# **THE MODELING OF AIRCRAFT DEICING**

# **FLUIDS DEPOSTION**

**00-Abstract- 66**

Roger L. Wayson, John D. Dietz, Brian Y. Kim

Civil and Environmental Engineering

University of Central Florida

P.O. Box 162450

Orlando, FL 32816-2450

\(407\) 823-2480

\(407\) 823-3315 Fax

wayson@pegasus.cc.ucf.edu

Neal Westlund, and Chris T. Quinn

###### Ricondo & Associates

20 North Clark Street

Chicago, IL.

## ABSTRACT

Glycol deposition near aircraft during deicing operations has become an
important consideration at major airports. A sampling process was used
to quantify glycol deposition from deicing operations at a major
international airport. The resulting data indicate that glycol
deposition was different for the two types of aircraft used in this
study (DC-9 and 757). The main reason appears to be the height of
release. Atmospheric dispersion modeling was not performed because of
the nature of the deposition mechanisms (i.e. overspray and mists).
Based on the collected data, deposition modeling was conducted using
three mathematical modeling approaches and the best overall model was
found to be a power series. The developed model predicts downwind ground
level concentrations of Type I, propylene glycol. In addition to
downwind distance, wind speed was incorporated into the model by
normalizing the measured deposition concentrations by their
corresponding average wind speeds. Separate models were developed for
each aircraft, but the engine conditions were aggregated to obtain
average models.

### INTRODUCTION

Concerns have been raised at a major international airport over deicing
operations at newly proposed sites. Concerns stem from the deposition of
deicing fluid due to overspray and wind-carried mist (fine droplets)
which could result in excessive ground concentrations at critical
locations. To investigate the deposition characteristics of the fluid on
the surrounding areas, several test sprays were conducted on two
aircraft types: a DC-9 and Boeing 757. Quantification of fluid
deposition (in mass per unit area) allowed models to be developed to
predict worst case downwind deposition for varying distances.

As alluded to, the deposition occurs in two ways; direct spray from the
nozzle (overspray) and mists carried by the local wind. The overspray
from the nozzle results in a large quantity of glycol being deposited
near the aircraft in a short period of time. The mist is created either
from spray impact on the aircraft surface or from the high pressure
nozzle itself. This mist is carried by wind currents to greater
distances than the direct spray. Fortunately, mists represent a much
smaller fraction of the total deposition than that due to the direct
spray.

This paper reports on the measurement and modeling effort of the glycol
deposition. The final, developed model is presented.

### BACKGROUND

The fluids commonly used in deicing operations are actually of two
types: Aircraft deicing fluids (ADF) and aircraft anti-icing fluids
(AAF) (McCready 1998). The most commonly used ADF is a Type I mixture of
50% water and 50% propylene (or ethylene) glycol by weight. This fluid
is essentially used to melt and remove ice that has already accumulated
on the outer surfaces of the aircraft and helps prevent additional
accumulation. In contrast, a Type IV AAF is used to prevent ice from
forming on an aircraft. This anti-icing agent is applied as a gel to
literally coat the aircraft. The terms "Type I\" and \"Type IV\" are
Society of Automotive Engineers (SAE) nomenclature used to identify
application conditions. Type I refers to applying fluids at higher than
ambient temperatures and Type IV refers to applying fluids at ambient
temperatures. Since only propylene glycol was used during testing at MSP
(no ethylene glycol or Type IV AAF), \"glycol\" will be used throughout
the rest of this paper to refer to the Type I propylene glycol mixture.

Propylene glycol (1,2-Propanediol) has an extremely low vapor pressure
of 1 mm Hg at 45.5^o^C, and a high boiling point of 189^o^C (Lide 1992).
These properties result in the evaporation rates being quite low with
little evaporation after a deicing event. Puddles usually form during
deicing and the deposited glycol can be collected with a vacuum and
consequently, recycled, or captured by sewers for treatment as
wastewater. The concerns of this study was the resulting wet surfaces
around the aircraft that could result in a potential safety hazard for
vehicles due to a loss of surface friction. This primary reason, as well
as some secondary reasons, resulted in this study to allow prediction of
glycol deposition near the deicing operations.

