The aerodynamic effect of heavy rain on airplane performance
 

The following extracts are from US Flying feature writer Thomas H. Block published in 1981 and are well worth studying. He states "Water, indeed can cause problems beyond hydro-planing and poor visibiility.

With regard to the aerodynamic effect of heavy rain on airplane performance. The National Aeronautics and Space Administration has been conducting a series of tests to determine the effect of heavy rain on airfoil aerodynamics. The results of these tests have shown that heavy rain can significantly increase drag as well as decrease lift and stall angle of attack. This paper describes a recent effort to use the heavy rain airfoil data to determine the aerodynamic effect on a conventional twin-jet transport. The paper reports on the method used to model the heavy rain aerodynamic effect and the resulting performance degradation.

The heavy rain performance effect is presented in terms of the diminished climb performance associated with increasing rain rates. The effect of heavy rain on the airplane's ability to escape a performance-limiting wind shear is illustrated through a numerical simulation of a wet microburst encounter. The results of this paper accentuate the need for further testing to determine scaling relationships and flow mechanics, and the full configuration three-dimensional effects of heavy rain.

Short extracts from the article follow: Intense rain beating continously against the wing seems to have the same effect on the airfoil as a layer of frost or ice. At higher angles of attack - the sort of profiles we fly during the slow airspeed maneuvers of takeoff, landing or missed approach, it is estimated that all that water can distort the airfoil shape and reduce lift by as much as 30 percent. That's enough to affect the performance of any aircraft.from light aircraft through to heavy jets. After reviewing the official reports on several airline accidents that occurred in heavy rain, the NASA-funded study by the University of Dayton Research Institute points out that "the effect of rain was not taken into account."

We believe that this heavy rain factor - totally neglected in accident investigation - produced significant aerodynamic penalties and resulted in a serious overestimate of wind shear as a cause of thunderstorm - related accidents. Penalties associated with heavy rain can be of the same order as those associated with windshear."

If further testing proves the theory correct, we need to get the word out that heavy rain on takeoff or landing requires additional precautions. By keeping bank angles shallower, pitch attitudes flatter and airspeed margins wider, we could easily avoid the risks. If we stay closer to the centre of the aircraft's performance box and don't depend on book-value performance for runway, climb gradient and airspeed requirments, we will finish our day's flying smiling.

Some pilots still don't believe that a thin layer of ice or frost can roughen the airfoil shape enough to be disastrous, and those are the people who litter our wintertime accident statistics. Now we are told by this new line of NASA-inspired research, too much water at the wrong time can be nearly as bad. Perhaps the time has come for us to add one more thing - -heavy rain - to that list of items we prefer to keep off the wings.

I have seen no literature on the effect of heavy rain on wing aerofoils and certainly never in the numerous flight simulator sessions I have conducted or observed. I think it is vital knowledge for pilots to be aware of.

I think that many pilots naturally do this. Fly conservatively, near the center of the box, keep margins in tact, divert when it feels right, not when the last letter of the law comes into effect. I think that this attitude is what keeps us mainly safe.
The latest Boeing FCTM’s ive read do make mention of avoiding heavy rain and have supplementary procedures available for situations where it is unavoidable.
​​​​​​​Thanks for the informative and thought provoking post.

More up to date research. There's quite a bit going on the the field.

https://www.noveltyjournals.com/upload/paper/Analysis%20of%20Airfoil-323.pdf

https://www.icas.org/icas_archive/IC...PAPERS/292.PDF

https://www.academia.edu/19821010/Two_dimensional_NACA_23012_airfoil_performance_degradation_b y_super_cooled_cloud_drizzle_and_rain_drop_icing

The FCTM I have is well out of date and doesn't specifically mention aerodynamic interference caused by heavy rain. It only mentions engine handling in heavy rain. For take off in heavy rain it must be difficult for the pilot to judge whether or not to delay take off as it is very much a subjective assessment especially if other aircraft are taking off under the same weather conditions with no apparent problem.

Times change
 

Many years ago, post accident, BAe looked at the problem of fuel flowing over the wing from a 'lost' over-wing refuelling cap - HS125. I cannot recall all of the conclusions, but alerts were published re the effect on lateral control.

Similarly there was considerable flight test investigation associated with type IV deicing fluids flowing off the wing inflight - BAe, Canadair, Saab, ATR. (Airbus maintained a watching brief).
Overall, performance was degraded. Some aircraft had performance adjustments after deicing. (ATR changed trim / handling techniques ?)

