P c/ 

U.S. Depmnent 

of Tmnsprtation 

FedemlAvOat&n 

Administfution 

Advisory 

Circular 

Subject: 

~~~ 

WATER INGESTION TESTING FOR Date: g/30/85 AC No: 20-124 

TURBINE POWERED AIRPLANES Initiated by: ANM-110 a-: 

1 PURPOSE. This advisory circular (AC) describes a method of demonstrating 

compliance with the requirements of the Federal Aviation Regulations (FAR) 

concerning the ingestion of water from the runway/taxiway surface into the 

airspeed system, the engine, and essential auxiliary power unit (APU)l air 

inlet ducts of turbine engine powered airplanes. Like all advisory circular 

material,it is not mandatory and does not constitute a regulation. It is .for 

guidance purposes and to provide an exam.ple of a method which has been found 

acceptable. 

2 l APPLICABLE FAR SECTIONS. 

a. Sections 23.1091(c)(2) and 25.1091(d)(2) of the FAR. 

b l Sections 23.1325(a), 25.1323(d), and 25.1325(b) of the FAR. 

3 RELATED ADVISORY CIRCULAR. 

dited May 24, 1978. 

AC 91,6A, Water, Slush, and Snow on the Runway, 

4 l BACKGROUND. 

a, The Problem. Airplane turbine engines are susceptible to surge, stall, 

and flameout when they ingest excessive quantities of water, All certificated 

turbine engines have demonstrated a capability of ingesting simulated rainfall 

without suffering operating problems. The quantities of water spray ingested by 

an engine as the airplane passes through water standing on runways and taxiways 

may exceed the water quantities used in the engine certification testing. The 

regulations require that the airplane design prevent hazardous quantities of 

water or slush from being ingested into engines and APUs during takeoff, 

landing, and taxiing. Also, water ingestion into the airspeed system may cause 

system malfunctions during takeoffs and landings. It is often practical to 

investigate these effects concurrently with the engine water ingestion 

evaluation. 

b Critical Case, If a critical water ingestion problem exists, it 

typically exists during takeoff when engine power demands are high. The 

airplane tires cause bow waves, side spray, and "rooster tails" (spray which is 

1 An essential auxiliary power unit produces bleed air and/or power to drive 

accessories necessary for the dispatch of the airplane to maintain safe 

operation. .e 
AC 20-124 g/30/85 

thrown off the tire as it rotates) which can collect into concentrated water 

streams that may be ingested by the engine. If the quantity of water is 

excessive, the operation of the engine may be affected sufficiently to cause an 

unsafe condition. Additionally, certain airplane designs may allow for 

ingestion of hazardous quantities of water during landing rollout while using 

reverse thrust. 

c. Limiting Water Depth. Airplanes should be capable of operating on wet 

runways with areas of standing water without creating adverse engine, APU, and 

airspeed system operating characteristics due to water ingestion. Takeoffs 

should not be attempted when standing water, slush, or wet snow greater than 

one-half inch in depth covers an appreciable part of the runway (reference 

AC 9196A, "Water, Slush, and Snow on the Runway," dated 5/24/78). Therefore, 

one-half inch water depth is the accepted test criteria for demonstrating 

compliance with the referenced regulatory requirements concerning water 

ingestion effects on engines, APUs, and airspeed systems. An applicant seeking 

airworthiness approval may demonstrate the water ingestion characteristics of 

the engine and airspeed system by conducting tests in a one-half inch deep 

water test bed. Upon a successful demonstration, the airplane is approved 

without an operating limitation on runway water depths. If the airplane can 

demonstrate successfully only at some shallower water depth, then the lesser 

depth becomes a limitation on 'that airplane's operation. 

5 . WATER INGESTION TEST PROGRAM. 

a. Determination of Need To Test. The airplane configuration generally 

determines the spray pattern of the water. The applicant may show compliance 

with §I 23.1091 and 23.1325 or §§ 25.1091, 25.1323, and 25.1325 of the FAR by 

analysis acceptable to the FAA or by test. 

b 

airplane 

Test Plan. If it is determined that a test is needed to evaluate the 

spray characteristics, the applicant should submit a test plan. The 

test plan should include, at least, data describing the test airplane's 

configuration, the test facility, the selected water test depth, the test 

instrumentation, and the detailed test procedure. 

c. Test Considerations. The demonstration tests should be extensive enough 

to clearly show the critical water ingestion characteristics of the airplane, if 

any. The number of test runs should be sufficient to demonstrate a17 critical 

phases of the takeoff, landing, and taxi operations. The test procedures 

outlined in paragraph 5d describe a preliminary test encompassing test runs at 

