Does air temp. make a difference in flight speed.

[tipsyMJT]

I was wondering, do pilots have to compensate for the less dense air that would happen with higher air temperatures. Do they have to fly faster? And if not, is there even a miniscule change. Does air temperature have any effect on lift?

[PakledHostage]

Yes, temperature affects the air density. "Density altitude" is the measure of equivalent air density, relative to standard. Higher temperatures result in higher density altitude (i.e. lower air density). Takeoff, landing and climb performance are a function of density altitude because lift is directly proportional to air density. For example, you need a longer runway to takeoff at high density altitudes than you do at sea level. Compare runway lengths in high elevation cities like Denver, CO or Calgary, AB to those in JFK or LAX. They're about 4000 feet longer in Calgary and Denver.

Temperature also affects the speed of sound. Airliners typically cruise at a mach number rather than an indicated airspeed. Higher temperatures result in higher speed of sound, although the effect isn't that significant at typical cruising altitudes. The speed of sound at 15°C is 340 m/s, while the speed of sound at -57°C is 295 m/s.

Edited July 3, 2013 by PakledHostage
Added sentence about runway lengths.

[K^2]

It quite measurably affects performance, as PakledHostage described. Everything from lift, to engine efficiency, to thrust from turbines or props will depend on outside temperature, among other factors. Even the way your air speed indicator works will depend on temperature, because it uses ram pressure, and that will depend on density. But variations in temperature do not really affect the way you fly all that much. If you are supposed to climb to a specific altitude, you always go by pressure altitude, which is what your instruments measure. If you need to go at certain air speed, you use Indicated Air Speed. Amount of lift and IAS are proportional to air density in the same way, so you don't have to worry about stalling at a lower IAS on a warm day.

Pretty much the only effect of density you are likely to really notice is the landing and takeoff roll distances. These are affected most dramatically, because they depend on the true ground speed. But wind and weather are much bigger factors in that.

[PakledHostage]

Pretty much the only effect of density you are likely to really notice is the landing and takeoff roll distances. These are affected most dramatically, because they depend on the true ground speed. But wind and weather are much bigger factors in that.

You are right that density affects engine performance as well as lift and drag, but density altitude is a very significant concern for pilots and aerospace engineers because it also affects climb performance. In extreme cases, an aircraft might be able to get off the ground but be unable to climb out of ground effect. Many, many accidents have happend this way.

In jet aircraft, air density also affects the balanced field length because of the detrimental effects of high density altitude on performance. Jets are required to be able to reach V1 (loosely defined as the speed at which they can safely continue a takeoff after an engine failure) and then stop again without going off the end of the runway. Wind affects balanced field length, but so do factors such as braking action (i.e. how slippery the runway is). In extreme cases, flights may need to depart with less fuel than needed for the whole trip and make an enroute fuel stop.

Edited July 3, 2013 by PakledHostage

[WestAir]

Jets are required to be able to reach V1 (loosely defined as the speed at which they can safely continue a takeoff after an engine failure) and then stop again without going off the end of the runway.

As a complete aside, the airspeed at which you can "safely continue a takeoff after an engine failure" is V2, not V1. V2 is usually refereed to as the Take-off safety speed in SOP's.

EDIT: As far as V1 is concerned, I've always been told it just means "Take your hand off the thrust levers and don't touch those brakes" once you reach it.

Edited July 4, 2013 by WestAir

[K^2]

No, PakledHostage is absolutely right. V₁ is how fast you must be going to continue takeoff. V₂ is the speed you must reach before you actually takeoff after critical engine failure. It comes down to governing ability of your rudder. V₁ assumes that gear traction is going to help counter engine torque, so it can be lower. V₂ means that torque is countered entirely by the rudder, and so it has to be a higher air speed.

And yeah, I was thinking mostly of light aircraft. Climb performance is still a big factor there, though, so shame on me for not considering it.

[WestAir]

No, PakledHostage is absolutely right. V₁ is how fast you must be going to continue takeoff.

If that's what PakledHostage was saying, then I misread his comment. I swore he wrote "loosely defined as the speed at which they can safely takeoff after an engine failure." My bad for misreading PakledHostage' post. It happens to me a lot.

V₂ is the speed you must reach before you actually takeoff after critical engine failure.

EDIT: As an additional aside, and at the risk of misreading someone again, that would be V_r where you lift off, irregardless of the critical engine failure. You'll just want to reach V₂ within a set amount of feet after wheels up. Sometimes it's 15 feet, usually 35, I've also read 50 on a pilot forum I visit.

Edited July 4, 2013 by WestAir

[PakledHostage]

As a complete aside, the airspeed at which you can "safely continue a takeoff after an engine failure" is V2, not V1. V2 is usually refereed to as the Take-off safety speed in SOP's.

I gather from some of your other posts that you're an ATP, but I think you're misunderstanding what I mean by "airspeed at which you can safely continue a takeoff". To be clear, I don't mean that you can lift off at that speed. You are correct that V2 is the takeoff safety speed.

V1, however, is the dividing line between being able to stop within the remaining runway distance and being able to safely reach V2 within the remaining runway distance after having experienced an engine failure. "Take your hand off the thrust levers and don't touch those brakes" would, presumably, be good advice once you reach that speed. My use of V1 is consistent with the FAA's use of the term in their definition of takeoff field length for transport category aircraft:

Note: V_EF and V_EVENT in the figure above are the velocity at which the aircraft is moving when the occurrence happens that leads to the go/no go decision.