BAe re-evaluated all Jetstream variants, HS 748, ATP, and 146/RJ aircraft, but as I recall the effects were within the allowed-for certification margins for performance, even with engine failure.
Thus fly the book, don't outthink the situation.
Concerns with deicing performance were somewhat alleviated by other type IV fluid activities; wash-off, application, to address residue dry-out and control freezing.

There was a very old Boeing report re legacy 737 aircraft. Their flight tests indicated that climb performance loss with deicing fluid was up to 6%, but I don't recall any adjustments being required.
Distant memories also recall discussion on heavy rain (Boeing) - but conclusions evade me. However, noting that other manufacturers didn't get too excited by this perhaps it was not a significant problem - margins as above.

But times change; new supercritical wing profiles, sensitive to the merest bug contaminant, similar with engine blades.
Climate change, more intense rain; recall the 737 forced landing when rain put the 'engine-fires' out.

Which one was that Safeypee?

Meteorological Question? Can heavy rain occur without the presence of big red blogs on the radar?

Memory - web ref fades; 737 off-airport landing in the USA with double engine failure. Landed in a wet field near a levee. (Additionally, flown out after repair - long time ago)

jimtx "rain without red"; early generations of wxr would 'black-out' at extreme rainfall rates. The implication was that there was a surrounding red zone.
Similarly, inability of wxr to see behind large storms, or mis-set gain, scan angle, low alt - over whelming ground returns.
Also note difference between aircraft wxr and some met satellite displays where red ≈ cold cloud tops / altitude.

Bingo. A very memorable off-airfield dead-stick landing.

https://en.wikipedia.org/wiki/TACA_Flight_110 (May 24, 1988)

I have had ATC ask me if I wanted a vector for weather avoidance, the path looked clear on radar. ……the path was not clear so I am not 100% convinced the radar is always right!

Also a dead stick ditching by a Garuda 737 into a river following a dual engine flame out in a thunderstorm in January 2002. See: Accident Boeing 737-3Q8 PK-GWA, Wednesday 16 January 2002

In the older thread ( https://www.pprune.org/pacific-gener...nderstorm.html ), you wrote that damage to the radome (and thus, unreliable/malfunctioning weather radar) was a contributing cause.

That's also relevant here, because you can only avoid heavy rain if you know where it is. And when that "gap between storm cells" is really water in your radome blocking the returns, well...

I'm not here to discuss the importance of the "investigation" of the aerodynamic effect of heavy rain. The question should be asked. However, if the question relates to operating the aircraft I do feel the same old question popping up: "how much safety do you need?"

There were multiple instances of these with engine flameouts in heavy rain. A story many times told to explain why the classic engines have a different cone (NG has pure pointy conical shape, on the classic has a rounded off conical shape). This is pure related to fan engine design, has nothing to do with aerodynamics. The problem was solved after the modifications (there were other problems by the way, including many false fire warnings up to the dangerous point pilots started to neglect them).

When it comes to aerodynamics, one thing you really have to keep in mind is that our "operational" envelope is already developed with safety margins. There is a remark about bank angles, well, we do have 25°+ 15° overshoot. Not sure if anyone ever tried 40° bank in heavy rain to be honest, the wing loading you would feel instantly and you would feel instantly uncomfortable. If you look at terminal maneuvering, just look at the difference between maneuvering speeds schedule during deceleration and acceleration and your actual minimum maneuvering speed. The difference is huge. You are more than protected aerodynamically.

Non-normal situations you might want to take into consideration. Just to give a realistic example from the top of my head: with flap problems on a 737 you might end up with 15°+15° limitations in the turn and that might catch people out in high speed final turns. Would you push on in heavy rain and take 30° bank despite what the NNC is saying in heavy rain, or skip it and come back for a second try? As said before, I think we all are adult enough to be risk avert and re-evaluate the situations a little bit (or at least I would hope).

So again "how much safety do you need?"

Forgive a GA perspective please, which is in fact relevant here: This prompted memories of This report into a non-fatal Extra 300 crash which included a section titled, ‘Effect of ‘wet wings’’ (Page 59 in the original bulletin). The reference source was a NASA Langley paper, AIAA-86-2606 ‘Potential Influences of Heavy rain on General Aviation Airplane Performance’ by RE Dunham Jr.

A fascinating investigation for two reasons, the other being the way it emphasised the value in a thorough on-foot exploration of the entire site and its surroundings.

Just a point of interest here (at least to me). After the TACA dual engine flameout event, CFM redesigned the spinner and "splitter" (the lip that separates the airflow into the core from the fan stream) which gave a dramatic improvement in the ability of the engine to tolerate high intake water levels (I don't recall if the Garuda event had the changes implemented).