different speeds, but at reduced thrust settings, in order to demonstrate that 

no engine inlet or airspeed port is wetted and that water ingestion is not 

1 ikely at higher thrust settings. If, after a sufficient number of test runs at 

different speeds, it is obvious by inspection of the wetted surfaces that water 

ingestion would not occur even at higher thrust settings, further testing is not 

warranted. 

Page 2 Par 4 
g/30/85 AC 200.124 

d . Suaaested Test Procedure. 

(1) Airplane Configuration. All portions of the airplane that may 

influence the water spray patterns should be the production configuration. If 

spray deflectors (chine tires, fenders, etc.) are used, the predicted in-service 

wear patterns should be included in the demonstration testing. If the engine 

pneumatic systems (air conditioning, anti-ice, etc.), the APU electrical and 

pneumatic loads, and the state of the engine and APU ignition systems could have 

an influence on engine and essential APU operating characteristics while the 

powerplants ingest water, the most critical powerplant operating configuration 

that is consistent with the applicant's recommended procedures for operation on 

wet runways should be used. In addition to testing the takeoff configuration, 

if the landing configuration redirects the water spray into a critical pattern, 

simulated landing tests should be conducted. 

(2) Test Facility. The test facility must have adequate space for 

simulated takeoff runs (also taxi and landing runs, if required). The water 

test bed should be constructed so that the selected water depth can be 

maintained over not less than 90 percent of the test bed area. Intermediate 

dams are allowed within the water trough in order to maintain a more consistent 

water depth on sloped or uneven surfaces. The test bed length should not be 

less than that required to produce a spray pattern of one second duration at 

critical test speeds. The test bed width need only accommodate one landing gear 

if acceptable data are presented to the FAA which show the nose gear spray 

combined with the main gear spray is not more hazardous than the spray produced 

by each separately. If a water depth less than one-half inch is used, then that 

depth becomes an operational limitation for the airplane. 

(3) Data Collection. The applicant is expected to provide adequate 

personnel and equipment to collect the test data. The following list is the 

suggested data which are to be collected: 

(i) Airplane Performance Data. The operating characteristics of 

the engine (and APU, if applicable) should be recorded. The airplane velocity 

should be recorded when it enters and exits the test bed. Any engine sounds 

which are indicative of engine distress should be noted. The airspeed system 

should be monitored to observe any fluctuations from water ingestion. 

(ii) Water Spray Pattern. The pattern. of water spray on the 

airplane should be recorded. This can be accomplished by coating the 

appropriate areas of the airplane with agents which identify the wetted areas 

and by using suitable high speed video recording equipment. The origin, 

trajectory, and configuration of the water spray should be recorded. 

(iii) Meteorological Data. The appropriate meteorological data 

. including wind velocity should be recorded for each test run. If no local 

airport facilities provide this data, appropriate equipment should be provided 

by the applicant. 

(iv) Pilot Comments. Pilot comments should be noted to determine 

if the takeoff woul?!%% been affected by abnormal audible engine sounds, 

airplane instrument anomalies, etc. 

Par 5 Page 3 
AC 20-124 g/30/85 

(4) Test Runs. The purpose of the test runs is to demonstrate 

compliance with the relevant regulations by determining that the water spray 

characteristics of the airplane are not hazardous to its operation. If the 

individual spray patterns indicate that the combined spray patterns of the nose 

and main landing gear do not combine to produce a more hazardous spray pattern, 

then each gear may be tested individually. If the nose gear or the main gear ( 

is determined to be the most critical, only that gear need be tested. However, 

if data from the individual tests show the possibility of a combined spray 

pattern presenting a hazardous condition, nose and main gear combination 

testing should be performed. 

(i) Engine Thrust or Power Settings. The tests should be 

. conducted with the appropriate thrust or power setting. However, during 

preliminary tests to determine airplane speeds for critical water spray 

patterns, reduced power settings may be used to facilitate establishing target 

test bed entry speeds. If any engine inlet or airspeed system port or probe is 

wetted during reduced power test runs, additional testing at the appropriate 

power setting should be conducted for that speed range. If the reduced power 