### METHODOLOGY

Glycol spraying was conducted on two different aircraft (McDonnell
Douglas DC-9 and Boeing 757) under several different conditions. The
various test scenarios are presented in Table 1. The power on state
refers to an idling condition where the aircraft engines are operating
at a low level. In contrast, the breakaway (power out) state refers to a
higher level that is required to move the aircraft. The table shows that
a total of 12 runs corresponding to 9 actual sprayings of aircraft were
conducted. The three non-spraying runs refer to blank samples taken for
quality assurance purposes. The test sprays indicated in Table 1 refer
to runs where the travel distances of the direct sprays were determined.
All tests were conducted between 12 midnight and 6 AM for safety
purposes. Different engine states for the DC-9 (off , on, and breakaway)
and the 757 (off and breakaway) were also tested to determine their
effects on deposition. No testing was done of the 757 with engines on
because this is not done due to impacts on the engines. Three deicing
vehicles were in use during each spraying run, and the operators of the
deicing equipment were instructed to conduct the sprayings as if under a
severe situation (i.e. heavily ice-coated aircraft). The sampling was
passive in that local air patterns were not influenced.

Table 1. Test Scenarios

  ------- ------------- ------------------- ------------- -------------------
  Run     Aircraft^a^   Engine State        Spray Time    Comments

  0       None          N/A                 N/A           Q/A Blanks

  1       DC-9          Off                 4 min         Quick Spray

  2       DC-9          Off                 8 min         Extended Spray

  3       DC-9          On                  7 min         Extended Spray

  4       DC-9          On + Breakaway^c^   7 min         Extended Spray

  5       None          N/A                 1 min         Test Spray

  6       None          N/A                 N/A           Q/A Blanks

  7       757^b^        Off                 9 min         Extended Spray

  8       757^b^        Off                 7 min         Extended Spray

  9       757^b^        Off + Breakaway^c^  4 min         Quick Spray

  10      None          N/A                 1 min         Test Spray

  11      None          N/A                 N/A           Q/A Blanks
  ------- ------------- ------------------- ------------- -------------------

> ^a^The DC-9 sprayings were conducted on the first night and the 757 on
> the second
>
> night.
>
> ^b^The 757 was not sprayed with the engine on due to safety concerns.
>
> ^c^Breakaway (pull-out) power was used immediately after spraying.

The actual collection of glycol was accomplished through the use of
teflon coated collection trays, each 0.127 m^2^ (1.37 ft^2^) in area.
These trays were strategically located downwind of the aircraft as shown
in Figure 1. The same orientation, as shown in Figure 1, was used on
both nights although the second night was a mirror image of the first.
However, in both cases, the aircraft faced into the wind at a 45 degree
angle. Site A was collected only at the end of each day\'s samplings
(i.e. after all the runs had been completed for one type of aircraft) in
an effort to determine the buildup due to multiple deicing runs. Sites B
and C were placed side-by-side to assess data collection precision. Site
D was elevated to 3 m (10 ft) above ground level to help determine
vertical distribution. Sites A-F were at a 45^o^ offset from the
aircraft\'s longitudinal axis and directly downwind. Sites G and H were
at a 90^o^ angle from the aircraft longitudinal axis and used to measure
possible upwind deposition. Sites I, J, and K were along the
longitudinal axis of the aircraft to allow the deposition due to jet
blast to be evaluated. In addition, Sites I, J, and K allowed the impact
from engine jet blast on glycol re-entrainment to be evaluated.

###### Figure 1. Sample Locations

After each run, the teflon surfaces were rinsed with deionized water
from teflon rinse bottles to remove glycol. The rinse mixture was
captured in glass collection bottles. The bottles had been previously
baked to remove any organic residue. The use of teflon and glass
surfaces was warranted to prevent contamination of samples since low
level detection would be used.

The samples were analyzed at the University of Central Florida (UCF) for
Total Organic Carbon (TOC). Since the teflon coated surfaces had been
cleaned before each test and no other primary sources of carbon were
nearby or upwind, the TOC was assumed to represent the glycol collected.
TOC characterization tests of several samples were conducted to
determine actual glycol results during TOC analysis. This allowed
results from TOC testing to be applied directly to glycol capture
through the use of a correction factor.

The result from a TOC analysis is essentially a concentration in mass
per volume of the carbonaceous material in the sample bottle. Since the
volume of the bottle and the collection area of the pan were known, the
mass of the glycol, and hence, the deposition concentration in mass per
unit area could be determined.

