How does ATC maintain separation between aircraft flying at different speeds?

Question

How are collisions avoided when 2 aircraft are flying same altitude and same direction but have different speed characteristics.

Example a B737 and an ATR are both flying at 25000 feet. 737 will cruise at 850 Km/h whereas the ATR will cruise at 550 Km/h and the 737 is approaching from behind.

How is this type of situation handled by the pilots and ATC?

My question is specific to airliners as I have seen from websites like flightradar there are thousands of planes flying at any given moment and was wondering how this problem is handled.

Answer 1

FL250 for a Boeing 737 in cruise at 850 kph/460 knots GS is an unlikely value. Aircraft in the same category of speed, such as the Boeing 737/747/767/777 usually fly at higher altitudes, e.g. FL300-FL400. Slower turboprop aircraft such as the ATR72 usually fly in the ranges of FL200-FL250. During cruise, airspeed will be matched by ATC for all aircraft where the trailing aircraft might catch up to the preceding aircraft.

ATC sees the ground speed on their radar screens and can thus compare and evaluate if they need to assign speeds to maintain separation.

^{(Image Source: What data does ATC see on their screens? - Speed: 395 knots GS)}

R: DLH123, report mach number.
A: Mach .78, DLH123
R: BER456, report mach number.
A: Mach .79, BER456
R: BER456, reduce speed to mach .78
A: Reducing mach .78, BER456

The same principle applies to aircraft in the terminal area, the lower you get, the more will aircraft have the same groundspeed when assigning the same indicated airspeed, as air density and wind approach the same values. ATC will sometimes assign a slower speed to trailing aircraft if they need to achieve the same groundspeed at different altitudes.

R: DLH123, descend to 3000ft, reduce speed to 210 knots
A: Descending to 3000ft, speed 210 knots, DLH123
R: BER456, descend to 4000ft, reduce speed to 200 knots
A: Descending to 4000ft, reducing speed to 200 knots, BER456

For aircraft that still have significantly different groundspeeds on the same airway, ATC can also vector aircraft around the preceding aircraft and put it back in front or they can issue a direct that will achieve the same effect, changing the sequence of the aircraft. A climb or descend can be coordinated with the pilots, e.g. the trailing aircraft can be asked if they are able to accept a climb to another flight level, maybe even temporarily.

Answer 2

Such situations usually occur when you intercepted the localizer and is coming in for a landing, where many aircrafts line up in the air one behind the other coming in to land. These situations are maintained by slowing down the aircrafts which are at the back of the first aircraft and so on. Hence you hear "(insert airline name) (insert number), you are number 2 (or number 3) for landing. Follow the company Boeing 777 on short final". And once the aircraft at number one is landed, the second and third and so on lands one by one. Should there be a delay in the aircraft vacating the runway, the landing plane initiates a go around. As far as planes at higher altitudes are concerned, you don't usually have such problems of one plane being exactly behind the other. The planes are slightly up or down or left or right and in this way it's easy for them to fly past one another. Hope this short answer helps.