You might be interested to read about a related incident that occurred about 18 years ago at YVR. In that occurence, a fully loaded DC10-30 went off the end of the runway after experiencing an engine failure two seconds after reaching V1. The Transportation Safety Board of Canada AVIATION OCCURRENCE REPORT NUMBER A95H0015 contains the details of what transpired next.

EDIT: Sorry, didn't refresh before posting this... I got distracted while looking for the references. Apparently it took longer than I thought to find them...

Edited July 4, 2013 by PakledHostage

[WestAir]

PakledHostage,

I met a lady who was friends with a guy that died in a similar crash where the pilots continued AFTER V1, rather than rejected. From what I've gathered, the engine separated from the aircraft on this other crash after V1, the pilot correctly continued the take-off at Vr and accelerated to V2 speeds then climbed at V2. Unfortunately the engine tore with it a hydraulic line that caused the SLATS to retract on the same (left) wing. It caused the stall speed of that wing to increase to a speed above V2. The left wing stalled, flipping the aircraft up-side-down and causing the crash. I also heard the stall horn or stick shaker was INOP for whatever reason and did not alert the pilots to a stall event.

EDIT: Here's a question that brings us slightly back on topic: Does the temperature of the air affect the stall speed of an aircraft?

Edited July 4, 2013 by WestAir

[OtherDalfite]

Here's a question that brings us slightly back on topic: Does the temperature of the air affect the stall speed of an aircraft?

I would say so, because warmer air = thinner air = Less air to cushion your fall.

[PakledHostage]

PakledHostage,

I met a lady who was friends with a guy that died in that crash.

Nobody died in the "crash" that I am referring to. 6 people were slightly injured escaping the aircraft because they fell into soft mud at the bottom of the escape slides. The aircraft remained in service with CP for many years afterwards until it was eventually returned to the leasing company and subsequently leased to Biman Bangladesh. I think it may have since been converted to cargo, but don't quote me on that.

[jwenting]

PakledHostage,

I met a lady who was friends with a guy that died in a similar crash where the pilots continued AFTER V1, rather than rejected. From what I've gathered, the engine separated from the aircraft on this other crash after V1, the pilot correctly continued the take-off at Vr and accelerated to V2 speeds then climbed at V2. Unfortunately the engine tore with it a hydraulic line that caused the SLATS to retract on the same (left) wing. It caused the stall speed of that wing to increase to a speed above V2. The left wing stalled, flipping the aircraft up-side-down and causing the crash. I also heard the stall horn or stick shaker was INOP for whatever reason and did not alert the pilots to a stall event.

EDIT: Here's a question that brings us slightly back on topic: Does the temperature of the air affect the stall speed of an aircraft?

That crash then was caused by the hydraulics failure, not the engine failure.

Small consolation for those who suffered I know, but the crew were correct in continuing the takeoff run. V1 is effectively the point of no return.

If an engine separates after V1 is reached, you go on and hope for the best. If one separates before V1, you abort.

[WestAir]

Nobody died in the "crash" that I am referring to. 6 people were slightly injured escaping the aircraft because they fell into soft mud at the bottom of the escape slides. The aircraft remained in service with CP for many years afterwards until it was eventually returned to the leasing company and subsequently leased to Biman Bangladesh. I think it may have since been converted to cargo, but don't quote me on that.

Once I saw "DC-10" and "Failure after V1" I thought of American Flight 191. Had to edit my post once I actually took a moment to read the link you provided. After this discussion I'm going to remove good reading comprehension and communication skills from my resume.

[PakledHostage]

I would say so, because warmer air = thinner air = Less air to cushion your fall.

It would depend on whether you're talking about indicated airspeed (IAS) or true airspeed. As K^2 mentioned, airspeed indicators measure dynamic pressure. Dynamic pressure is a function of true airspeed and air density. Wings produce lift as a function of dynamic pressure too. In other words, a wing will behave the same at a given IAS, but not necessarily so at a given true airspeed.

This is the source of the infamous "coffin corner" of the U2 spy plane. At high altitude, air density is low so dynamic pressures and IAS are low for a given true airspeed. The aircraft has to fly faster and faster as it climbs to maintain an IAS above stall. The U2 is a subsonic airplane however, and the speed of sound is basically a function of air temperature not density. Eventually the aircraft reaches an altitude where it must fly at high subsonic Mach numbers to avoid stalling. But subsonic aircraft are generally not designed to approach the speed of sound. They have an MMO above which they may encounter all sorts of nasty effects (i.e. flutter, etc)

Edited July 4, 2013 by PakledHostage
removed some non pertinent information

[PakledHostage]

@Westair: I just realised that I misread your post. (Edit: OK, maybe I didn't misread it. You snuck in and changed it...)

You are talking about the Chicago crash. That was the result of a structural failure of the pylon, not an engine failure. AA had done an inspection on the pylon using an unapproved method. They unknowingly caused the rear bulkhead of the pylon to crack. It failed shortly thereafter and took out the slat mechanism when it and the attached engine departed the aircraft.

Edited July 4, 2013 by PakledHostage