### RESULTS OF MEASUREMENTS

During the tests, several observations were noted. First, the low
evaporation rate of glycol was confirmed when the pavement surface was
observed to still be damp from the first night\'s sprayings even after a
vacuuming operation and 19 hours that included sunny and windy daytime
hours. Second, testing verified that the direct spray could travel well
over 30 m (100 ft). If an aircraft\'s vertical stabilizer is being
treated, overspray is almost certain to occur as the leading and
trailing edges are treated. Third, as expected, the overspray was
observed to be dominant in its contribution to glycol deposition
compared to the mists that were formed. Lastly, the engine operation
mode was found to significantly affect glycol deposition but differently
depending on aircraft type. The jet blast can cause mists to settle at a
much farther distance than if the engines are off.

The characterization testing from the TOC testing, using pure glycol,
indicated that a factor of 0.314 mg TOC per mg glycol occurred. This
factor was used to determine deposition rates of glycol based on the
measured TOC results.

Sites A, B, and C had the highest glycol depositions most likely due to
their proximity to the aircraft and the direction of the prevailing
wind. Although site D was also close to the aforementioned sites, it did
not experience as much deposition because it was elevated 3 m above the
ground. This tends to point out the quick settling rates. The data also
confirms the observation that the jet blast significantly affects the
settling distance of mists. This effect is exemplified by the increase
in deposition for sites downwind of the jet blast as the engine
condition is changed from the off to the on condition (from run 2 to run
3), and from the off or on condition to breakaway (run 3 to 4 or run 8
to 9). The data also shows that when comparing different aircraft types
(i.e. runs 2 and 7), the 757 deicing operations resulted in much greater
deposition of glycol than that of the DC-9. This discrepancy is thought
to be caused by the greater height and overall size of the 757 resulting
in a higher release point causing greater areal dispersion of glycol and
the engine orientation. The DC-9 engines are located along the fuselage
while the B757 engines are under the wings. The differences in the
aircraft also resulted in different spray patterns as the results show.
It should also be noted that the aforementioned comparisons were made
for similar spray times. The difference in spray times for each runs are
further elaborated in the discussion of uncertainties.

### DEPOSITION MODELING

Atmospheric dispersion modeling was not applied in this situation
because of the complexities (i.e. random droplet size) involved with
deposition mechanisms. As previously discussed, deposition away from the
aircraft occurs either through overspray which accounts for the bulk of
the deposition or through mist settling. Further complications are that
aircraft type and engine condition can also affect deposition.
Accordingly, deposition models were developed separately for each
aircraft to predict ground level deposition rates downwind of the
aircraft. Due to the nature of the spraying operations, spray directions
constantly change and are not included in the aforementioned models. All
results in this study correspond to complete deicing operations on
aircraft.

Modeling was accomplished by relating wind speed and distance to
deposition concentration. Near site wind data were taken and compared to
tower data for the two days of sampling. A normalization process was
used in that deposition rates were divided by their respective average
measured wind speeds. These normalized values and their respective
deposition distances were regressed using different model forms:

Polynomial: y = ax^2^ + bx + c (1)

Power Series: y = ax^b^ (2)

Exponential: y = ae^bx^ (3)

The use of these model types was warranted due to the non-linear
appearance of the plotted data (Figures 2 to 5). For each run, only data
from sites within the corresponding wind direction were included in the
regression analysis since this was the dominant deposition direction and
represents the worst case scenario. Data was included if the site was
within 30^o^ ([+]{.underline}30^o^) of the wind direction. Sites within
this angular band were assumed to be equally affected by the wind. In
order to use data with similar test conditions, only runs 2-4 (DC-9) and
7-9 (757) were used in such a way that deposition was analyzed with and
without the engine breakaway condition. All of this data corresponds to
ground level receptor locations. Site D was the only receptor location
that was raised (i.e. 3 m) above the ground. Since the data from this
site was not considered sufficient to incorporate height into the
deposition models and the primary concern in this study was ground
deposition, the models developed in this study correspond to ground
level depositions only.

