holding aircraft. Note. See chapter 6 for the application of radar and chapter 5 for application of non-radar separation. 7 3.2.15.3 Levels at holding fixes shall as far as practicable be assigned in a manner that will facilitate clearing each aircraft to approach in its proper priority. Normally, the first aircraft to arrive over a holding fix should be at the lowest level, with following aircraft at successively higher levels. 3.2.15.4 When extended holding is anticipated, turbojet aircraft should, when practicable, be permitted to hold at higher levels in order to conserve fuel, while retaining their order in the approach sequence. 3.2.15.5 If an aircraft is unable to comply with the published or cleared holding procedure, alternative instructions shall be issued. 3.2.15.6 For the purpose of maintaining a safe and orderly flow of traffic, an aircraft may be instructed to orbit at its present or at any other position, provided the required obstacle clearance is ensured. 3.2.15.7 In the event an aircraft is held en route or at a location or aid other than the initial approach fix, the aircraft concerned shall, as soon as practicable, be given an expected onward clearance time from the holding fix. The aircraft shall also be advised if further holding at subsequent holding fix is expected. Note. “Onward clearance time” is the time at which an aircraft can expect to leave the fix at which it is being held. 3.2.15.8 In the event of extended delays, aircraft should be advised of the anticipated delay as early as possible and, when practicable, be instructed or given the option to reduce speed en-route in order to absorb delay. 3.2.15.9 Holding other than Published The controller may require or approve a request for an aircraft to hold or orbit in a manner different from that published, not below FL200, provided that: a) preferably the specified holding pattern will not take the aircraft outside controlled airspace; b) terrain clearance will be maintained; and c) no active SUA will be infringed. 3.2.15.10 Holding due Weather 3.2.15.10.1 When radar service is not provided and aircraft are holding because weather conditions are worse than the prescribed landing minima of an applicable instrument approach, the pilot shall be instructed to report at scheduled intervals not exceeding 15 minutes. TEHRAN ACC OPERATIONS MANUAL 117 8 3.2.15.10.2 If the pilot of an aircraft in an approach sequence has indicated an intention to hold for weather improvement, or for other reasons, such action shall be approved. However, when other holding aircraft indicate intention to continue their approach to land, provision of standard radar or geographical separation is mandatory either by clearing the pilot desiring to hold to an adjacent holding pattern which has a standard lateral/geographical separation or by application of other separation methods. 3.2.15.11 Published En-route Holdings As specified in AIP ENR 3.6, five en-route holdings are defined as follows: * - Hours of OPR: Below FL 300 H24, Above FL 300 HN. * - Holding not available below FL 300 when OID22 is active above FL140. 3.3 Horizontal Speed Control Instructions 3.3.1 General Note. Short explanations regarding various types of speed commonly used in flight operations are as follows: 3.3.1.1 Indicated Air Speed (IAS) IAS is the speed shown to the pilot on the airspeed indicator. It is a safety speed for handling the aircraft, particularly, stall speeds (Vs, Vso) or gear and flaps limitation speeds (Vle, Vlo, Vfe) are indicated airspeeds. In pilot/controller communications it is referred to as "airspeed" and when expressed in knots, the IAS is often written as “KIAS”. 3.3.1.2 True Air Speed (TAS) 3.3.1.2.1 TAS is the speed of an aircraft into the air mass it is in, whatever the density of this air mass is. It is often shown on FMS or GPS or can be calculated from IAS (see below). TEHRAN ACC OPERATIONS MANUAL 118 3.3.1.2.2 TAS is a flight planning and navigation speed which enables calculations of deviation or ETA (see ground speed (GS)). 3.3.1.2.3 When expressed in knots, TAS is often written as “KTAS”. 