Plane speed question?

Question

Ok 2 questions here: (based on flight simulator observations lol)

How come the higher you go the lower your maximum speed is?

and

When i look up cruising speed for an aircraft for example the 737-400 is 439 kts and cruising height is like 35 000 feet and when i try this in flight simulator it says over speed then the aircraft crashes from stress?

Thanks

Answer 1

1. the higher you go up the lower your speed because the pressure is lower. Now when we talk about speed we need to make sure we know what speed we're talking about be it IAS, Mach or TAS. in this case, it's your IAS that you're talking about.

2. the 439kts is the TAS. you are confusing it with your IAS and therefore you will exceed your max speed and crash. The aircraft climbs around 310kts until it intercepts the cruising mach number of 0.86 and then continues to climb and eventually cruise at 0.86, this transition happens around 33,000ft. just look at your PFD, the speed tape tells your IAS and under it is the mach number. on your fmc climb page make sure you set (or normally defaulted) the transition speed which looks like "310/.86". once you are happily cruising at 0.86 you can look at your ND on the top left and it will show a TAS of around 490kts (which is a more accurate speed compared to your 439kts). good luck.

Answer 2

You are crashing the aircraft because you are exceeding  'Mach Crit' factor. Just for example, if 439 knots at 35,000 feet is giving 0.8 Mach, then this same indicated air speed at a higher altitude will give you a higher Mach number because of the less denser air. So, in a nutshell, you are exceeding the safe structural limits of the aircraft not by virtue of maintaining the same IAS, but because you are exceeding the safe Mach number for the aircraft.

Answer 3

Do any of these amazing flight simulator programs have a Cessna 150 in them? You people keep asking about concepts involving sophistcated swept wing large jet transports before you even understand aerodynamics and the atmosphere that an airplane works in.

Kind of like dropping in the last week of med school and saying, "OK, I'm ready to be a doctor now." You may have skipped a few of the basics, ya think?

Answer 4

Theres a difference in Indicated Airspeed (IAS) and actual airspeed as noted above.  Think of it this way, a car on a highway goes 70 mph right?  Put the car on ice and make the speedometer say 70 mph but really, you should only be spinning out techincally so your actual speed is somewhat lower than your indicated speed at which the car's tires are moving.  This works the same for the plane the higher up the slower they have to go simply because there is less air to move or thrust backwards (you would be spinning out or overheating your engines trying to go the same speed at 100 feet compared to 40,000 feet) to make you move forwards, this is why helicopters have a limited ceiling.

Answer 5

Both of your questions have the same answer -- you are confusing true airspeed with indicated airspeed. Indicated airspeed is the airspeed your airspeed indicator will show. Actual airspeed is the actual speed difference between the air and the plane.

An airspeed indicator works on pressure difference. A static port measures the ambient pressure and a pitot tube measures ram pressure. The faster you go, the more pressure in the pitot tube.

But as you get higher up, the pressure goes down. So the same actual airspeed will result in a much lower indicated airspeed.