Landing in hot weather
Discussion
Walking across the tarmac at the supermarket carpark today, I could really feel a lot of heat rising from it, my dad commented that it would be a good day for gliding.
It made me wonder whether aircraft much bigger than gliders have to take thermals into account when landing.
For example, does the pilot of a heavy jet have to use a defined lower landing speed if touching down on a particularly dark runway in a very hot country?
It made me wonder whether aircraft much bigger than gliders have to take thermals into account when landing.
For example, does the pilot of a heavy jet have to use a defined lower landing speed if touching down on a particularly dark runway in a very hot country?
goldblum said:
SlipStream 77
"For example, does the pilot of a heavy jet have to use a defined lower landing speed if touching down on a particularly dark runway in a very hot country?"
Would a heavy jet use a lower landing speed?
I don't know, that's why there's a question mark after it. "For example, does the pilot of a heavy jet have to use a defined lower landing speed if touching down on a particularly dark runway in a very hot country?"
Would a heavy jet use a lower landing speed?
Logically, if thermals provide a significant amount of lift I would have thought the pilot would do something to reduce that lift in order to maintain the correct angle of approach.
I'm not a pilot, hence the question, but there are several things that, AFAIK can be done to reduce lift, including retracting flaps, deploying spoilers, changing the AOA and reducing speed.
goldblum said:
SlipStream 77
"For example, does the pilot of a heavy jet have to use a defined lower landing speed if touching down on a particularly dark runway in a very hot country?"
Would a heavy jet use a lower landing speed?
No."For example, does the pilot of a heavy jet have to use a defined lower landing speed if touching down on a particularly dark runway in a very hot country?"
Would a heavy jet use a lower landing speed?
Vat (Velocity at threshold) is based upon the weight of the a/c (I believe in Civil flying this is termed Vref). The heavier you are the higher the Vat because the heavier you are the higher the stalling speed, and Vat is based upon a margin of safety above Vstall (1.3 x Vstall).
Generally speaking an approach will be flown at Vat +10kts to Vat +15kts, with the touchdown being 5 to 10kts below Vat.
If there was sufficient thermal activity at low level to cause significant turbulence on the approach you would be thinking about adding speed to the Vat not reducing it (to give you a margin of safety if shear is encountered).
Because Vat vs AUW is worked out by the manufacturer based upon TAS (True Airspeed) for sea level in a Standard ICAO Atmosphere, but the pilot(s) fly IAS (Indicated Airspeed), where things get trick are hot and high airfields because:
1. You have less engine power available for any possible overshoot (and the wings are producing less lift in the less dense air).
2. The IAS / TAS relationship means that your TAS at touchdown will be far higher than it would at sea level. On a C130 for eg, landing at Nairobi in the summer you cant put the nosewheel down straight away after landing because you would overstress the nosegear and possibly cause the tyres to dismount.
Edited by Ginetta G15 Girl on Thursday 24th June 21:05
Ginetta G15 Girl said:
goldblum said:
SlipStream 77
"For example, does the pilot of a heavy jet have to use a defined lower landing speed if touching down on a particularly dark runway in a very hot country?"
Would a heavy jet use a lower landing speed?
No."For example, does the pilot of a heavy jet have to use a defined lower landing speed if touching down on a particularly dark runway in a very hot country?"
Would a heavy jet use a lower landing speed?
Vat (Velocity at threshold) is based upon the weight of the a/c (I believe in Civil flying this is termed Vref). The heavier you are the higher the Vat because the heavier you are the higher the stalling speed, and Vat is based upon a margin of safety above Vstall (1.3 x Vstall).
Generally speaking an approach will be flown at Vat +10kts to Vat +15kts, with the touchdown being 5 to 10kts below Vat.
If there was sufficient thermal activity at low level to cause significant turbulence on the approach you would be thinking about adding speed to the Vat not reducing it (to give you a margin of safety if shear is encountered).
Because Vat vs AUW is worked out by the manufacturer based upon TAS (True Airspeed) for sea level in a Standard ICAO Atmosphere, but the pilot(s) fly IAS (Indicated Airspeed), where things get trick are hot and high airfields because:
1. You have less engine power available for any possible overshoot.
2. The IAS / TAS relationship means that your TAS at touchdown will be far higher than it would at sea level. On a C130 for eg, landing at Nairobi in the summer you cant put the nosewheel down straight away after landing because you would overstress the nosegear and possibly cause the tyres to dismount.
I was imagining something like a secondary ground effect occurring as soon as the aircraft crosses the runway theshold and is over a surface of significantly high temperature.
Thinking about it, the hot and high scenario you mention must be very hard on brakes, with the high ambient temperature, higher landing speed and reduction in aero drag. Having said that, I expect a Hercules loses speed pretty fast with its nose in the air.
Aren't there two issues here, namely (a) thermals (b) air temperature?
From my position (as armchair pilot), thermals will make for turbulence as I fly through them but air temp and therefore density is another kettle of fish. I recall a programme where they retraced Empire flying boat route through Africa in a Catalina. They landed on a high altitude lake and couldn't take off until they left the camera crew behind. Hence more power and more speed to compensate for the lower air density?
From my position (as armchair pilot), thermals will make for turbulence as I fly through them but air temp and therefore density is another kettle of fish. I recall a programme where they retraced Empire flying boat route through Africa in a Catalina. They landed on a high altitude lake and couldn't take off until they left the camera crew behind. Hence more power and more speed to compensate for the lower air density?
I can't comment about flying jets but I can tell you if its warm and you're trying to land a little'un like a C152, there is noticable sink when you go over areas of trees (which insulate the radiation of heat from the ground).
Per GG15's post - if it's a bit choppy you'd add more throttle not less.
