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I was on a Dash-8 back in July and August. Looking at Flightradar 24, the speed is non constant and very choppy. It even goes down momentarily during the takeoff process. I believe that the Dash-8 does not have auto-throttle if that makes a difference.

Is this normal for this aircraft, or do I have bad data?

QF2142

QF2143

The KGC end of each trip is unavailable probably due to a lack of infrastructure at a very remote airport.

Bianfable
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phkc070408
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    You were on the airplane... did the acceleration *feel* like they were slamming on the brakes at times (which is what it would take to reduce speed gain to zero or negative) followed by strong bursts of speed? Or did it feel like a constant & relatively even acceleration the whole time? – Ralph J Oct 15 '22 at 12:14
  • I'd also add that winds aloft can change as the airplane climbs, resulting in varying ground speed for a constant airspeed. Similarly change in heading can also drastically change ground speed (e.g. taking off south into a 20 knot headwind which turns into 20 knot tailwind when turning on course to the north would cause a 40 knot change in ground speed). – Jeff B Oct 21 '22 at 16:45

1 Answers1

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The aircraft ground speed will continuously increase during the takeoff roll and then remain constant or only change slightly during the initial climb. The speed should never decrease during these stages of flight. Airspeed can vary a bit during the takeoff, e.g. caused by a sudden gust of wind. This might result in "choppy" airspeed data, but due to the inertia of the aircraft not really ground speed. FR24 always shows ground speed and may sometimes also include airspeeds (see Understanding Extended Mode S Data in Flightradar24 for more details).

The ground speed and even the position shown by FR24 can have inaccuracies due to the way it is determined. We already have a few questions here that cover the details, e.g.:

Update after seeing the FR24 screenshot:

The groundspeed during the takeoff roll is actually quite smooth and increases continously:

FR24 screenshot

The slight decrease in ground speed during the climb could be the result of a change in wind speed or direction, possibly caused by turning into a different direction after takeoff.

Regarding the Dash-8, you said

I believe that the Dash-8 does not have auto-throttle if that makes a difference.

That is true, the Dash-8 does not have autothrottle. But at least the Q400 variant has FADEC (full authority digital engine control). For takeoff, the pilots will smoothly advance the power levers into the rating detent. The FADEC then controls the engines to produce the rated power, typically NTOP (normal takeoff power):

The power lever system is used to initiate power demand through the Full Authority Digital Elec- tronic Control (FADEC) to meter fuel to the engine in the forward and reverse ranges. [...]

Power lever movement between FLIGHT IDLE and RATING results in the FADEC modulating the power proportionally between flight idle power and the selected rated power.

(Dash-8 Q400 FCOM - Powerplant)

The Q400 can also reduce the takeoff power when full power is not needed:

Prior to take-off, the power may be reduced for take-off in the NTOP rating using the power der- ate function. To decrement (or derate) the requested power, the pilot presses the DEC discrete with the condition lever at the 1020 RPM position (NTOP rating) and the power lever below the rated power detent. Selection of the DEC discrete, which is a momentary switch, decreases the NTO requested power in steps of 2% to a limit of 10%. The selection of the Power Derate RESET discrete at any time resets the derate to 0%.

(Dash-8 Q400 FCOM - Powerplant)

This system ensures a smooth and constant power for takeoff. It is actually quite similar to how it works in modern Airbus aircraft, which have autothrust. The thrust levers are placed into the TOGA or FLEX/MCT detents for takeoff and the FADEC does the rest. Autothrust is not used for takeoff.

Bianfable
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  • +1 but "Airspeed can vary a bit during...", I beg to differ on that. Airspeed can vary alot during takeoff or approach/landing if it is gusty. This will of course be compensated for to keep flying safely, for example by adding half the gust speed to target speed. – Jpe61 Oct 15 '22 at 15:15
  • @Jpe61 A gust of 20 kt will change airspeed by ~20 kt. Compared to typical takeoff speeds ~120 kt for the Dash, it's not that much. And I wouldn't take off in 120 kt gusts :D Unfortunately, the OP didn't mention how "choppy" the speed data is in the FR24 recording. – Bianfable Oct 15 '22 at 15:31
  • I consider a sudden change of ~15% a lot. We can agree to disagree on this. – Jpe61 Oct 15 '22 at 19:01
  • If the reported speed is GPS speed then I know experimentally that this is speed in the horizontal plane (for the GPS types I’ve checked), so if the climb is steep then you might see a reduction in groundspeed with no reduction in airspeed. In the extreme case if the aircraft ascends vertically then the groundspeed will be zero. – Frog Oct 15 '22 at 19:54
  • @U_flow Takeoff roll for the Dash should be around 25-30s and typical takeoff speeds are around 120kt (~140mph). Seems perfectly normal to me. – Bianfable Oct 17 '22 at 10:40
  • @Bianfable yes looking at it again, that is probably correct. First the taxi, then the acceleration to take-off speed. I will delete my other comment. – U_flow Oct 17 '22 at 11:51