3.3.1.3 The Relation between TAS and IAS 3.3.1.3.1 For IASs of between 240 kt and 400 kt and flight levels of between 50 and 250, a good approximation is given by “TAS=IAS+FL/2” Example: For an aircraft steady at FL120 and 320 KIAS, KTAS equals 320 + 120/2 = 380 kt. 9 3.3.1.3.2 For IASs of lower than 240 kt, “TAS=IAS+(1.5%IAS*altitude)” the altitude expressed in thousands of feet. Example: For an aircraft steady at FL150 and 220 KIAS, KTAS equals 220 + ((1.5% x 220) x 15) = 270 kt. 3.3.1.3.3 Generally, the lower and slower the aircraft is, the closer to the IAS is the TAS. 3.3.1.4 Ground Speed (GS) 3.3.1.4.1 GS is the TAS corrected with the wind speed and represents the speed of the aircraft relative to the ground. It is shown on FMS, GPS, radar situation displays or can be calculated from TAS when wind direction and speed are known (see below). 3.3.1.4.2 GS is the speed needed to calculate the estimated time of arrival (ETA) over a point. Example: For an aircraft whose TAS is 260 kt with a 20 kt headwind, GS equals 260-20 = 240 kt. It means that the aircraft is flying 4 NM a minute (240/60=4). If the pilot reads on DME a distance of 30 NM from a VOR/DME and the controller asks for the time estimate over this NAVAID, the answer will be 30/4 = 7.5 minutes added to present time. ETA is calculatable using the Base Factor. By definition, Base Factor is “BF=60/GS”. In this example it equals to (60/240=0.25). The ETA over the NAVAID is “ETA=d*BF”, in which “d” is the distance, i.e. (30*0.25=7.5 minutes) added to present time. 3.3.1.5 Mach Number 3.3.1.5.1 By definition, “M=TAS/a”, where "a" is the local speed of sound. Approximatively, we can consider that (Mach 0.10 = 60 KTAS). Technically, Mach number is the TAS expressed as a fraction of the local speed of sound. 3.3.1.5.2 The speed of sound (Mach 1) is a function of temperature. Where it is colder, i.e. at higher altitudes, it is slower. In international standard atmosphere (ISA) conditions, at sea level Mach 1 is a little over 661 kt TAS, but at FL360 it has decreased to 572 kt and remains constant up to around FL600 to FL700 (See figure 18). TEHRAN ACC OPERATIONS MANUAL 119 Figure 18. Mach Number – Level Relationship Example: If an aircraft climbs from sea level to FL350 at a constant Mach number of 0.70 then the TAS will decrease from 463 to 400 kt. Similarly, a constant Mach number descent will result in 10 increasing TAS. 3.3.1.5.3 Since the air density is lower at higher altitudes, airspeed indicators read less than the actual speed of the aircraft, but at sea level there is virtually no difference between them. Therefore, an aircraft climbing at a constant IAS will have an increasing TAS. Example: For example, under ISA conditions 250 kt IAS at sea level equates to 250 kt TAS, but at FL430 an IAS of 250 kt equals a TAS of 502 kt. Conversely, if a descent is carried out at a constant IAS the TAS will decrease as altitude is lost. (See figure 19) Figure 19. TAS – Altitude Relationship TEHRAN ACC OPERATIONS MANUAL 120 3.3.1.6 Other speeds They are all indicated airspeeds, shown on the airspeed indicator onboard. 3.3.1.6.1 Take-off a) V1= take-off decision speed (or critical speed). Before V1, the pilot can abort take-off. After V1, the pilot “Must” take off. b) VR= take-off rotation speed at which the pilot pulls the flight controls to rise the nose and take off. c) V2= take-off safety speed to be reached before passing 35 ft above runway altitude. 3.3.1.6.2 Cruise a) Va= Maneuvering speed, max speed at which flight controls can be fully maneuvered. b) Vno= Normal Operation speed, max cruise speed. c) Vne= Never Exceed speed. d) Vmo= Maximum Operating speed (KIAS). e) Mmo= Maximum Operating Speed (Mach). 3.3.1.6.3 Approach and Landing a) Vfe= Maximum speed with Flaps Extended. b) Vlo= Maximum speed for Landing Gear Operations (retraction and extension). c) Vle= Maximum speed when Landing Gear Extended. d) Vs= Stalling speed (at max weight). e) Vso= Stalling speed with gear and flaps Out (at max weight). f) Vref= Reference speed (or landing speed) =1.3 x Vso (Vref is also known as Vat). 