Per GG15's post - if it's a bit choppy you'd add more throttle not less.
Edited by mattdaniels on Thursday 24th June 21:01
Simpo Two said:
Aren't there two issues here, namely (a) thermals (b) air temperature?
From my position (as armchair pilot), thermals will make for turbulence as I fly through them but air temp and therefore density is another kettle of fish. I recall a programme where they retraced Empire flying boat route through Africa in a Catalina. They landed on a high altitude lake and couldn't take off until they left the camera crew behind. Hence more power and more speed to compensate for the lower air density?
You're right of course.From my position (as armchair pilot), thermals will make for turbulence as I fly through them but air temp and therefore density is another kettle of fish. I recall a programme where they retraced Empire flying boat route through Africa in a Catalina. They landed on a high altitude lake and couldn't take off until they left the camera crew behind. Hence more power and more speed to compensate for the lower air density?
The question is does the lift created by thermals equal or exceed the reduction in lift caused by the reduction in air density.
However, I suspect there are many variables on which the answer depends.
ETA. Ginetta Girl has already answered my question with regards to the landing scenario, I think this is a different question now.
Edited by SlipStream77 on Thursday 24th June 21:40
SlipStream77 said:
Thinking about it, the hot and high scenario you mention must be very hard on brakes, with the high ambient temperature, higher landing speed and reduction in aero drag. Having said that, I expect a Hercules loses speed pretty fast with its nose in the air.
Yup very hard on brakes. With a Herc, of course, you have oodles of blade drag when you retard the throttles (because the Angle of Attack of the blades at flight idle vs the actual TAS is too course); additionally you have lots of reverse thrust available.However the problem with a hot and high landing (given the fact that you are landing above nosewheel limiting speed) is that if you horse the power levers straight back into reverse you will cause the a/c to pitch down (because the centre of pressure (lift) moves forward as you do this. This is the last thing you want. Additionally, at such high TAS(s), if you are rough with the power levers you are more likely to have a blade 'hang up' on the Low Pitch Stop, rapidly followed by the a/c departing the runway stage left (or right, dependent upon which blade hangs up). This you do not want!
As an example, I landed a C130 at Khamis Mushait in Saudi (near the Yemeni border and where the F117s were based during Gulf War I) which is, IIRC around 7000' amsl. OAT was around +35C, so our touchdown TAS was around 155kts (with a nosewheel limiting speed of 135kts). Given the high TAS (and associated lower drag from the thinner air) we used up over 1/3 of the 12,0000ft runway before I could put the nosewheels on the ground (and select reverse thrust). An almost unheard of landing roll for Albert!
Simpo Two said:
Aren't there two issues here, namely (a) thermals (b) air temperature?
Yes.Simpo Two said:
From my position (as armchair pilot), thermals will make for turbulence as I fly through them but air temp and therefore density is another kettle of fish. I recall a programme where they retraced Empire flying boat route through Africa in a Catalina. They landed on a high altitude lake and couldn't take off until they left the camera crew behind. Hence more power and more speed to compensate for the lower air density?
Spot on! Thermals may/will cause low level turbulence and/or shear, hence you increase your speed down the slope.High temp and/or altitude means less lift and less engine power/thrust because the air is less dense.
Thus you tend to use higher power settings (if available) or you reduce the weight of the a/c.
WRT to speeds, Vat will not change being, as it is, based upon a/c weight and the margin above Vstall (which is based upon TAS). However V1 (stop go decision speed) and Vr (rotate speed) will alter based upon air temperature and pressure (ie density).
V1 will go down because your TAS vs IAS relationship has increased (and you thus need more braking energy to stop the a/c on the take off roll, as well as the fact that the a/c accelerates on the T/O run more slowly and thus uses more runway to achieve Vr), and Vr will increase (because you are developing less lift, and Vr is based upon 1.1 Vmca [Velocity Minimum Control Airborne, ie engine out, which is based on IAS).
Ginetta G15 Girl said:
SlipStream77 said:
Thinking about it, the hot and high scenario you mention must be very hard on brakes, with the high ambient temperature, higher landing speed and reduction in aero drag. Having said that, I expect a Hercules loses speed pretty fast with its nose in the air.
Yup very hard on brakes. With a Herc, of course, you have oodles of blade drag when you retard the throttles (because the Angle of Attack of the blades at flight idle vs the actual TAS is too course); additionally you have lots of reverse thrust available.However the problem with a hot and high landing (given the fact that you are landing above nosewheel limiting speed) is that if you horse the power levers straight back into reverse you will cause the a/c to pitch down (because the centre of pressure (lift) moves forward as you do this. This is the last thing you want. Additionally, at such high TAS(s), if you are rough with the power levers you are more likely to have a blade 'hang up' on the Low Pitch Stop, rapidly followed by the a/c departing the runway stage left (or right, dependent upon which blade hangs up). This you do not want!
As an example, I landed a C130 at Khamis Mushait in Saudi (near the Yemeni border and where the F117s were based during Gulf War I) which is, IIRC around 7000' amsl. OAT was around +35C, so our touchdown TAS was around 155kts (with a nosewheel limiting speed of 135kts). Given the high TAS (and associated lower drag from the thinner air) we used up over 1/3 of the 12,0000ft runway before I could put the nosewheels on the ground (and select reverse thrust). An almost unheard of landing roll for Albert!
Simpo Two said:
Life gets so dull when everything is converted to acronyms! You'll have to be at Duxford on 11 July for a certain book signing...
Sorry! Too many years in the RAF has reduced me to using TLAs (Three letter Abbreviations), especially when I type. Apparently I upset people on this section of the Forum with my writing style being too brief and direct.Ho hum...
Book signing?
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