For Pilots We have all received it. The command issued to us on approach of Maintain XXX Speed and Do not Exceed XXX Speed/Maintain speed above XXX kts, Maintain best forward speed/Maintain Slowest speed possible. But what is the difference and what does that mean for us pilots? This post will look at what a pilot needs to be aware of when issued on of these commands
First lets look at Maintain XXX Speed. This command is issued when ATC wants you to maintain a certain speed. The reason can be to fit you into an already existing approach/departure line, maintain a fixed separation distance between aircraft, enable the pilot the speed needed to maneuver approach vectors, and/or to prevent collisions between aircraft on different vectors. When it comes down to it this is the easiest command to understand because ATC is giving you a set speed to be flying at. In this case it becomes more of ATC’s responsibility to maintain separation and give future speed commands.
Now lets look at the Do Not Exceed XXX and Maintain speed above XXX kts commands. These commands are a lot less common on Live and either has a floor speed or ceiling speed. When issues this command, ATC is giving you a hard cap to how fast or slow you can fly. However, just because you are given a cap on how fast you can fly doesnt mean that you should be flying at this cap. This is a mistake I have commonly seen people make on Live and this creates problems. When given this command ATC is creating a fixed distance between you and the aircraft ahead or behind you, but the responsibility to maintain proper separation becomes more of your responsibility rather than ATC’s. These commands I have seen used more often in heavily congested approaches. ATC most likely is issuing this command because they are focused on giving vectors/commands to a plethora of aircraft under their control and do not have the ability to focus on the separation between every aircraft. That is why it is your responsibility to raise or lower your speed within the limits they have given you.
An Example: ATC issues you a command on approach to not exceed 230kts. At the same time ATC has not issued any speed command to the aircraft ahead of you. The aircraft ahead of you begins to slow to a speed far lower than the do not exceed command capped you at. What are you suppose to do? Maintain your separation and slow to the a speed that keeps separation between the aircraft ahead but leaving you wiggle room to shorten or lengthen the separation within safe distance. This applies in reverse to the Maintain speed above XXX command. This is less commonly used but can be used to maintain separation between an aircraft not on the same approach frequency but is within the same area of control.
Finally, let’s look at Maintain best forward speed/Maintain slowest practical speed commands. These commands are issued by ATC when you are operating in perimeters where speed may result in stalls or collisions. Usually the tower will issue you this command when on final approach, but it is not unheard of to receive this from Approach/Departure. When this command is issued speed and spacing becomes almost entirely the pilots responsibility. The reason this is issued is because ATC is not aware of what your flap speeds are for your final approach and as thus that discretion becomes the pilots, yet the pilot must make accommodations to the other aircraft and as such cannot fly atthe rate within the flight envelope that he/she is most comfortable at.
When the command is issue Maintain Slowest Practical Speed ATC is telling you to fly as close to stall speed as you safely can on the fullest of flaps that you possibly can use within your flight envelope. This doesnt mean use full 40 degree flaps and near stall with the 737-700 when on final for a 10,000ft+ runway. What it does mean is using the accepted flaps measure for the situation and maintaining the slowest speed you can safely maintain at that setting without stalling the aircraft.
When issued the Maintain best forward speed, the pilot must maintain the highest allowed speed for full flaps in the situation. This will usually mean you must maintain the speed at or just below the speed where you are allowed to use flaps at full. For example (hypothetical), if the flaps speed ceiling for 30 degrees on a 777-300ER is 140KIAS then with the maintain best forward speed command you should be flying at around 139KIAS-140KIAS with the flaps at 30 degrees.
Important These commands do not mean that you immediately go to full landing configuration. What it does mean is that for your selected flaps you are either at the high limit of your speed for your configuration or at the low limit of your configuration.This is while making the necessary adjustments for your final approach. So just because you are on flaps 5 maintaining best forward speed does not mean that you are not allowed to still slow down to get set up into full landing configuration (flaps set for landing, gear extended, spoilers armed), what it means is that you are to do your best to accommodate for the aircraft ahead or behind you.
I know this is a lot to wrap your head around and at times it can be confusing. The most important thing is to be aware of your surroundings and paying attention to what ATC is trying to do with the airspace they are responsible for.