test runs demonstrate that no critical speed exists, and would not likely exist 

at higher power settings, further testing of that configuration is not 

necessary. 

(ii) Test S eeds 

+* 

To establish the critical speed range, test runs 

through the water trough s ould be conducted in speed increments of not more 

than 20 knots. If a critical speed region is determined, smaller test speed 

increments may be required throughout that speed range. If preliminary tests or 

analysis, acceptable to the FAA, satisfactorily show that a test speed range is 

not critical with respect to engine or airspeed system water ingestion, test 

runs need not be conducted at speeds within that range. 

(iii) Test Configurations. The following ground operational phases 

. . should be tested for their water Ingestion characteristics, unless it has 

previously been determined that a hazard does not exist. 

(A) Simulated Takeoff (if required). If tests of the takeoff 

configuration are deemed necessary, the runs through the test trough should be 

conducted at the critical speeds. If the rotated airplane attitude is suspected 

to be critical, at least one run at VR should be conducted with the airplane 

rotated to the normal rotation angle to show what influence this combination of 

power, flaps, and angle has on the spray pattern. 

(B) Simulated Landing (if required). If tests of the landing 

configuration are deemed necessary, then the runs through the test trough should 

be conducted at any critical speeds from the touchdown speed to the lowest 

appropriate speed using the normal approved landing configuration. Engine power 

settings appropriate for landing and roll-out should be used. If thrust reversers 

or reversing propellers are provided, additional water ingestion tests should be 

considered to evaluate the use of these systems while operating on wet runways. 

(C) Taxiing (if required). Airplane speeds, power settings, 

. and configurations appropriate for normal taxi operations should also be tested. 

This requirement is usually covered during takeoff tests. 

Page 4 Par5 .
g/30/85 AC 20-124 

(iv) Pretest Activity. The water depth, wind velocity, 

and ambient temperature should be recorded before each test run. The 

water depth should meet the 90 percent criteria before beginning any test run. 

Winds parallel to the water trough may interfere with maintaining consistent 

water depth, unless dams are used throughout the trough. 

(v) Test Activity. Outside observers should be provided to 

determine if the airplane remains in the test bed. The observers should also 

note the water spray pattern, listen for abnormal engine sounds, and observe the 

affect of the wind on the spray pattern. The motion/video cameras, if used, 

should record the test run. The preliminary test data collected on each test 

run should be provided so that a determination can be made that the test run 

parameters were met. Usually, it is apparent to the observers at the completion 

of each test run whether the run was valid and if a critical speed was 

identified. Failure to attain the desired speed or stay within the test bed 

may disqualify the run. 

6. FINDING. 

a. No hazardous quantities of water are considered to have been ingested 

. when review of the collected test data shows that: 

(1) No engine flameout, performance degradation, distress, or airspeed 

fluctuations occurred which would create a safety hazard during any of the test 

runs through the selected water depth; and 

(2) If engine sounds denoting engine distress, such as pops or bangs, 

. 

or cockpit instrument indications of engine surge or stall occurred, they were 

not of sufficient magnitude to cause a pilot in service to abort the planned 

operation. 

b If the above criteria are not met using the prescribed water depth, test 

runs it another water depth must be performed to establish an operational limit. 

7. AIRPLANE FLIGHT MANUAL. If compliance with the water ingestion test 

. . criteria is demonstrated at a water depth less than that specified, the lesser 

depth becomes a limitation in the Airplane Flight Manual. The limitation 

should specify the water depth for which the airplane was found acceptable. For 

all airplanes, any special instructions for operation in runway/taxiway standing 

water should be included (e.g., special tires, flap settings, engine ignition 

jetting, or reverse thrust or propeller setting, etc.). Any special airplane 

handling techniques necessary to ensure compliance with the water ingestion test 

requirements should be compatible with the handling techniques assumed in 

establishing the Airplane Flight Manual performance characteristics. 

~1. CiiA1G ‘BEARD 

Director of Airworthiness 

Par 5 *KJB.oovERNyElvTPRxMTxNooFFIcE: 1985 - 491-810 - 313/20643 

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