Are we losing any lift - Frost on bottom of wing
 

There seems to be a lot of aircraft that are approved for flight with a significant part of the bottom of the wing covered in frost. Is there any lift loss for this situation?

That’s not entirely the case.
Aircraft with cold soaked fuel which can very well be -15C or colder after prolonged flight at altitude will get frost in the area of the fuel tanks with any level of humidity.
This may be in an area which is of lesser consequence during take off and climb stages of flight.

I would only worry about frost on the bottom of a wing if I were about to fly an air show that involved sustained* inverted flight. ;) In, say, an Extra with a symmetrical wing.

Frost (or other contamination) on the wing is an issue when it will cause early flow separation and an early stall. The bottoms of wings do not stall, due to the asymmetry of gravity and lift - unless, as I said, one is flying upside down. Thus contamination on the bottom is not much of an issue except as a producer of drag.

* the momentary inverted flight during a barrel-roll or loop does not count, since those are still usually positive-G maneuvers (i.e. lift is still in the direction of the pilot's head).

As an aside, a symmetrical wing isn't required for sustained inverted flight.

Different rules may apply though for the horizontal stab.

Cough
 

Indeed!
You do not get any NEI or frost on the underside of the stab as it has no fuel in it!
Inverted airfoil aside!

Quite a few jets do carry fuel in the horizontal, no? 747-400, some versions of A330, VC-10...

Honorable mention to the EE Lightning, which had fuel tanks in the flaps.

Yes, but the problem comes from cold soaked fuel after the flight- The 747 feeds its Stab fuel into the centre tanks so is highly unlikely to have any fuel in it by top of climb, let alone post flight.

Underwing CSFF can be indicative of the upper, more critical surface.

RVF750

Correct, 747's equipped with stab fuel automatically use that fuel first - short of a failure which prevents fuel transfer there will never be usable fuel in the stab at cruise or after landing.
I believe the A330 can use stab fuel for cruise trim, so it potentially can have cold soaked fuel in the stab after landing.

Super VC10 had a fin tank containing 1,350 Imp gals. But no tank in the horizontal stabiliser.

BECOME A TEST PILOT.
 

Hoar frost and refrozen melt water running down the dihedral is unique to your airframe that day that hour.

This wing profile has probably neither been windtunnel tested nor flight tested so welcome to the wonderful world of the test pilot!!

The 73NG allows a certain depth of hoar frost (3mm?) but no more, so best sling some warmer fuel in the wing tanks or take the risk, preferably without me aboard?!

Usually AOM or company SOPs will specify and anyway, from basic aerodynamics, there will be some form drag, lower surface flow disruption and extra weight unaccounted for in the loadsheet.

Do these add up to a lift loss?

A330 will always have fuel in the Stab as that is how the APU is fed. If due to a malfunction, manual transfer from the Stab is selected then the tank will drain as will the APU feed line. Maintenance action is then required to bleed the line.
Anyhow I digress.
It is common on the 330 to arrive at the gate with a couple of hundred pounds in the Stab. This will create frost most days on the upper and lower surfaces. However by the time the refuelling truck has arrived the proactive Eng/Tech/Mech will have transferred the fuel out so that warm fuel will defrost it. Same with the outer wing cells.
Re: lower surfaces. Ridging must be removed. Causes drag and disturbed airflow. Does it reduce "lift" on the Stab, yes. On the wings, I wouldn't have thought so.

If no fuel in the stab, the fwd apu fuel pump feeds from the left inner tank!

When the aircraft has been parked in -20 C or less overnight it's quite frosty. Best option to put it in the hangar. If not available, engine covers keep the fan blades clean. Cabin heating is comfortable to have. Some places put a preventive layer of thickened fluid on, but I think the "hold over" is something like 8 hours(?), perhaps reducing with really cold temp. So may be a waste if put on the night before.

I have to admit, I am yet to see anyone do a under wing de-ice despite there being a thin layer of hoar frost. Acceptable on the fuselage, according to the books, but as I read the FCOM frost on the underside of the wing is really only approved if caused by cold fuel. Anyone can fill in with their experience?

Yes deiced underwing, you are allowed 3mm of frost however you are not allowed ridges, sometimes water runs down from further up and refreezes in ridges.
Yes you can anti-ice an overnight a/c but it has to be deiced/cleared before flt, the idea being it uses less fluid, we had to do it last year as there was a fluid shortage and unless you anti-iced you would not be deiced the next day.

spanner,

Did you use the normal de-icing rig to spray it? I can see how you could get in under a 747 wing, possibly, but what do you do with the underside of a frosty 737 wing? I am thinking an overnight parked aircraft in low temp, not cold soaked fuel frost.

I have seen underwing de-icing as a pax on an A320 using what appeared to be the normal de-ice vehicle.

Thanks for the answers. I was intentionally a little vague on my question.

Without admitting violation, a person that I know very well went flying in a C172 the other day in a winter location. There had been a lot of frost accumulation but fortunately for the pilot, there were wing covers on the aircraft. However, they did not prevent most of the bottom of the wing from being covered in thin but clearly visible frost. As well, there was not cover on the tail which had frost on both sides of the horizontal stab.

The frost was removed from the tail due to concerns about lift but the wing is quite large and it would take a seriously long time to clean the bottom of the wing. The instructor wasn't concerned about the frost on the bottom of the wing and the pilot agreed to go flying but mentioned that he would rotate at a higher than normal speed at a lesser than normal rotation rate and climb out at a higher than normal airspeed.

Keep in mind that the wing on a C-172 is quite high lift as compared to some other aircraft where one might not want to attempt this sort of thing. As well, the takeoff weight was well below maximum and the OAT was well below freezing allowing for better than normal performance, Everything went fine and a check of the wing after a few minutes in cruise showed that all frost was gone.

I was just curious if there is any performance penalty for a situation like this?

Same lift more drag!
 

Jamstab
Thanks for clarifying! We just went from A330 with frost on the Stab.
With a Vr at 150kts plus , to a C172 with that as a Vne.

Same same but different !
Turbulent air under a C172 wing due to frost is for sure not going to give you MORE lift.
What it will give you is some more drag! How much ? Quite a lot in some cases.

In this case I have to say, Laziness and madness!