3.3.1.6.4 Approach Regulation a) Minimum clean speed=minimum speed with gear/flaps/slats up and airbrakes in, usually about 1.5 x Vso. b) Minimum approach speed=Vref (see above), 1.3 x Vso. 3.3.2 Horizontal speed control instructions 3.3.2.1 General 11 3.3.2.1.1 In order to facilitate a safe and orderly flow of traffic, aircraft may, subject to conditions, be instructed to adjust speed in a specified manner. Flight crews should be given adequate notice of planned speed control. Note 1. Application of speed control over a long period of time may affect aircraft fuel reserves. Note 2. Provisions concerning longitudinal separation using the Mach number technique are contained in 5.4.1.2.1.1.4. TEHRAN ACC OPERATIONS MANUAL 121 3.3.2.1.2 Speed control shall not be applied to aircraft entering or established in a holding pattern. 3.3.2.1.3 Speed adjustments should be limited to those necessary to establish and/or maintain a desired separation minimum or spacing. Instructions involving frequent changes of speed, including alternate speed increases and decreases, should be avoided. 3.3.2.1.4 The flight crew shall inform the ATC unit concerned if at any time they are unable to comply with a speed instruction. In such cases, the controller shall apply an alternative method to achieve the desired spacing between the aircraft concerned. 3.3.2.1.5 At levels at or above FL250, speed adjustments should be expressed in multiples of 0.01 Mach; at levels below FL250, speed adjustments should be expressed in multiples of 10 kt based on indicated airspeed (IAS). Note 1. Mach 0.01 equals approximately 6 kt IAS at higher flight levels. Note 2. When an aircraft is heavily loaded and at a high level, its ability to change speed may, in cases, be very limited. 3.3.2.1.6 Where the controller only requires speed control to be applied following the point at which the aircraft changes over from Mach number to IAS, the phraseology to be used by the controller is: “DESCEND to (level), ON SPEED CONVERSION [or TRANSITION SPEED], (speed) KNOTS”. 3.3.2.1.7 Where the controller requires the aircraft to fly at a specific Mach number until the changeover to IAS and then fly a specified IAS, the phraseology to be used by the controller is: “MACH DECIMAL (Much number), ON SPEED CONVERSION [or TRANSITION SPEED], (speed) KNOTS”. 3.3.2.1.8 The procedures stated in 3.3.2.1.6 and 3.3.2.1.7 may be used in reverse for the application of speed control to aircraft currently below FL250 but climbing to a level above 12 FL250 with the following phraseologies: a) CLIMB to (level), ON SPEED CONVERSION [or TRANSITION SPEED], MACH DECIMAL (Much number); and b) (speed) KNOTS, ON SPEED CONVERSION [or TRANSITION SPEED], MACH DECIMAL (Much number). 3.3.2.1.9 Aircraft shall be advised when a speed control restriction is no longer required. 3.3.2.2 Methods of application 3.3.2.2.1 In order to establish a desired spacing between two or more successive aircraft, the controller should first either reduce the speed of the last aircraft, or increase the speed of the lead aircraft, then adjust the speed(s) of the other aircraft in order. 3.3.2.2.2 In order to maintain a desired spacing using speed control techniques, specific speeds need to be assigned to all the aircraft concerned. Note 1. The true airspeed (TAS) of an aircraft will decrease during descent when maintaining a constant IAS. When two descending aircraft maintain the same IAS, and the leading aircraft is at the lower level, the TAS of the leading aircraft will be lower than that of the following aircraft. The distance between the two aircraft will thus be reduced, unless a sufficient speed TEHRAN ACC OPERATIONS MANUAL 122 differential is applied. For the purpose of calculating a desired speed differential between two succeeding aircraft, 6 kt IAS per 1000 ft height difference may be used as a general rule. At levels below FL80 the difference between IAS and TAS is negligible for speed control purposes. Note 2. Time and distance required to achieve a desired spacing will increase with higher levels, higher speeds, and when the aircraft is in a clean configuration. 3.3.2.3 Descending and arriving aircraft 3.3.2.3.1 An aircraft should, when practicable, be authorized to absorb a period of notified terminal delay by cruising at a reduced speed for the latter portion of its flight. 