The coefficients and goodness of fit criteria for the models are
presented in Table 2. Although the polynomial models produced the
highest coefficient of determination (R^2^), the parabolic nature of the
curves restricts its use to distances from zero to that corresponding to
the minimum point on the curve. Therefore, these models could not be
used to represent the overall deposition mechanism. The coefficient of
determination and the mean squared error (MSE) are similar for the power
series and exponential models, but the power series models generally
appear to produce the best results. The R^2^ values for these models are
acceptable (i.e. 0.6011 and 0.5757 for DC-9, and 0.6844 and 0.8025 for
757), and the relatively small root MSEs and the significance of the
p-values (i.e. less than 0.05 for 95% confidence) indicate that using a
power series model to predict glycol deposition is justified. Since the
deposition concentrations were normalized with their corresponding wind
speeds, the calculated results from the models must be multiplied by
wind speeds. Therefore, the functional form of the recommended power
series model would be as follows:

C = (ae^bx^)u (4)

where C = deposition concentration (L/m^2^)

x = distance from aircraft (m)

u = wind speed (m/min)

a, b = coefficients (dimensionless)

In order to visually compare the effects of different engine conditions,
the data used in the regression analysis was plotted according to
aircraft type and engine condition. While not shown here, the graphs
support the initial observations that engine condition affects glycol
deposition concentration. In general, deposition seems to increases as
engine condition changes from the off to the on condition, and increases
even further when breakaway power is applied. General models were
developed by aggregating data corresponding to different engine
conditions. However, breakaway power was thought to be significant, and
an attempt was made to separate out its effects by deriving models
corresponding to data sets with and without breakaway power (runs 4 and
9). corresponding to the use of breakaway power.

### DISCUSSIONS OF UNCERTAINTY

The precision involved with the sample collection was determined by
comparing duplicate samples from the same collection bottle, and also by
comparing samples taken from adjacent locations (i.e. sites B and C).
The results of the duplicate sample comparisons provide an indication of
analytical (measurement) error reported as a relative percent difference
(RPD). In contrast, the results of the adjacent sample data provide
errors associated with field sampling procedures. The average analytical
and sampling RPDs (4.46% and 17.42%, respectively) give an overall
indication of acceptable precision.

The accuracy of the data was characterized by quantifying glycol
recoveries from known samples. Since glycol concentrations in field
samples were unknown prior to laboratory analysis, they were
inappropriate for determining accuracy. Therefore, samples fabricated in
the laboratory were used. To determine analytical accuracy, a known
concentration was spiked with a known quantity of glycol. This glycol
was taken directly from the

Figure 2. Deposition Model for the DC-9 without Breakaway Data

![](media/image1.jpeg){width="5.989583333333333in" height="3.25in"}

Figure 3. Deposition Model for the DC-9 with Breakaway Data

![](media/image2.jpeg){width="6.010416666666667in"
height="3.4270833333333335in"}

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Figure 4. Deposition Model for the 757 without Breakaway Data

![](media/image3.jpeg){width="6.010416666666667in"
height="3.4270833333333335in"}

Figure 5. Deposition Model for the 757 with Breakaway Data

![](media/image4.jpeg){width="6.010416666666667in"
height="3.4270833333333335in"}

spray trucks on site to ensure consistency such that the same glycol
mixture was used for both quality assurance testing and field sampling.
Sampling accuracy was determined by reproducing field sampling
procedures in the laboratory but under controlled conditions. A known
glycol quantity was applied to a sampling apparatus which was then
rinsed using procedures employed in the field to generate a sample for
analysis. The recovered sample was compared to the known quantity placed
on the apparatus. Temperature was also varied to see if it had any
effect on recovery. The average recoveries of 105.15%, 93.69%, and
85.05% indicate reasonable levels of analytical and sampling accuracy
for the lab.

In analyzing the samples collected in the field, contamination had to be
quantified. Both opened and unopened blank bottles were analyzed to show
that the TOC, and hence, the glycol levels in these blanks were
virtually zero (i.e. below the detection limit). Blanks collected from
washings at the beginning of a sampling day and second washings after a
spray run were used to determine detection limits. The term, detection
limit, as used here refers to a contamination level that makes a data
value unusable if it is below the detection limit. The average value for
the blanks at the beginning of the first and second day (run 6) of
sampling were 0.56 mg/L and 0.88 mg/L (as pure propylene glycol),
respectively. To be conservative, the second day's value (0.88 mg/L) was
used as the detection limit for all of the samples collected on the
first day. The determination of the detection limit for the second day
is more complicated. The detection limit of 0.88 mg/L is appropriate for
the first run (run 7) on the second day, but not for runs 8 through 10
because runs 7 through 9 appear to have produced high concentrations
that were not uniformly experienced at all sites. Thus, the use of a
single detection limit was not considered appropriate. Instead, blank
levels after the second day (run 11) of sampling were used as the
detection limits based on site location:

