Hi.My plane (Boeing 777 200ER) flies at an altitude of 4280 feet. The speed of the plane is 230 knots. Isn’t the plane slow?
No, its correct.
The 230 knots that you’re seeing is the “Indicated Air Speed (IAS)” which comes from the amount of air that goes through the sensors. Note that when you’re at a high altitude, the air is thinner, so while you are flying very fast, the thin air translates to not as much air that goes through the sensor, so that’s why it’s showing 230 knots.
If you want to know the actual speed, you need to look at the “True Air Speed (TAS)”. If you want to know the actual speed in relative to the ground, you need to look at the “Ground Speed (GS)”
1 Like
@Loorman
That’s very slow, but why are you flying so low?
@Neo4316 At this altitude 230kias equal 244ktas. Not a very big difference.
Except for a few Heavies.
They have to fly faster than 250kts.
1 Like
Not in all airspace classes…
I read on the Net that it can fly 480 Knots or so…
Ground speed.
Quick airspeed lesson.
There are 5 different ways to measure your speed.
Indicated Airspeed - IAS
True Airspeed - TAS
Calibrated Airspeed - CAS
Ground Speed - GS
Mach - % of the speed of sound
Obviously the Airspeed Indicator indicates IAS. The IAS is measured using two pressure sensors in the aircraft. They measure the ambient pressure outside the aircraft, static air pressure, measured by the static port, through a small hole, usually in the side of the airframe; and the pressure created by moving thru the air, ram air pressure, measured using a pitot tube. The pitot tube is like a straw pointed into the oncoming air.
As the aircraft accelerates, the ram air increases the pressure in the pitot tube. The difference between that pressure and the static port moves the needle on the airspeed indicator. As the altitude increases, the pressure outside decreases as the density of the air becomes less. Because the ram air pressure is a measure of the number of air molecules being crammed into the tube, that pressure falls as the number of air molecules captured decreases.
TAS (true airspeed) is IAS adjusted for the change in air density. If an aircraft was travelling at a fixed speed as it climbs, the IAS will fall. In a no wind condition the TAS will not be dissimilar to the ground speed. GS is just as it sounds, the speed over ground, just like the speedometer on your car.
CAS is IAS adjusted for errors created by changes in airflow and mechanical effects. Generally CAS will be very close to IAS.
Your question is bit difficult in that 4280 feet would be a very unusual altitude for a 777 to be flying unless landing or taking off. If that is the altitude you meant then, yes, 230 knots would be slow for a cruising speed. I would expect to see that if the aircraft was either on descent for a landing, (although by 4000 feet I expect you would have slowed well below 230) or if climbing. As mentioned above, in most cases there is a speed limit below 10000’ of 250 knots so not uncommon to climb the first stage at a reduced speed.
If you meant you were at 43000 feet then 230 knots indicated is just fine for a cruise speed. In Standard conditions the 230 knots indicated would be 484 knots TAS.
Pretty close.
5 Likes
I thought you said “quick.”
1 Like
Tough to explain this stuff in layman’s terms. Still shorter than the textbook lesson.
Ground speed is never used to measure performance since it depends on wind.
Performance is always given in true Airspeed.
1 Like