3.3.2.3.2 An arriving aircraft may be instructed to maintain its “maximum speed”, “minimum clean speed”, “minimum speed”, or a specified speed. Note. “Minimum clean speed” signifies the minimum speed at which an aircraft can be flown in a clean configuration, i.e. without deployment of lift-augmentation devices, speed brakes 13 or landing gear. 3.3.2.3.3 Speed reductions to less than 250 kt IAS for turbojet aircraft during initial descent from cruising level should be applied only with the concurrence of the flight crew. 3.3.2.3.4 Instructions for an aircraft to simultaneously maintain a high rate of descent and reduce its speed should be avoided as such maneuvers are normally not compatible. Any significant speed reduction during descent may require the aircraft to temporarily level off to reduce speed before continuing descent. 3.3.2.3.5 Arriving aircraft should be permitted to operate in a clean configuration for as long as possible. Below FL150, speed reductions for turbojet aircraft to not less than 220 kt IAS, which will normally be very close to the minimum speed of turbojet aircraft in a clean configuration, may be used..4 Vertical Speed Control Instructions 3.4.1 General 3.4.1.1 In order to facilitate a safe and orderly flow of traffic, aircraft may be instructed to adjust their rate of climb or rate of descent. Vertical speed control may be applied between two climbing or two descending aircraft in order to establish or maintain a specific vertical separation minimum. Note. It does not mean that vertical speed control shall not be applied between more than two aircraft simultaneously, but it means that for arranging the rates of climb and descent, aircraft are to be taken into account two by two. TEHRAN ACC OPERATIONS MANUAL 124 3.4.1.2 Vertical speed adjustments should be limited to those necessary to establish and/or maintain a desired separation minimum. Instructions involving frequent changes of climb/descent rates should be avoided. 3.4.1.3 Flight crew shall inform Tehran ACC if unable, at any time, to comply with a specified rate of climb or descent. In such cases, the controller shall apply an alternative method to achieve an appropriate separation minimum between aircraft, without delay. 3.4.1.4 Aircraft shall be advised when a rate of climb/descent restriction is no longer required. 3.4.2 Methods of Application 3.4.2.1 An aircraft may be instructed to expedite climb or descent as appropriate to or through a specified level, or may be instructed to reduce its rate of climb or rate of descent. 14 Note. It is strongly recommended not to use the phrases “EXPEDITE CLIMB/DESCENT” but to specify a specific rate of climb or descent, as the situation warrants. It should be noted that only an increase of 100 ft/min may be considered expedition by flight crew concerned. 3.4.2.2 Climbing aircraft may be instructed to maintain a specified rate of climb, a rate of climb equal to or greater than a specified value or a rate of climb equal to or less than a specified value. 3.4.2.3 Descending aircraft may be instructed to maintain a specified rate of descent, a rate of descent equal to or greater than a specified value or a rate of descent equal to or less than a specified value. Phraseology: CLIMB (or DESCEND) TO (level) AT (number) FEET PER MINUTE [OR GREATER or (OR LESS)] Example: IRA245 CLIMB TO FL220 AT 1500 FEET PER MINUTE OR GREATER. 3.4.2.4 In applying vertical speed control, the controller should ascertain to which level(s) climbing aircraft can sustain a specified rate of climb or, in the case of descending aircraft, the specified rate of descent which can be sustained, and shall ensure that alternative methods of maintaining separation can be applied in a timely manner, if required. Note 1. Controllers need to be aware of aircraft performance characteristics and limitations in relation to a simultaneous application of horizontal and vertical speed limitations. Note 2. Controllers should keep in mind the rate of climb/descent restrictions while applying the vertical speed control. To reduce false TCAS RA, crew must reduce the aircraft rate of climb or descent as applicable to 1000 feet per minute or less, in their last 1000 feet. 3.5 Visual Approach Should any request by flight crew be received by Tehran ACC as for performing visual approach, the clearance to do so shall be withheld until approval is obtained from controlled aerodromes or coordination is made with uncontrolled ones. Tehran ACC shall not issue any clearance for visual approach without fulfilling either of the aforementioned options. 15