Site A 32.46 mg/L

Site B 35.45 mg/L

Site C 128.49 mg/L

Site D 12.49 mg/L

Site E 21.80 mg/L

Site F 14.19 mg/L

Site G 0.72 mg/L (use 0.88 mg/L)

Site H 0.37 mg/L (use 0.88 mg/L)

Site I 1.68 mg/L

Site J 0.65 mg/L (use 0.88 mg/L)

Site K 0.94 mg/L

Site L 243.39 mg/L

All of the aforementioned detection limit values were used in
classifying data as being real or below detection, and hence, invalid if
the measured values were less than those shown.

A discrepancy that needs to be addressed concerns the fact that the
models were derived from the values which were based on varying spray
times. Although the usage of only runs 2-4 and 7-9 minimizes the
variation in spray times, the differences can still be problematic since
glycol usage, and hence, deposition rates are directly related to spray
times. The most significant difference occurred when comparing runs 7
and 9 (9 and 4 minutes, respectively) which would reasonably be
considered significant. However, the subjective nature of spraying
operations by human operators tends to diminish the importance of the
difference in spray times. During deicing processes, it was noted that
spraying was not continuous; rather, the operators would intermittently
discontinue spraying for up to several seconds in order to reposition
the spray vehicle and spray personnel bucket. The inconsistencies in
these off-times also reduced the possibility of using exact spray times
for a field use model. Furthermore, the time spent deicing different
sections of an aircraft may not be consistent. Therefore, even if the
spray times were equal for two runs, the amount of glycol sampled at the
same site could be different. Considering these factors, the deposition
models developed in this study should be considered as estimation tools
for predicting average deposition concentrations.

# **CONCLUSIONS**

From observations and the collected data, direct sprays were found to
reach distances of up to 53.3 m (175 ft). At these distances, it was
clear from observations that most of the deposited glycol reached the
ground very quickly while a fraction was carried further downwind as
mist. Although deposition due to mist appeared to be relatively small,
even mist depositions can become significant as they add up over time. A
strong factor affecting deposition concentration is the initial height
of the spray. With a higher nozzle position, deicing a 757 appears to
produce greater glycol deposition with distance than a DC-9. Also,
increasing receptor height appears to dramatically decrease the
deposition rate as shown with site D which was raised 3 m (10 ft) above
ground level. It is also important that deposition is affected by engine
mode. However, more data will be required to better quantify this
finding. The deposition models were developed separately for each
aircraft since the spray height appeared to affect deposition. Power
series models appeared to produce the best overall results. They
generally had the highest R^2^ values and their coefficients tested
significant with p-values less than 0.05.

In order to obtain an overall deposition concentration for several
consecutive deicing operations at the same location, the concentrations
measured or predicted could be multiplied by the number of aircraft
(taking into account that different aircraft will produce different
concentrations). This simple method is reasonable since glycol
evaporation is negligible. The movement of aircraft and other vehicles
would have to be taken into account since they will carry off some of
the deposited glycol.

### RECOMMENDATIONS

Abatement at the deicing site is suggested in two steps. First, spray
crews should be instructed to spray at a downward angle when possible
and to spray at the minimum height where good coverage still occurs.
Second, the data from the elevated site show that even at closer
distances (here 36.6 m; 120 ft) deposition is generally less than
one-fourth the ground level deposition. This implies that a blast fence
could be effective in reducing the impact away from the aircraft. In
addition, a typical blast fence with wide slot openings is not
recommended. The fine mist could easily travel through these slots
carried by the drag force of the wind. Many designs are possible, but
for each, the wind flow should be forced to make abrupt changes in
direction in an attempt to capture the mists using impaction.

######### REFERENCES

1\. Lide, David R., Editor-in-Chief. [CRC Handbook of Chemistry and
Physics]{.underline}. 73^rd^

Edition. CRC Press, Inc., Florida. 1992-1993.

2\. McCready, David. \"Estimation of Glycol Air Emissions from Aircraft
Deicing.\" A

Paper Presented to the Air & Waste Management Association 1998 Annual
Meeting,

San Diego, California, June 14-19, 1998.

######### KEY WORDS

Aircraft, glycol, deposition, modeling, airports, deicing fluids