id: 6.14-aircraft-speed title: 6.14 Aircraft Speed meta: Relationship between Mach Number (M), Indicated Airspeed (IAS) and Groundspeed (GS) on Infinte Flight. order: 14 #
6.14 Aircraft Speed #
Indicated Airspeed (IAS) Vs. Groundspeed (GS) #
6.14.1 #
There is a common misconception between the relationship between Indicated Airspeed (IAS) and Groundspeed (GS). For simplicity the reasons behind the difference are not covered, however there are two things that will ultimately affect the relationship: wind and altitude!
6.14.2 #
Wind: if there is a tailwind of 20kts, that means there is 20kts of wind pushing the aircraft along in addition to its normal speed, so 180kts IAS will become 200kts GS. Similarly, if there is 20kts of headwind, that means there is 20kts of wind pushing against the aircraft, so 180kts IAS becomes 160kts GS.
6.14.3 #
Altitude: as the altitude of aircraft increases, so will the difference between IAS and GS. Below are some very “ball-park” figures which can be used for reference. With the aircraft flying at 250kts IAS with no wind:
- 3000ft ~ 260kts GS (+10)
- 6000ft ~ 270kts GS (+20)
- 9000ft ~ 290kts GS (+40)
- 12000ft ~ 300kts GS (+50)
6.14.4 #
An aircraft’s current IAS (Mach Number above FL280) is displayed on their information tab when opened along with other data, all speed commands are referenced to IAS or Mach Number although GS is displayed on the radar screen to assist controllers in the effects of altitude and wind (see 6.14.2 and 6.14.3 above).
Mach Number (M) #
6.14.5 #
IAS is converted to Mach Number at FL280, with all speed referred to as Mach above this Flight Level. Below is a table that highlights typical cruising Mach Numbers for different aircraft types. Controllers should{.red} note that altitude and weight have a huge influence on Mach Number and therefore if aircraft are asked to perform outside of the speed ranges below, the aircraft may be unable to comply:
| Category | Aircraft Types | Speed Range (M) |
|---|---|---|
| Turbo Prop | e.g. TBM-930, Dash-8 Q400 etc | .51 - .53 |
| Narrow Body Jet | e.g. CRJ, E-Jet, A320, B737 etc | .75 - .80 |
| Wide Body Jet | e.g. A330, B777, MD-11 etc | .82 - .85 |
Speed Control #
Tip
: Controllers should{.red} choose 2-3 common speeds (such as 250kts, 210kts and 180kts) which can be added to the quick command tab list when in use. Using the same speed assignments helps standardize the approach and manage traffic!
6.14.6 #
When issuing speed control to aircraft, Controllers must{.red} utilize the following techniques:
- Compensate for closure rate when assigning speed adjustment in an “in-trail” situation by either reducing the trailing aircraft speed first, or increasing the lead aircraft speed first (see 6.14.7 below)
- Implement specific speed control if required to maintain spacing
- Allow sufficient time and distance to achieve speed adjustments (especially at higher altitudes, greater speed differentials and when aircraft are in a clean configuration)
- Ensure that aircraft are able to operate in a clean configuration for as long as possible (see 6.14.8 below)
6.14.7 #
It is important to note that if you have only given a speed assignment to a single aircraft (when multiple aircraft are involved), it is unlikely this will provide the necessary separation, for example:
- Aircraft #1 can’t be reduced unless aircraft #2 has been given a limit that won’t overtake aircraft #1
- Aircraft #2 can’t reduce to follow aircraft #1, if aircraft #1 has not been issued with any speed control
In addition, “Adjust Speed to Follow Aircraft Ahead” can be useful but does not replace the necessity for specific speed commands, therefore Controllers should{.red} use it sparingly. As well as this, if used without assigning a specific speed command to the aircraft ahead, it is likely that the lead aircraft will start to dictate the entire flow into the airport, Controllers must{.red} avoid this.
6.14.8 #
When aircraft slow down, they may need to configure; Controllers should{.red} avoid slowing aircraft down where they will have to configure until within 20 track miles to touchdown - expeditious approaches are only possible if aircraft keep their speed up until close to the airport.
6.14.9 #
Once aircraft are cleared for the approach, speed is at their discretion unless they are subsequently assigned a speed command. In this instance Controllers must{.red} use the “maintain XXXkts until X mile final” option (see 6.13.4 above).